# Could this three moons system be stable?

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In my fictional world I have selected all parameters to maximize the Hills Sphere of my planet. The planet has three times Earth's mass and it is located in 2 AU from its star that's 1.4 times more massive than our Sun.
I assume that there are more planets on the system but in such way to have minimum interaction with my planet.

I want to have three moons on a 1:2:4 resonance. I also want:

• the first one to be as large as our moon to the sky
• the second 1.5 times larger
• and the third, half our moon's size.

To achieve this I have assumed:

• the first moon having 1.1 lunar radius,
• the second 2.3
• and the third 1.1.

Their orbits are 18, 36, 72 days.

I don't really care about their density and I assumed that their masses are

• 1 lunar mass (probably rocky)
• 3 lunar mass (water or ice world)
• 0.3 lunar mass (probably ice world)

I guess that to be more stable i should have eccentricities smaller than 0.05 and inclination equal to zero.
I am not sure about the spin of my moons, is there a chance being tidal locked?
This is my effort to achieve maximum stability of the system, what parameters should I reconsider to make it more stable? I am willing to change almost everything even the resonance, but I want them to be visible from my planet in similar way moon is to Earth.
Have in mind this question is about a fictional world for a fantasy story so even the minimum possibilities for that senario to happen is enough for me. So any ideas?

P.S I have asked a similar question on worldbuild.stackexchange but they directed me also here for a better answer.

Nothing in your description sounds wildly implausible. I'll just go through and extend some of your properties to make sure they make sense though.

The planet has three times Earth's mass

I'll assume that your new planet is Earth-like in composition and density. That implies that it's radius should be about $R_p approx 1.4 : R_oplus$. Let's put that aside to use for later.

Now let's look at your moons.

• the first one to be as large as our moon to the sky
• the second 1.5 times larger
• and the third, half our moon's size.

To achieve this I have assumed:

• the first moon having 1.1 lunar radius,
• the second 2.3
• and the third 1.1.

The first moon, you want to be as large as our current Moon in the sky and you say it has a physical radius of $R_{M1} = 1.1:R_{M}$. Our current moon subtends about $30:arcsec = 8.73 imes10^{-3}:rad$ on average. You can calculate the distance, $d_{M1}$, the moon must be from the planet to have the same apparent size as our moon with the following equation.

$$d = frac{R}{tan(delta/2)}$$

Using $R_{M1}$ for $R$ and $8.73 imes10^{-3}:rad$ for $delta$, your moon must be $d approx 4 imes10^8:m = 44.5:R_p$. Your first moon must be about 44.5 planetary radii out. Compare this to the Moon's distance of about 60 planetary radii out.

Now, we want to know how long it would take for such a moon to orbit, given the planet's mass, the moon's mass, and orbital distance. You can get that from Kepler's third law.

$$P = sqrt{frac{4pi^2}{G(M_p + M_{M1})}d^3}$$

We can say $M_p = 3:M_oplus$ (as you specified) and we calculated $d$ just now. We only need to specify the mass of the moon. You provided the desired masses for your moons, so we're all set. Note $G = 6.67 imes10^{-11} m^3kg^{-1}s^{-2}$ is the gravitational constant. I find that $P_{M1} = 1.44 imes10^6:s=16:days$.

Let's stop right here for a second now. We've reached a point where your numbers are in conflict. Given your planet and your first moon's apparent size, mass, and radius, we found that it would have to orbit the planet in 16 days. If you want to keep the 1:2:4 resonance, then you need your other moons to orbit in 32 and 64 days, respectively.

I won't go through the rest of the math, I'll leave that up to you. But what you can do, if you want to ensure everything is consistent with real physics, is say I know how long my remaining two moons need to orbit for (i.e., you know $P$ for them), then at what distance must they orbit (i.e., what is the value of $d$ for them)? Work Kepler's third law backwards to get that. Then, given your desired apparent sizes in the sky, determine how large must they be physically by working the angular size equation backwards. You'll get new radii for the moon. You might then want to verify that the new mass and radii for your moons correspond to densities that match the desired moon types. E.g., if your third moon is going to be an ice moon, it should have a density of $sim1:g/cm^3$. You can play with your numbers until you get a system that matches your requires and fits the equations.

An important note: All these equations use MKS units. That is, masses should be in kilograms ($kg$), distances and sizes in meters ($m$), time in seconds ($s$), and angles in radians ($rad$).

I see no reason why this wouldn't work. The innermost Galilean moons are in a 1:2:4 resonance so it's clearly a stable orbital configuration. They could all be tidally locked if you want, but the planet itself can't be tidally locked with all of them. If the planet were tidally locked it would likely be locked with the innermost moon.

Also of note, the Galilean moons all have eccentricity less than 0.01 and inclinations less than 0.5 degrees.

## Moons Can Help Planets Remain Stable Long Enough for Life to Form

The Moon is more than just Earth’s partner in space — it may have helped stabilize Earth’s orbit enough for it to become hospitable for the evolution of complex forms of life.

A new study suggests that large moons can form and remain stable for long times around distant planets as well, potentially helping alien life evolve.

Researchers also suggest that if the recently discovered rocky alien planet Kepler-62f has a moon, the moon could last more than 5 billion years, perhaps long enough to help foster the evolution of complex life. The investigators detailed their findings in the International Journal of Astrobiology.

In the past two decades or so, astronomers have confirmed the existence of more than 1,700 planets beyond the Solar System, and they may soon prove the existence of thousands more of such exoplanets. Of special interest are distant planets in habitable zones, the regions around stars just warm enough for worlds to possess liquid water on their surfaces, as there is life virtually wherever liquid water is found on Earth.

Artist’s conception of super-Earth Kepler-62f, which some astronomers say is a misleading term because these worlds are closer in kin to Neptune. Credit: NASA

To support complex forms of life, a world needs more than just an orbit within its star’s habitable zone. It probably also needs a climate that remains stable over long time spans as well. One major factor controlling a world’s climate is its obliquity, also known as axial tilt, which has to do with the amount its axis of rotation is tilted in relation to the path it takes around its star.

Earth’s seasons, for example, depend on the axial tilt, as the amount of light hitting the northern and southern hemispheres varies with the way the northern and southern hemispheres point toward or away from the Sun.

Earth’s axial tilt was stabilized with the help of the gravitational pull of its large moon, which is roughly one-quarter the diameter of the Earth.

“If the Earth did not have the Moon, the Earth’s axial tilt would have changed rapidly and the climate of the Earth would have changed often,” said lead study author Takashi Sasaki, a planetary scientist at the University of Idaho.

In contrast, Mars has relatively small moons, and its axial tilt has changed substantially over long periods of time, fluctuating chaotically on a 100,000-year time scale. These wobbles in Mars’ axial tilt might help explain why vast underground pockets of ice have been discovered far from the Red Planet’s poles. In the distant past, Mars’ axis might have been tilted at a significantly more extreme angle than it is now, and ice caps were able to reach across the planet. Even after Mars’ axial tilt became less extreme, this ice far from the poles survived, protected by subsequent layers of dust.

View of a planetary system seen from Kepler-62f. Credit: Danielle Futselaar/SETI Institute

A planet whose axial tilt fluctuates wildly like Mars may not maintain a favorable climate for a long enough time for complex forms of life to evolve. For example, it took about 3.8 billion years for life on the 4.6-billion-year-old Earth to evolve from single-celled organisms to multicellular life such as plants, animals and fungi.

“Because the Earth has had a long-term stable climate, life on the Earth has had time to evolve from single cells to complex life forms,” Sasaki said.

Since the Moon is a key reason why Earth has had a relatively stable climate for a long time, the Moon is one of the key factors in Earth’s evolution of complex life forms, he said.

Sasaki and his colleague Jason Barnes sought to understand how long moons might last around rocky planets in habitable zones, given varying masses and compositions of moons, planets and stars. They focused on systems where moons could last 5 billion years, assuming that such a duration is long enough for complex life to evolve.

Their model accounted for how a planet or moon’s gravitational pull increases in relation to increasing mass. In addition, their calculations factored in how gravitational tidal forces are greater the closer two bodies are to one another. The gravitational pull of a planet’s star can also influence the relationship between that world and its moon.

Mars’ moon Phobos as seen by the Mars Express spacecraft. Credit: G. Neukum (FU Berlin) et al., Mars Express, DLR, ESA

Three potential scenarios were possible. First, a moon could get closer and closer to its planet until it breaks apart or collides with its host, as Mars’ moon Phobos is predicted to do about 50 million years from now. Next, a moon could get farther and farther away until it escapes the planet. Last, a moon can reach a stable distance from its planet, as is the case for the dwarf planet Pluto’s moon Charon.

The rate at which a moon gets closer to or farther away from its planet depends on the extent to which the tidal forces they exert on each other dissipates and slows their rates of spin. For instance, as the Moon’s orbit has taken it farther away from Earth over time, the Moon’s rate of spin has slowed to the point that it now always shows just one side to Earth. Eventually, the Earth will also slow its rate of spin enough to always show just one side to the Moon.

An artist’s concept of Pluto as viewed from the surface of one its moons. Pluto is the large disk at the center of the image. Charon is the smaller disk to the right. Image Credit: NASA, ESA and G. Bacon (STScI)

The degree to which a moon and its planet dissipate the tidal forces they exert on each other relies greatly on the mass, compositions and structures of those bodies. For instance, the way tides slosh water around in the shallow seas of Earth dissipates large amounts of tidal energy. Planets with no oceans or with deep oceans would dissipate less tidal energy than Earth.

The researchers examined four typical planet compositions: Earth-like planets composed of 67 percent mostly silicon-based rock and 33 percent iron planets with 50 percent rock and 50 percent ice planets with 100 percent rock and planets with 100 percent iron. These planets were one-tenth to ten times Earth’s mass and orbited the habitable zones of stars that ranged from 40 percent to equal the mass of the Sun.

The scientists found that stars with less than 42 percent of the Sun’s mass may not be good places to look for complex life because moons cannot survive for more than 5 billion years in these systems. This is because the habitable zones are closer to stars that have dimmer and lower masses than in brighter, higher-mass star systems. For instance, in solar systems with stars 40 to 50 percent of the Sun’s mass, the habitable distance is approximately one-quarter of the distance between the Sun and Earth. Since these planet-moon systems are so close to their host stars, their stars gravitational pull perturbs the planet-moon systems too much for the moons to remain around their planets, Sasaki said.

New Horizons LOng Range Reconnaissance Imager (LORRI) composite image showing the detection of Pluto’s largest moon, Charon. When these images were taken on July 1 and July 3, 2013, the New Horizons spacecraft was still about 550 million miles (880 million kilometers) from Pluto. On July 14, 2015, the spacecraft is scheduled to pass just 7,750 miles (12,500 kilometers) above Pluto’s surface, where LORRI will be able to spot features about the size of a football field. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute

This finding runs counter to the belief that lower-mass stars are good for habitable planets because they live longer than higher-mass stars, potentially giving them more time for life to evolve. For example, while the lifetime of a planet with a mass twice the Sun’s mass is about 1.2 billion to 1.3 billion years, the lifetime of a star with half the Sun’s mass is about 80 billion years, Sasaki said. However, he noted “our results show that small-mass stars may not be good parent stars for habitable planets.”

For stars more than 42 percent the Sun’s mass, whether a moon survives depends on factors such as the planet’s composition and how well the planet dissipates tidal energy. A moon has a longer lifetime the higher the density of its host planet.

The researchers also investigated the prospect for moons in the Kepler-62 system, which at a distance of 1,200 light years from Earth has a star that is a bit cooler and smaller than the Sun, as well as two planets in the habitable zone: Kepler-62e and Kepler-62f. The planets are 1.4 and 1.6 times Earth’s diameter, respectively.

The diagram compares the planets of the inner solar system to Kepler-62, a five-planet system about 1,200 light-years from Earth in the constellation Lyra. Credit: NASA/Ames/JPL-Caltech

The scientists found that Kepler-62e would have to be composed almost entirely of a high-density material, such as iron, for a moon orbiting it to exist for more than 5 billion years. They also discovered that Kepler-62f could have a moon for more than 5 billion years if it had a variety of different compositions, particularly if it had an absence of oceans or only deep oceans, either of which would cause the planet to dissipate less tidal energy.

In the future, instead of looking at moons around Earth-sized planets in habitable zones, Sasaki said they would like to investigate moons around giant planets in habitable zones.

“If a giant planet at habitable zone has a big enough moon, there may be life on the moon,” Sasaki said. “Finding the conditions favorable for habitable moons is a direction we might go.”

What you are describing is a rogue planet with moons. This is a planet that does not orbit any star, either having been ejected from its original solar system, or never having belonged to a solar system in the first place. This would not be aptly described as a solar system, since there is no star, but you could have a rogue planet with moons orbiting it. A rogue planet could perhaps be described its own system, as it does not belong to any solar system.

Rogue planets are naturally dark and cold, since they do not receive significant insolation from any star. Starlight, though weak, is still light however, so the planet would not be entirely pitch black. Any heat would have to come from geothermal or radioactive activity within the planet itself, as there is no significant external source of energy like the Sun. Rogue planets that don't have their own internal sources of energy will be dead. I wouldn't say it's impossible for life to exist on a rogue planet, but I'd expect the odds to be much lower simply due to far lower abundance of available energy.

### A single big planet and a single big star orbiting around? No.

Wikipedia has a list of star extremes. This star is the smallest, 7% the mass of our sun. (So about $7*10^<28>$ kg)

This planet is the largest, 20 * times the mass of jupiter (So about $3.7*10^<28>$ kg)

These are very close in mass, they'll orbit around a point 2/3rd the distance from the sun to the planet.

### But can we get a planet with a star orbiting around it?

Yes There are some precise configurations that have a massive planet in the centre and a big sun orbiting around it.

The simplest would be 3 massive planets, and a tiny sun. The tiny sun is twice the weight of each planet, and all 3 planets are the same weight. The tiny sun and 2 of the big planets share an orbit, with the two planets close together. The forces should cancel out, leaving the big planet in the barycentre of the system.

Note this isn't called a solar system - technicality.

Life on the planet will be very similar to if the sun was in the centre and the planet orbited around it. Ironically, if they're aware of normal solar systems It may take them a while to realise the sun isn't the centre, actually)

This system is highly unlikely to occur naturally - perhaps a supernova blew out a chunk of gas cloud, forming a ring of gas, which formed the star and 2 gas giants? It's a bit of a stretch. Perhaps the bodies were precisely caught in the right way, or perhaps aliens built it. This would also not be stable for millions of years, it would decay over time.

### Can we do it slightly more stable?

If we allow the planet to move, but meet the "at the centre" requirement by having nothing closer to the centre than the planet, we can have a slightly more stable system by having the big planet in a tight multi-body orbit around nothing (ie "at the centre"), with the sun in orbit around that mutual centre.

Huge gas giant rotating fast.

Behold the glowing sky of Io!

This eerie view of Jupiter's moon Io in eclipse (left) was acquired by NASA's Galileo spacecraft while the moon was in Jupiter's shadow. Gases above the satellite's surface produced a ghostly glow that could be seen at visible wavelengths (red, green, and violet). The vivid colors, caused by collisions between Io's atmospheric gases and energetic charged particles trapped in Jupiter's magnetic field, had not previously been observed. The green and red emissions are probably produced by mechanisms similar to those in Earth's polar regions that produce the aurora, or northern and southern lights. Bright blue glows mark the sites of dense plumes of volcanic vapor, and may be places where Io is electrically connected to Jupiter.

Io has different types of aurora. They are produced by interactions with Jupiter. Jupiter has a tremendous magnetic field and given off loads of radiation in the form of charged particles. Ultimately I think the energy fueling this is the rotational momentum of the Jupiter and maybe the residual heat of condensation when it was formed.

Your central planet is a colossal gas giant, 20 times the size of Jupiter. Its great mass and fast rotation generate huge magnetic fields. Your moons also have magetic field and atmospheres - these moons are the size of Earth. They also have magnetic fields which they need to protect them from the radiation emitted by their giant. The charged particles splash against the magnetic fields of the moon, lighting the sky just as Jupiter's particles light the skies of Io.

Just yeet out a Gas Giant

So in general, you could have such a System. If you look at the Jupiter System, it is pretty much its own "Solarsystem". You got a bunch of Moons around a Central Object with a shi´tton of debris flying around.

This could just from natrually in the middle of nowhere. One might argu that such a System is just like or one. Only smaller and with a failed star at its center.

But how do you get light ?

Good question. Id say since the Gas Giant is the middle of nowhere, it probably wont emit any light by itself. If it would, it is a Mini Star. So one way of getting a bit of light would be to have a Planet VERY close to the Gas giant. In fact so close that it melts dou to the tidal forces. But even this would be really dark. Not to mention that such a close Planet would just fall into the Gas Giant in a very short amount of time.

You could try to go all Meta and have a lifeform on the Gas Giant that is Bioluminescent for some reason. Depending on the sizes of the Gas Giant that may create enough light for something. But i am not quiet sure why any life form would decide to go that way in the Darkness of Interstellar Space. Maybe because of some Aurora but even that is a real streatch because, where does the Aurora come from ?

But i would still assume that a Bioluminescent Gas Giant is probably your best bet of getting any amount of light. Even if it still is almost nothing. Such a Gas Giant would probably by hardly any more bright than or Moon.

Your main source of Energy in such a System isnt light anyways. Its the tidal forces. And the first life on earth really didnt need light so it might very well still start. But i dont see how life would get complex if every Moon around the Gas Giant is a frozen Ice ball.

## The best places to look for alien life in our Solar System

### Venus

Location: 108 million kilometres from the Sun

Pros: May have harboured oceans for a long time

Cons: Hellishly hot on surface, clouds of concentrated sulphuric acid

Missions planned: DAVINCI+ (2026 launch, not confirmed)

You’d have to have been living under a rock on a distant planet to have missed the news in September 2020 about the unexpected – and as yet unexplained – discovery of the gas phosphine in the atmosphere of Venus.

By October, there were some doubts creeping in about whether phosphine had really been detected, but either way there’s definitely some previously unknown chemistry going on in the Venusian atmosphere. Perhaps it could even be biochemistry – is the phosphine a telltale signature of Venusian life?

The problem with Venus, at least for astrobiologists, is that it’s a truly hellish world. The planet is smothered in an exceptionally thick atmosphere of carbon dioxide, which creates a powerful greenhouse effect. The surface temperature is over 460°C: hot enough to melt lead.

As you rise to higher altitudes the temperature grows cooler (just as experienced by mountain climbers on Earth), and by around 55km the temperature and pressure are similar to Earth’s surface: T-shirt weather. But the droplets making up the clouds here are concentrated sulphuric acid – far more extreme than could be survived by any hardy life known on Earth.

Perhaps Venusian life – if it exists – evolved to tolerate much higher acidities than us wimpy terrestrials, and migrated up into the cloud layer from ancient oceans before the planet underwent its runaway greenhouse effect.

But no matter how unlikely the prospect of life in an aerial biosphere on Venus might be, the discovery has certainly stoked interest in the further exploration of the planet. Luckily, there’s already a mission being considered by NASA’s Discovery Program.

DAVINCI+ was shortlisted at the beginning of 2020, and if selected could launch as early as May 2026. The mission will release a probe into the Venusian atmosphere that will take measurements with its sensitive spectrometer instruments as it parachutes down.

Dr Melissa Trainer, a space scientist at the NASA Goddard Space Flight Center, helped propose DAVINCI+. “Finally we’ll get a clear picture of the mix of gases down through the atmosphere from the cloud-tops to the near-surface,” she says.

For example, DAVINCI+ will make detailed measurements of water vapour in the atmosphere, and so hopefully will reveal how much water the planet has lost over its history, and for how long it might have possessed an extensive ocean. And with any luck, it’ll get to the bottom of the phosphine mystery.

“I think it’s urgent to get back to our sister planet Venus now, and to take the right measurement tools with us so we can decipher what is going on in its atmosphere,” Trainer says.

Location: 228 million kilometres from the Sun

Pros: Extensive evidence of ancient liquid water, organic molecules, energy sources

Cons: Extremely cold and dry surface

Missions planned: Tianwen-1, Al Hamal, Perseverance (en route) Rosalind Franklin (2022 launch)

While some 19th-Century astronomers may have convinced themselves they could see canals criss-crossing the surface of Mars, our first close-up look at the Red Planet with flyby probes in the 1960s plainly revealed the Martian surface to be a freeze-dried desert.

Mars has a thin atmosphere, which means it is exceedingly cold. Liquid water isn’t stable over most of its surface, and it is also bathed in ultraviolet radiation from the Sun.

But Mars hasn’t always been this inhospitable – there are extensive signs of ancient river valleys, deltas, lakes, and possibly even an ocean over its northern hemisphere, which indicate a warmer, wetter primordial Mars. Did life get started during this earliest phase of the planet’s history, and might ‘biosignatures’ of these microbes remain preserved in sedimentary deposits?

Scientists interested in the chances of life on Mars explore extreme environments here on Earth, and investigate what sorts of microorganisms are able to survive. Dr Claire Cousins is an astrobiologist at the University of St Andrews.

“While nowhere on Earth can be exactly like Mars, there are places that have enough similarities to make them valuable comparisons,” she says. “If you wanted to get a feel for what the bone-dry Martian surface is like today, you could go to the Atacama Desert in Chile. Alternatively, to understand the environment of early Mars – some three to four billion years ago – you could study volcanically active places like Iceland.”

Mars is exciting not only because it seems to have once offered a habitable environment for life, but being our planetary neighbour it is relatively easy to get to and explore with robotic probes. In July 2020, no fewer than three separate missions were launched to Mars: China’s Tianwen-1 orbiter and rover, the United Arab Emirates’ Al Amal orbiter, and NASA’s latest car-sized rover, Perseverance.

And when the launch window next opens in 2022, the European Space Agency (ESA) and Russia’s Roscosmos will be sending their own biosignature-hunting robot, the ExoMars rover Rosalind Franklin.

Cousins is also a member of the camera team for ExoMars. “The next rovers heading for Mars will probe the chemistry of Martian rocks in incredible detail. This is important because we’re trying to find evidence of tiny microscopic life that lived a few billion years ago – not easy!” she says.

“We’ll be looking for trace amounts of organic material left behind by any microorganisms that have been preserved that whole time.”

### Enceladus

Location: Saturn system 1,400 million kilometres from the Sun

Pros: Subsurface sea, organic chemistry, energy sources

Cons: Sealed beneath ice shell

Missions planned: None currently selected

Enceladus, one of Saturn’s moons, is a tiny snowball of a world. Its diameter would fit comfortably between London and Edinburgh, its minuscule gravity cannot cling onto any meaningful atmosphere and its surface is hard-frozen ice. Astrobiologists didn’t give it a second’s thought, until a surprise discovery in 2005.

The Cassini probe saw that fractures near the moon’s south pole were spewing glittering geysers of water ice out into space. Over time, the out-jetting of these ice crystals have built up the E ring around Saturn, and it’s believed they are being squirted from a large body of liquid water lying beneath the moon’s icy crust.

After this stunning discovery, Cassini was ordered to skim low over the surface of Enceladus and plunge straight through these diffuse water jets to analyse their composition. The fountains were found to contain sodium and grains of silica-rich sand – Enceladus’ sea is salty, and this is important because it means the water must be in contact with the rocky core of the moon to dissolve out minerals.

Cassini also detected simple organic compounds like formaldehyde and acetylene, as well as some larger molecules. These aren’t signs of life, but are just the sort of precursor chemistry that is thought to be important in the development of biology.

Then, in April 2017 – shortly before the mission ended in a dramatic plunge into the crushing atmosphere of Saturn – the Cassini team announced the discovery of possible hydrothermal activity on Enceladus’s seafloor.

Hydrothermal vents form oases for microbial life in the dark depths of Earth’s oceans, and the hydrogen gas detected in the plumes of Enceladus is an available food source for life. On Earth, certain microbes derive the energy they need by combining hydrogen with carbon dioxide, producing methane in the process.

So Enceladus seems to tick all the necessary boxes for providing a habitable environment suitable for life: liquid water, organic compounds and energy sources.

Several robotic missions for taking a closer look have been proposed in recent years. Enceladus Life Finder (ELF) and Enceladus Life Signatures and Habitability (ELSAH) missions were both proposed to the most recent round of NASA’s New Frontiers Program but lost out to Dragonfly.

Explorer of Enceladus and Titan (E2T) was proposed as a joint ESA-NASA mission, but in May 2018 wasn’t shortlisted for the latest round of ESA’s Cosmic Vision programme. The competition is fierce for the funding of space missions, but there is enough excitement about Enceladus that we will surely return there soon enough.

Read more about extraterrestrials:

### Europa

Distance from Earth: Jupiter system 778 million kilometres from the Sun

Pros: Subsurface sea, possible organic chemistry, possible energy sources

Cons: Sealed beneath ice shell

Missions planned: JUICE (2022 launch), Europa Clipper (2024 launch)

Space probes have revealed the surface of Europa, one of Jupiter’s moons, to be relatively fresh and young. It is scarred by few impact craters, which means the moon is geologically active. Europa is criss-crossed with long fractures from where the moon’s surface is being stretched and flexed by the powerful gravity of Jupiter.

The Galileo orbiter also noticed the moon distorting Jupiter’s magnetic field. This implied that a magnetic field was being created within Europa by an electrically conductive substance – an ocean of salty water beneath Europa’s surface being the ideal candidate.

There even appear to be regions where this ocean may have melted through to the surface, breaking off icebergs, before rapidly freezing over again with exposure to the cold of outer space. Therefore, in terms of the potential habitability of Europa, we know it harbours a great subsurface ocean of liquid water.

But that’s just about all we can be sure of. We don’t know how thick the ice shell on top of the ocean is, or what organic chemistry may be there, or whether there is any hydrothermal activity on the seafloor, or whether the pH or saltiness of the seawater is suitable for life.

If this ocean is habitable, then Europa offers much better prospects for extraterrestrial life surviving today than Mars (which is now exceedingly cold and dry), but the moon is tricky to explore with robotic probes.

Europa is much further away than Mars or Venus, it orbits within the intense radiation belt of Jupiter, and the moon has no atmosphere for parachuting to the surface. And even if we can get a hardy probe safely down onto the face of Europa, it might need to drill or melt down through many kilometres of rock-hard ice to reach the subsurface ocean.

In some respects, Enceladus would be much easier to check for life because it is conveniently squirting its seawater out into space for us – a probe could swoop through this water plume to collect a sample before looping back to the Earth for analysis. There is hope for Europa, however, after the Hubble Space Telescope spotted what seems to be water plumes erupting from near the moon’s south pole.

ESA’s Jupiter Icy Moons Explorer (JUICE) is launching in 2022, but will only make two flybys of Europa, whereas NASA’s Europa Clipper will make multiple passes of the moon and should launch in 2024. If the Europa Lander mission receives funding it could launch in 2025 and will be able to scoop 10cm into the surface ice to test for signs of life.

### Titan

Distance from Earth: Saturn system 1,400 million kilometres from the Sun

Pros: Geologically active, organic chemistry

Cons: Very cold, liquid hydrocarbons

Missions planned: Dragonfly (2027 launch)

Saturn’s largest moon, Titan, is enormous, larger even than the planet Mercury. When ESA’s Huygens descent probe parachuted down through Titan’s hazy orange atmosphere in 2005, it discovered a landscape with rolling hills, networks of river valleys, and smoothed pebbles strewn across the ground.

Flybys from the Cassini spacecraft subsequently found great lakes and signs of rain near the moon’s north pole. Titan is sodden wet and smothered with the sort of simple organic chemistry thought to have been important for the origin of life on primordial Earth – surely this is a surefire winner for hosting extraterrestrial biology?

The problem with Titan is that it is really cold. It orbits Saturn, nine times further from the Sun than the Earth and so only receives about 1 per cent the amount of solar warming. The surface is a numbing -180°C, and Titan’s rivers and lakes don’t slosh with liquid water, but liquid hydrocarbons like methane and ethane. This means that any life on the surface would have to be ethane-based rather than water-based, and molecules like DNA won’t work. Titan life would be truly alien.

Astrobiologists are keen to return to Titan. In June, NASA selected Dragonfly as the latest mission to be funded by its New Frontiers Program. Dragonfly is a truly innovative endeavour – where other planetary probes have involved a static lander or a rover to trundle slowly across the surface, Dragonfly is an octocopter drone.

Titan’s combination of low gravity and thick atmosphere makes it suited for exploration by air, and the craft will be able to fly faster than 30km/h, and take off and land vertically, giving it an unprecedented capability to pinpoint sites of interest.

Trainer is also deputy principal investigator on this mission. “While Dragonfly is not a life-detection mission, we are going after really fundamental questions about how far prebiotic chemistry may have progressed in this environment. We will characterise the products of millions of years of chemical synthesis, and search for biologically relevant molecules.”

## Colonizing the Moon?

Darby Dyar, professor of astronomy and geology at Mount Holyoke College, says the moon is to people today what the New World was to Europeans 600 years ago. “They had been there a few times,” said Dyar, “but it took time to work up the courage to send people there to stay.”

It’s no fantasy. Scientists like Dyar have been working on the prospect of colonizing the moon for decades. “In my lifetime,” she said, “we will establish some kind of permanent station on the moon. Mind you, I plan to live another 50 years!”

Now Dyar is serving on the Solar System Exploration Research Virtual Institute. The “virtual” part refers to the fact that the monthly meetings and collaboration between team members takes place mostly through video-conferencing.

The project involves nine teams around the country, of which Dyar serves on three. She will be studying minerals on the moon and other airless bodies such as asteroids.

Among her tasks: Figure out how future residents on the moon can get at that chemical compound that is essential to human existence – water. No water, no life.

“The moon is a very dry place,” said Dyar. “That’s why it’s difficult to imagine living on it.”

The challenge is to find out where the water is and how to tap it, said Dyar. “We have to understand how water got to the moon, how much is still there, and how hard it would be to extract water for human consumption for a settlement,” she said.

Some water was formed at the same time as the moon was formed, she said, and is “locked” in its minerals in tiny amounts. It’s a concept that’s hard to understand for people who are used to water flowing freely.

Water would also come from comets that have crashed on the moon. Comets are made of ice, said Dyar, and the heat of the impact melts the ice. Some of the water is preserved in “permanently shadowed craters” where the sun cannot reach it.

“By far the most common way water gets to the moon is by solar wind,” said Dyar. “Solar wind is composed of highly charged particles, some of which are hydrogen ions that bond with microscopic particles. They are spraying the moon all the time, and sometimes they stick.” Hydrogen is one of the components of water – the “H” in H20.

Getting water from moon rocks would involve heating them in a still – a daunting process.

One reason for serious space exploration is global politics. Americans may think the moon is theirs because they were the first to plant a flag on it. No such thing, says Dyar. “Who owns the moon is still up for grabs,” she said.

The Outer Space Treaty of 1967, signed first by the major powers and subsequently by about 100 other countries, governs exploration and use of celestial bodies. Among the rules: No nuclear weapons up there.

Another reason for serious space exploration: “If an asteroid were to hit the earth, people could survive temporarily on the moon,” said Dyar.

She is referring to the kind of asteroid that killed the dinosaurs. “If you read the literature, it’s very pragmatic,” she said. “We all know the U.S. and other countries monitor the skies. What would we do?”

One of the three teams to which Dyar is assigned is based at Stony Brook University in New York. It studies how to extract as much information as possible from very small rock samples from outer space.

Many of the techniques that have been used for such analysis require a pretty big sample,” said Dyar, who serves as co-leader of this team, and a big sample is not always available. Mount Holyoke lab instructor and asteroid expert Tom Burbine is also on that team.

Another team, based at Brown University in Providence, R.I., works on how to identify minerals long-distance from an orbiting spacecraft. Dyar also has a lead role in this one. She and her Mount Holyoke students will train Brown faculty and graduate students on how to use complicated data processing equipment to conduct the research.

Dyar is a spectroscopist, which means that she analyzes of the distinct patterns that light makes when it bounces off surfaces.

The third team project, based at Johns Hopkins University in Baltimore, Md., studies how much hydrogen is trapped in minerals on the moon.

Though she holds the august academic title of Kennedy-Schelkunoff Professor of Astronomy at Mount Holyoke, Dyar is as lively and excited as a kid when she talks about her work.

“It’s a fun project,” she said. “You gotta remember—I started working on lunar samples in 1979. I’ve had a lifetime to get used to how amazing this is!”

## Stable moons

Over nearly a decade of study, Kepler identified 10 exoplanets in orbit around nine pairs of stars. The planets lie close to their stars, zooming around in no more than seven Earth days. Each pair of stars is in a tight configuration, separated by only about a tenth of the distance between the Earth and sun, a number known as one Astronomical Unit (AU). The planets themselves orbit their stars' centers of mass at a distance of about one AU, making these worlds circumbinary. (Planets can also orbit a single star in a binary pair if the pair is far enough apart, the planet may act more like it is circling a single star.)

While the exomoons of planets that orbit a single star is awell-studied phenomenon, Hamers said, less work has been done for exomoons in binary systems. A handful of circumbinary worlds have been discovered using other telescopes, but the researchers in the new study were particularly interested in the newfound Kepler planets.

"We were curious which orbits of exomoons around these circumbinary planets would be dynamically stable," Hamers said.

The scientists ran multiple simulations of the moons of planets around stellar pairs. Results showed that stable simulated moons remained close to their planets, at about 0.01 AU apart, so that these moons were less affected by the gravity of the stellar pairs. Moons were also more stable when they circled more massive planets. The angle of the moon's path around the planet compared to the planet's path around the suns proved important, as well. When a moon circled at a 90-degree angle compared to the planetary path, the moon oscillated widely before becoming unstable, crashing into the planet or, on rare occasions, one of the stars.

What might it look like to stand on the moon orbiting a planet with two stars in the sky? That would depend strongly on the moon's orientation and rotation period, Hamers said. If the moon resembles the moons of Jupiter, its "day" will likely span several Earth days. The tight orbits of these exomoons mean they should whip around their giant planets over about 10 Earth days, he said.

"During the 'day' on the exomoon, there will be two stars visible in the sky, separated by about 40 degrees, which will noticeably move during the course of the 'day,'" Hamers said. "Also, there will be times that the binary stars eclipse each other, [with] only one star visible for a limited amount of time."

If all three objects travel along the same plane, the planet itself will obscure the stars roughly every 10 days. If they are tilted in relation to one another, however, eclipses may be avoided.

Although the new research did not directly hunt for exomoons, the findings could help aim future hunts for the tantalizing objects. By determining the regions around a circumbinary planet where an exomoon would be unable to survive, Hamers said, this research can help scientists discount ambiguous signals. Such misleading signals could be effects created by stellar activity or star spots.

The new findings also reveal the limits on exomoon stability around double stars depending on the ratio of the planet&rsquos mass to that of its stars. "This relation likely applies to any circumbinary system," Hamers said. However, he did add the caveat that the team focused on Kepler binaries the researchers didn't thoroughly investigate outside binaries.

Telescopes such as NASA's recently launched Transiting Exoplanet Survey Satellite and the upcoming European CHEOPS and PLATO spacecraft may be good for hunting down exomoons, Hamers said.

The research was detailed Nov. 1 in the journal Monthly Notices of the Royal Astronomical Society.

## Moonless Earth Could Potentially Still Support Life, Study Finds

Scientists have long believed that, without our moon, the tilt of the Earth would shift greatly over time, from zero degrees, where the Sun remains over the equator, to 85 degrees, where the Sun shines almost directly above one of the poles.

A planet's stability has an effect on the development of life. A planet see-sawing back and forth on its axis as it orbits the sun would experience wide fluctuations in climate, which then could potentially affect the evolution of complex life.

However, new simulations show that, even without a moon, the tilt of Earth's axis — known as its obliquity — would vary only about 10 degrees. The influence of other planets in the solar system could have kept a moonless Earth stable. [10 Coolest New Moon Discoveries]

The stabilizing effect that our large moon has on Earth's rotation therefore may not be as crucial for life as previously believed, according to a paper by Jason Barnes of the University of Idaho and colleagues which was presented at a recent meeting of the American Astronomical Society.

The new research also suggests that moons are not needed for other planets in the universe to be potentially habitable.

As the world turns

Due to the gravitational pull of its star, the axis of a planet rotates like a child's top over tens of thousands of years. Although the center of gravity remains constant, the direction of the tilt moves over time, or precesses (as astronomers call it).

Similarly, a planet's orbital plane also precesses. When the two are in synch, the combination can cause the total obliquity of the planet to swing chaotically. But the gravity of Earth's moon has been shown to provide a stabilizing effect. By speeding up Earth's rotational precession and keeping it out of synch with the precession of Earth's orbit, it minimizes fluctuations, creating a more stable system.

As terrestrial moons go, Earth's moon is on the large size — only about a hundred times smaller than its parent planet. In comparison, Mars is over 60 million times more massive than its largest moon, Phobos.

The difference is substantial, and with good cause — while the Martian moons appear to be captured asteroids, scientists think that Earth's moon formed when a Mars-sized body crashed into the young planet, blowing out pieces that later consolidated as the lunar satellite — a satellite which affects the planet's tilt.

Scientists estimate that only one percent of any terrestrial planets will have a substantial moon. This means that most such planets are expected to experience massive changes in their obliquity.

The pull of the planets

While Earth's moon does provide some stability, the new data reveals that the pull of other planets orbiting the sun — especially Jupiter — would keep Earth from swinging too wildly, despite its chaotic evolution. [10 Extreme Planet Facts]

"Because Jupiter is the most massive, it really defines the average plane of the solar system," said Barnes.

Without a moon, Barnes and his collaborators have determined that Earth's obliquity would only vary 10 to 20 degrees over a half a billion years.

That doesn't sound like much, but the changes of 1 to 2 degrees the planet presently exhibits are thought to be partly responsible for the Ice Ages.

According to Barnes, the present shift is "a small effect, but in combination with Earth's present climate, it causes big changes."

Still, a 10-degree change is not a huge problem when it comes to life. "(It) would have effects, but not preclude the development of large scale, intelligent life."

Furthermore, if Jupiter were closer, Barnes explains, the Earth's orbit would precess faster, and the moon would actually make the planet fluctuate more wildly, rather than less.

"A moon can be stabilizing or destabilizing, depending on what's going on in the rest of the system," he said.

The benefit of a backspin

The team also determined that planets with a retrograde, or backward, motion should have smaller variations than those that spin in the same direction as their parent star, a large moon notwithstanding.

"We think the initial rotation direction should be random," Barnes said. "If it is, half the planets out there would not have problems with obliquity variations."

What determines which way a planet spins? He suspects that "whatever smacks the planet last establishes its rotation rate."

A 50/50 shot at retrograde precession, combined with the likelihood of other planets in the system keeping the planet from tipping on its side, means more terrestrial planets could be potentially habitable. Barnes ventured an estimate that at least 75 percent of the rocky planets in the habitable zone may be stable enough for life to evolve, though he notes that additional studies are needed to confirm or disprove that.

In comparison, the previous idea that a large moon was necessary for a constant tilt meant that only about 1 percent of terrestrial planets would have a steady climate.

"A large moon can stabilize (a planet)," Barnes said, "but in most cases, it's not needed."

This story was provided by Astrobiology Magazine, a web-based publication sponsored by the NASA astrobiology program.

## Moons in Our Solar System that Could Support Extraterrestrial Life

The search for extraterrestrial life forms has to begin from our vicinity. The first logical assumption is our neighbor Mars, where scientists believe that liquid water existed billions of years ago. Climate changes stripped most of the Red Planet’s atmosphere, but there is a possibility that simple forms of life exist within the ice hidden under its rocky surface.

Some forms of life may still exist in our solar system on worlds other than Earth or Mars. While there aren’t too many candidates in the “golden zone”, where the temperature is just right for liquid water to exist, there are reasons to believe that alien organisms could live on some of the moons around us. The things that are the most important for these natural satellites to support life are liquid water, orbital stability, suitable atmosphere, favorable tidal effects, stable axial tilt and climate.

### Europa

System: Jupiter
Diameter: 0.25 Earths (

90 % of our own Moon)
Mass: 0.008 Earths
Atmosphere: Very thin, mostly oxygen

Europa is one of the most exciting prospects for extraterrestrial life in the Solar System. First, because there is a vast ocean buried beneath its icy surface. Heating caused by the tidal forces of Jupiter keeps large portions of these oceans liquid. This effect may provide a source of energy for life, while vents on the seafloor could provide food. Plumes of water have been seen erupting 160 kilometers (100 miles) above the surface. Oxygen, hydrogen and other compounds could also be supplied to living organisms from the water-ice surface. This outer shell is constantly “bombarded” with radiation from the giant planet, but this could be a shield for any life below. While Europa is only one-fourth the diameter of Earth, its ocean may contain twice as much water as the oceans on our planet.

### Enceladus

System: Saturn
Diameter: 0.04 Earths (500 kilometers / 300 miles)
Mass: 0.000018 Earths
Atmosphere: Mostly water vapor (also nitrogen, carbon dioxide)

The tiny natural satellite Enceladus is another top candidate for finding life. It does not only have an ocean beneath the surface, but scientists believe that the icy crust is also thinner compared to other worlds where life might exist. Additionally, it is actively and regularly firing out plumes of water from its south pole. This means that materials from the ocean are dumped on to the surface. So, studying it may not be beyond the realms of possibility. Data from the Cassini spacecraft even showed that materials form the ocean contained complex organic molecules, which may suggest that the ocean is habitable. Hydrothermal vents on the sea floor could also provide food for life.

### Titan

System: Saturn
Diameter: 0.4 Earths (larger than Mercury)
Mass: 0.02 Earths
Atmosphere: Thick, mostly nitrogen (also methane, hydrogen)

Saturn’s largest moon has unique qualities that have not been observed anywhere else in the universe so far. Namely, Titan is the only satellite in our family of planets known to have a substantial atmosphere. Additionally, it is the only world besides Earth known to have a system of liquid rivers, lakes, and seas. It can even rain and snow. The twist is that the liquid is not water, but methane, ethane, and other hydrocarbons. Normally, water is the key element that should be present somewhere if we expect to find life, but what if it’s not actually necessary? Carbon is a primary component of all known life on Earth and is the second most abundant element in the human body by mass after oxygen. It is a unique element that can bond to nearly anything, creating a wide variety of molecular structures. Therefore, some scientists suggest that methane and other hydrocarbons could be used as a solvent for life on Titan and similar worlds instead of water. So, if life exists on Titan, it would be very different from anything we have ever known before.

### Ganymede

System: Jupiter
Diameter: 0.4 Earths
Mass: 0.025 Earths
Atmosphere: Very thin, mostly oxygen

The largest moon in the Solar System is also the only one to have a significant magnetic field. This is crucial for keeping life on Earth safe from radiation, so it could have a similar role on Ganymede as well. Because of this commonality, auroras can be observed on its poles just like the northern lights can be seen on our planet. Interestingly, research studies have shown that Jupiter’s massive satellite could have layers of ice and liquid water between its surface and core. Tidal forces from Jupiter could keep this water in a frigid liquid form, so perhaps life could have evolved underneath the surface.

#### Other potentially habitable moons

##### Callisto

This is the most distant of Jupiter’s four largest moons, which means less radiation than the others. It is believed that Callisto may also contain a subsurface ocean, potentially habitable by living organisms. Its atmosphere consists of carbon dioxide, hydrogen, and oxygen, making this moon more hospitable to life as we know it.

##### Triton

There is a possibility that Neptune’s largest moon is home to alien life. Scientists are not completely sure if an ocean exists beneath its frozen crust, but there are some cracks and volcanic features on this world which suggest it is warmed by tidal heating from its planetary companion. Even though the surface of Titan is one of the coldest places in the solar system, the inner heat and geological activity could potentially provide conditions for water to exist in liquid form.

##### Io

Io is the most volcanically active world in the entire Solar System, so at first glance, it doesn’t look very hospitable and habitable. However, it could have had liquid water in the past, which in combination with the heat could have supported life.

##### Dione

This icy moon which orbits around Saturn is also thought to have an ancient ocean under the surface. However, the crust could be thick as much as 100 kilometers. Still, some form of life could theoretically exist down there.

##### Charon

A canyon and suspected cryovolcanic activity may suggest that Pluto’s largest moon once had an ancient internal ocean of water and ammonia. Whether it could have been habitable, remains a mystery.

## Wandering Promise: Study Says Moons of Rogue Exoplanets Could Be Habitable, Host Liquid Water

The cosmic universe is vast, with countless worlds scattered around billions of distant galaxies—each different and unique from the other. The planets beyond the bounds of our solar system are known as exoplanets, and astronomers have long suspected that some of them may hold the potential to host some forms of life.

Even among these, not all cosmic worlds are loyally bound to host stars! Some of them are rogue exoplanets, which wander the dark cosmic space without a host. The absence of a heat source excludes them from possessing suitable conditions to host life.

But, there is a twist! Some of these rogue planets have a natural satellite like the Moon to Earth. And it turns out, these moons or exomoons could be as warm and wet as Earth. For the first time, a recent study has determined that some of the exomoons of rogue exoplanets could hold habitable conditions.

### How can exomoons be habitable?

Scientists from the University of Concepción in Chile explored the possibilities of life on exomoons, equivalent to Earth’s mass, orbiting the rogue gas giants of mass comparable to Jupiter. The researchers modelled the probability of such exomoons hosting an atmosphere composed of 90% carbon dioxide and 10% hydrogen over its evolutionary history.

They further looked into the possible presence of an atmosphere and liquid water to find the ideal exomoon candidate. Finally, to understand the formation of these two life-supporting conditions, the team explored cosmic radiation and the gravitational effect of the rogue exoplanet on the exomoon.

And this was when they decoded the conditions conducive to life! The authors of the study explain: “We find that, under specific conditions and assuming stable orbital parameters over time, liquid water can be formed on the surface of the exomoon. The final amount of water for an Earth-mass exomoon is smaller than the amount of water in Earth oceans, but enough to host the potential development of primordial life.”

Researchers reveal that cosmic radiation can help execute the chemical reaction required to form water by converting hydrogen and carbon dioxide. “The chemical equilibrium time-scale is controlled by cosmic rays, the main ionisation driver in our model of the exomoon atmosphere,” says the study. Moreover, the tidal force will act as the source to keep it liquid.

### The world of exomoons

According to the observations in the study, the combination of two factors—cosmic radiation and the gravitational effect of the rogue planet—can create the settings just right enough to sustain liquid water and the atmosphere. On Earth, the heat helps to keep the process of photosynthesis going and help maintain the surface water in the liquid state.

The study highlights that there could be at least one rogue Jupiter-sized gas exoplanet for every star in our home galaxy: Milky Way. Earlier studies have estimated that there could be over 100 billion rogue exoplanets. There are high chances that many of them would have moved from their original location along with an exomoon.

The temperatures beyond the limits of a star system are incredibly frigid. Despite this, there are some known worlds where water has been discovered in liquid form. In fact, there are some icy moons in the solar system as well—like the Ganymede and Europa that orbit Jupiter and Enceladus that orbit Saturn—which are thought to host liquid oceans beneath the thick ice shells.

For decades, astronomers have speculated that Europa and Enceladus might host some form of alien life. The speciality of these worlds is the retention of water in liquid form due to the gravitational tug of respective planets. Likewise, a substantial amount of water could exist in the exomoon's atmosphere. With this study, the possibilities of exploring the world of exomoons become wide open in search of alien life.

The results of the study have been published in the International Journal of Astrobiology and can be accessed here.

## Revealed: Why We Should Look For Ancient Alien Spacecraft On The Moon, Mars And Mercury According To NASA Scientists

From UFO crash sites on other planets and aliens “lurking” on asteroids to a permanent radio . [+] telescope on the far side of the Moon, a new NASA-funded study into the search for intelligent extraterrestrial life (SETI) details how future NASA missions could purposefully look for “technosignatures.”

From UFO crash sites on other planets and aliens “lurking” on asteroids to a permanent radio telescope on the far side of the Moon, a new NASA-funded study into the search for intelligent extraterrestrial life (SETI) details how future NASA missions could purposefully look for the “technosignatures” of advanced alien civilizations.

Described as evidence for the use of technology or industrial activity in other parts of the Universe, the search for technosignatures has barely begun, but could unearth something surprising without much additional spend, says the study.

After more or less ceasing its search for technosignatures in 1993 after pressure by politicians, NASA has become increasingly involved in SETI.

Published in the specialized journal Acta Astronautica, the study includes a list of what’s NASA missions could detect as observational “proof of extraterrestrial life” beyond Earth.

Perhaps most intriguingly, the paper suggests that interstellar probes might have been sent into the Solar System a long time ago, perhaps during the last close encounter of our Sun with other stars.

### Explained: Why This Week’s ‘Strawberry Moon’ Will Be So Low, So Late And So Luminous

The closest star to the Sun right now, Proxima Centauri, is over 4.2 light-years distant, but roughly every 100,000 years a star comes within nearly a light-year from the Sun. There have therefore been “tens of thousands” of opportunities for technologies similar to ours to have launched probes into our Solar System, according to the paper.

“Such artifacts might have been captured by Solar System bodies into stable orbits or they might even have crashed on planets, asteroids or moons,” reads the paper. “Bodies with old surfaces such as those of the Moon or Mars might still exhibit evidence for such collisions.”

The Moon, Mars, Mercury or Ceres could contain evidence of impacts or existing artifacts that may . [+] have been preserved for between millions or billions of years.

The paper’s nine suggestions for technosignature-hunting missions include:

### Mission 1: search for crash sites on the Moon, Mars, Mercury or Ceres

The surfaces of these places are ancient and unchanging. Evidence of impacts or existing artifacts might be preserved for between millions and billions of years—so we should scan the Moon and Mars in ultra-high resolution.

### Mission 2: look for pollution using Earth as a template

As recently published for NASA by the same authors, the JWST could find CFC gases—proof of civilization—around exoplanets if it was 10 times more common than on Earth. It could also find nitrogen dioxide (NO2), produced as a byproduct of combustion or nuclear technology.

### Mission 3: search for Dyson spheres

A so-called “waste heat mission” to pick-up technological waste heat would require an all-sky survey using a space telescope with sensitivity at many infrared bands.

A permanent dish on the “radio-quiet” far side of the Moon would be free of contamination from human . [+] radio emissions, so enable super-sensitive searches. (Photo by NASA via Getty Images)

### Mission 4: build a radio telescope on the Moon’s far side

The search for technosignatures so far has been conducted largely via radio astronomy—and continues to be so via the Breakthrough Listen project. However, a permanent dish on the “radio-quiet” far side of the Moon would be free of contamination from human radio emissions, so enable super-sensitive searches.

### Mission 5: look for ‘lurkers’ on asteroids

We may be being watched by aliens concealed on resources-rich near-Earth objects (NEOs)—possibly even asteroids that orbit the Sun with Earth.

### Mission 6: intercept missions to ‘interstellar interlopers’

‘Oumuamua for 2I/Borisov passed through the Solar System without us able to conclusively establish their nature and origins. So we should have an intercept mission ready to launch when a target next presents itself—and that could be soon after the Vera C. Rubin Observatory’s all-sky surveys begin later in 2021.

Illustration of Oumuamua. In 2017, astronomers discovered an object in the Solar System which seemed . [+] out of place. Its orbit is highly hyperbolic, not parabolic, which implies it originated outside of the Solar System and is just passing through. The interloper has been named Oumuamua Hawaiian for scout or messenger. Follow-up observations have revealed that Oumuamua is very oddly shaped, like a cigar, more elongated than any known Solar System object. Estimates put its size at 200 x 30 x 30 m, and its rotational period at 8.14 hours. An alternative possibility, however unlikely, has been mentioned in a scientific paper - that the object might actually be an alien spacecraft such as a solar sail (left).

### Mission 7: search existing data

Such as objects in orbit around exoplanets, pollution in exoplanet atmospheres and the detection of night-time illumination on exoplanets.

### Mission 8: conduct all-sky laser searches

Short laser pulses could be searched for in visible light and in wide regions of the infrared with a single instrument.

### Mission 9: study small asteroids

Asteroids under 10m in diameter may be artificial, but we’ve never looked. Anything with very flat metallic surfaces will high reflectivity polarize reflected light.

Wishing you clear skies and wide eyes.

I'm an experienced science, technology and travel journalist and stargazer writing about exploring the night sky, solar and lunar eclipses, moon-gazing, astro-travel,