Is the newly discovered exoplanet 'Teegarden b' habitable?

Is the newly discovered exoplanet 'Teegarden b' habitable?

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A new exoplanet has been just discovered around the star Teegarden, at only 12 light years away.

It is potentially habitable.

Teegarden b has a minimum mass almost identical to Earth and it orbits within the star's habitable zone.

The exoplanet has a 60% chance of having a temperate surface environment where temperature should be closer to 28°C assuming a similar terrestrial atmosphere.

Moreover, it is the exoplanet with the highest Earth Similarity Index discovered so far: 95%. This means that it has the closest mass and insolation to terrestrial values.

TLDR version: at this point there are too many unknowns to determine whether this planet candidate is habitable.

The discovery paper for this system, Zechmeister et al. (2019) "The CARMENES search for exoplanets around M dwarfs. Two temperate Earth-mass planet candidates around Teegarden's Star" is available on the arXiv. Table 1 lists the properties of the star and table 2 lists the properties of the planets.

The planetary quantities we have reliable estimates for are the minimum masses, the orbital periods and the distances. The eccentricities are consistent with zero so I'm going to assume for the moment that the orbits are circular. The reason that minimum mass is relevant is that this is an example of a radial velocity detection, which results in a degeneracy between the inclination of the planet's orbit to the line-of-sight and the mass of the planet.

Habitable zone

According to figure 13 in the discovery paper, Teegarden b is too close to the star to be in the conservative estimate of the habitable zone, though it is in the optimistic habitable zone. The Earth Similarity Index (ESI) appears to be a fairly misleading metric (I am of the opinion that ESI is a bad metric that should be abandoned), from the discovery paper:

In addition, planet b receives nearly the same stellar flux as the Earth and therefore has a nearly identical equilibrium temperature, but is outside the conservative HZ. As discussed by Kopparapu et al. (2014), this is caused by a runaway greenhouse effect that is due to water vapour starting at lower insolation for low-mass stars. This effect is neglected in the ESI calculation and leads to the curious fact that planet b has a higher ESI value than planets inside the conservative HZ.

The position of Teegarden b therefore suggests it may have undergone a runaway greenhouse effect, though this might be avoided if it can build up enough reflective cloud cover on the daylight hemisphere, or if it is a desert planet without enough water to destabilise the climate.

Even if a planet is located in the habitable zone, it is not necessarily habitable. An obvious counterexample would be the Moon.

Terrestrial or not?

The next question: are these planets terrestrial? Unfortunately we don't know: Teegarden b does not transit the star, and there has so far been insufficient coverage of Teegarden c to say whether it transits. As a result, we do not know the radii of the planets so we cannot tell whether or not the planets have densities compatible with being terrestrial. The minimum masses are low enough that these planets do likely fall below the transition region between rocky and non-rocky planets suggested by Kepler data, see for example Rogers (2015) "Most 1.6 Earth-Radius Planets are not Rocky" - using the mass-radius relationships of Zeng et al. (2016) "Mass-Radius Relation for Rocky Planets based on PREM" the transition is somewhere around 3-5 Earth masses.

Another point of comparison here is the other known system of low-mass planets around an ultracool dwarf star: TRAPPIST-1. In this case, despite the small size of the planets, the inferred masses indicate the presence of substantial amounts of volatiles, e.g. Dorn et al. (2018) "Interior characterization in multiplanetary systems: TRAPPIST-1" where the volatile contents of the planets are estimated to correspond to a few percent by mass, compared with 0.02% by mass for the Earth. A planet like this would be very different to the Earth, in an optimistic scenario it might result in an ocean planet. The habitable zone for ocean planets would be different to that for Earthlike planets, since the negative feedback of continental weathering would not occur.

Red dwarf stars

Red dwarf stars have a variety of problems when it comes to habitability. One issue is that they have an extended high-luminosity pre-main sequence phase which might result in runaway greenhouse effects and/or severe water-loss on close-in planets, see Ramirez & Kaltenegger (2014) "The Habitable Zones of Pre-Main-Sequence Stars".

An additional problem is that red dwarf stars have severe stellar activity that would erode an Earth-like atmosphere in a very short amount of time, as an example Garcia-Sage et al. (2017) "On the Magnetic Protection of the Atmosphere of Proxima Centauri b" estimate an Earthlike atmosphere at Proxima Centauri would be lost in less than 400 million years. Teegarden's Star is fairly quiet for its spectral type, but it is an old star and was likely far more active in its youth.

These issues are not necessarily fatal, outgassing from volcanism might be able to replenish the atmosphere to some extent after the stellar activity dies down, or if the planet was born with a large amount of volatiles it might be able to emerge as a habitable planet. Unfortunately we don't have sufficient data on habitable zone planets around red dwarf stars to know for sure.

A further issue is that the atmospheric chemistry on planets around stars like Teegarden's Star would likely result in substantial quantities of carbon monoxide accumulating in the atmosphere, see Schwieterman et al. (2019) "A Limited Habitable Zone for Complex Life". This would be lethally unpleasant for any human visitors (or other organisms that use haemoglobin as their oxygen carrier) caught outside without an air supply. Note that haemoglobin is not the only possible oxygen carrier: some groups of life on Earth use alternatives (haemerythrin, haemocyanin) that are less vulnerable to carbon monoxide poisoning, as noted by Howell (2019) "Comment on "A Limited Habitable Zone for Complex Life" (Schwartzman et al. 2019 ApJ 878, 19)":

In short, there is a notable diversity of extant invertebrates on this planet that would be quite comfortable outside of the range of CO concentrations proposed by Schwieterman et al. to constrain the habitable zone. There is no a priori reason to rule out the possibility that these lineages could have given rise to intelligent life during the history of life on this planet. A significantly higher atmospheric CO concentration would have prevented the evolution of hemoglobin, but not necessarily prevented astronomically observable biosignatures from Earth.


It is too early to be able to say whether the planets around Teegarden's Star are habitable. There are plenty of reasons to be sceptical but worlds like Teegarden b are still promising enough to be worth investigating in more detail. It is unfortunate that it is not a transiting planet, but similar systems like TRAPPIST-1 should be helpful in constraining what the possibilities are.

ESO Discovers Earth-Size Planet in Habitable Zone of Nearest Star

A newly discovered, roughly Earth-sized planet orbiting our nearest neighboring star might be habitable.

A newly discovered, roughly Earth-sized planet orbiting our nearest neighboring star might be habitable, according to a team of astronomers using the European Southern Observatory's 3.6-meter telescope at La Silla, Chile, along with other telescopes around the world.

The exoplanet is at a distance from its star that allows temperatures mild enough for liquid water to pool on its surface.

"NASA congratulates ESO on the discovery of this intriguing planet that has captured the hopes and the imagination of the world," says Paul Hertz, Astrophysics Division Director at NASA Headquarters, Washington. "We look forward to learning more about the planet, whether it holds ingredients that could make it suitable for life."

The new planet circles Proxima Centauri, the smallest member of a triple star system known to science fiction fans everywhere as Alpha Centauri. Just over 4 light-years away, Proxima is the closest star to Earth, besides our own sun.

"This is really a game-changer in our field," said Olivier Guyon, a planet-hunting affiliate at NASA's Jet Propulsion Laboratory, Pasadena, California, and associate professor at the University of Arizona, Tucson. "The closest star to us has a possible rocky planet in the habitable zone. That's a huge deal. It also boosts the already existing, mounting body of evidence that such planets are near, and that several of them are probably sitting quite close to us. This is extremely exciting."

The science team that made the discovery, led by Guillem Anglada-Escudé of Queen Mary University of London, will publish its findings Aug. 25 in the journal Nature. The team traced subtle wobbles in the star revealing, the presence of a star-tugging planet.

They determined that the new planet, dubbed Proxima b, is at least 1.3 times the mass of Earth. It orbits its star far more closely than Mercury orbits our sun, taking only 11 days to complete a single orbit -- a "year" on Proxima b.

Long list of unknowns

The stunning announcement comes with plenty of caveats. While the new planet lies within its star's "habitable zone" -- a distance at which temperatures are right for liquid water -- scientists do not yet know if the planet has an atmosphere.

It also orbits a red-dwarf star, far smaller and cooler than our sun. The planet likely presents only one face to its star, as the moon does to Earth, instead of rotating through our familiar days and nights. And Proxima b could be subject to potentially life-extinguishing stellar flares.

"That's the worry in terms of habitability," said Scott Gaudi, an astronomy professor at Ohio State University, Columbus, and JPL affiliate credited with numerous exoplanet discoveries. "This thing is being bombarded by a fair amount of high-energy radiation. It's not obvious if it's going to have a magnetic field strong enough to prevent its whole atmosphere from getting blown away. But those are really hard calculations, and I certainly wouldn't put my money either way on that."

Despite the unknowns, the discovery was hailed by NASA exoplanet hunters as a major milestone on the road to finding other possible life-bearing worlds within our stellar neighborhood.

"It definitely gives us something to be excited about," said Sara Seager, a planetary science and physics professor at the Massachusetts Institute of Technology, Cambridge, and an exoplanet-hunting pioneer. "I think it will definitely motivate people to get moving."

'Not completely unexpected'

Statistical surveys of exoplanets -- planets orbiting other stars -- by NASA's Kepler space telescope have revealed a large proportion of small planets around small stars, she said.

The Kepler data suggest we should expect at least one potentially habitable, Earth-size planet orbiting M-type stars, like Proxima, within 10 light-years of our solar system.

So the latest discovery was "not completely unexpected. We're more lucky than surprised," Seager said. But it "helps buoy our confidence that planets are everywhere."

It's especially encouraging for upcoming space telescopes, which can contribute to the study of the new planet. The James Webb Space Telescope, launching in 2018, may be able to follow-up on this planet with spectroscopy to determine the contents of its atmosphere. NASA's Transiting Exoplanet Survey Satellite (TESS) will find similar planets in the habitable zone in the stellar backyard of our solar system in 2018.

One of TESS's goals is to find planets orbiting nearby M-dwarf stars like Proxima Centauri.

"It's great news just to know that M-dwarf planets could be as common as we think they are," Seager said.

Another possible inspiration Proxima b could reignite: the admittedly far-off goal of sending a probe to another solar system.

Bill Borucki, an exoplanet pioneer, said the new discovery might inspire more interstellar research, especially if Proxima b proves to have an atmosphere.

Coming generations of space and ground-based telescopes, including large ground telescopes now under construction, could yield more information about the planet, perhaps inspiring ideas on how to pay it a visit.

"It may be that the first time we get really good information is from the newer telescopes that may be coming online in a decade or two," said Borucki, now retired, the former principal investigator for Kepler, which has discovered the bulk of the more than 3,300 exoplanets found so far.

"Maybe people will talk about sending a probe to that star system," Borucki said. "I think it does provide some inspiration for an interstellar mission, because now we know there is a planet in the habitable zone, probably around the mass of Earth, around the closest star. I think it does inspire a future effort to go there and check it out."


As alien planet-hunting missions continue to add more worlds to the vast menagerie of known exoplanets that exist in our galaxy, an increasing number of them are falling inside the “habitable zone” category.

The habitable zone around any star is the distance at which a rocky planet can orbit where it’s neither too hot or too cold for liquid water to exist on its surface (if it has water, that is). Liquid water is the stuff that Earth-like biology has an affinity to without it, life on Earth wouldn’t have evolved. So, even before we have any clue about its H2O-ness, if an exoplanet is seen to have an orbit around its star that is deemed habitable, that’s +1 point for habitability.

Now, the next point can only be won if that world is also of approximate Earth-like size and/or mass. There would be little reason in getting too excited for a Jupiter-sized exoplanet sitting in the habitable zone possessing liquid water on its “surface” (because it won’t have a surface). That’s not to say there can’t be some gas giant-dwelling balloon-like alien living in there, but we’re looking for Earth-like qualities, not awesome alien qualities we read in science fiction. (I’d also argue that these kinds of exoplanets might have habitable Earth-sized moons — like Avatar‘s Pandora — but that’s for another article…)

The two key methods for exoplanet detection is the “radial velocity” method and the “transit” method. The former — which precisely measures a star’s light to detect tiny stellar wobbles as an exoplanet gravitationally “tugs” at it as it orbits — can deduce the exoplanet’s mass, thereby revealing whether or not it has an Earth-like mass (Teegarden’s two worlds were discovered using this method). The latter — which was employed by NASA’s Kepler space telescope (and now NASA’s Transiting Exoplanet Survey Explorer, among others) to look for the slight dips in brightness as an exoplanet passes in front of its star — can deduce the exoplanet’s physical size, thereby revealing whether or not it has an Earth-like size. Should a habitable zone exoplanet possess either one of these Earth-like qualities, or both (if both methods are used on a target star), that’s another +1 point for its habitability.

There’s a few other measurements that astronomers can make that may add to a hypothetical world’s habitability (such as observations of the host star’s flaring activity, age, or some other derived measurement), but until we develop more powerful observatories on Earth and in space, there are several factors that quickly cause our hypothetical exoplanet to diminish in habitable potential.

Astronomers Detail the Newly Discovered and Potentially Habitable Super-Earth LHS 1140 b

An artist’s impression of the newly-discovered rocky exoplanet, LHS 1140b. This planet is located in the liquid water habitable zone surrounding its host star, a small, faint red star named LHS 1140. The planet weighs about 6.6 times the mass of Earth and is shown passing in front of LHS 1140. Depicted in blue is the atmosphere the planet may have retained.

Astronomers are moving to the next phase of studying alien worlds – searching for signs of life on planets outside our Solar System. And the newly discovered “super-Earth” LHS 1140 b, which orbits in the habitable zone of a nearby small star, is at the top of that list.

“This is the most exciting exoplanet I’ve seen in the past decade,” said lead author Jason Dittmann of the Harvard-Smithsonian Center for Astrophysics (CfA). “We could hardly hope for a better target to perform one of the biggest quests in science − searching for evidence of life beyond Earth.”

Astronomers found the planet using the transit method – when a star dims as a planet crosses in front of it as seen from Earth. By measuring how much light this planet blocks, the team determined that it is about 11,000 miles in diameter, or about 40 percent larger than Earth. They also reveal that it is located just 40 light-years away.

The team has also weighed the planet to be 6.6 times the mass of Earth, showing that it is dense and likely has a rocky composition. Small, potentially habitable planets have been found in the TRAPPIST-1 system, located a similar distance from Earth, but only one of those worlds has had its density measured accurately, showing that it isn’t rocky. Therefore, some or all of the others also might not be rocky.

Since this planet transits its star, unlike the closest world to the solar system Proxima Centauri b, it can be examined for the presence of air. As the planet moves in front of the star, the star’s light will be filtered through any atmosphere and leave an imprint. Large, next-generation telescopes will be needed to tease out these subtle signals.

“This planet will be an excellent target for the James Webb Space Telescope when it launches in 2018, and I’m especially excited about studying it with the ground-based Giant Magellan Telescope, which is under construction,” said co-author David Charbonneau of the CfA.

The planet orbits a tiny, faint star known as LHS 1140, which is only one-fifth the size of the Sun. Since the star is so dim and cool, its habitable zone (the distance at which a planet might be warm enough to hold liquid water) is very close. This planet, designated LHS 1140 b, orbits its star every 25 days. At that distance, it receives about half as much sunlight from its star as Earth.

Although the planet is potentially habitable now, it might have faced a hellish past. When the star was young, it would have bathed the planet in a harsh ultraviolet glare that could have stripped any water from the atmosphere, leading to a runaway greenhouse effect like we see on Venus.

However, since the planet is larger than Earth, it might have possessed a magma ocean on its surface for millions of years. Powered by heat from naturally radioactive elements, that churning ocean of lava may have fed steam into the atmosphere long after the star calmed to its current, steady glow. This process could have replenished the planet with water, making it suitable for life as we know it.

This artist’s impression video shows an imaginary trip to the exoplanet LHS 1140b, which orbits a red dwarf star 40 light-years from Earth and may be the new holder of the title “best place to look for signs of life beyond the Solar System.”

“Right now we’re just making educated guesses about the content of this planet’s atmosphere,” said Dittmann. “Future observations might enable us to detect the atmosphere of a potentially habitable planet for the first time. We plan to search for water, and ultimately molecular oxygen.”

In contrast with the TRAPPIST-1 star, LHS 1140 spins slowly and does not emit much high-energy radiation, which also may help the likelihood of life on its planet.

LHS 1140 b was discovered using the MEarth-South telescope array at Cerro Tololo Inter-American Observatory. This collection of eight telescopes, with its companion facility MEarth-North, studies faint, red stars known as M dwarfs to locate orbiting planets using the transit method.

In follow-up work the team was able to detect LHS 1140 wobbling as the planet orbits it, using the High Accuracy Radial velocity Planet Searcher (HARPS) installed on the European Southern Observatory’s 3.6m telescope at La Silla Observatory in Chile. This information was combined with data from the transit method, allowing the team to make good measurements of the planet’s size, mass and density.

The world's attention is now on Proxima Centauri b, a possibly Earth-like planet orbiting the closest star, 4.22 light-years away. The planet's orbit is just right to allow liquid water on its surface, needed for life. But could it in fact be habitable?

If life is possible there, the planet evolved very different than Earth, say researchers at the University of Washington-based Virtual Planetary Laboratory (VPL) where astronomers, geophysicists, climatologists, evolutionary biologists and others team to study how distant planets might host life.

Astronomers at Queen Mary University in London have announced discovery of Proxima Centauri b, a planet orbiting close to a star 4.22 light-years away. The find has been called "the biggest exoplanet discovery since the discovery of exoplanets."

RELATED: A Possibly Habitable Planet Was Found Around our Nearest Neighbor Star

Rory Barnes, UW research assistant professor of astronomy, published a discussion about the discovery at, a website dedicated to the search for life around Proxima Centauri. His essay describes research underway through the UW planetary lab -- part of the NASA Astrobiology Institute -- to answer the question, is life possible on this world?

"The short answer is, it's complicated," Barnes writes. "Our observations are few, and what we do know allows for a dizzying array of possibilities" -- and almost as many questions.

The Virtual Planetary Laboratory is directed by Victoria Meadows, UW professor of astronomy. UW-affiliated researchers include Giada Arney, Edward Schwieterman and Rodrigo Luger. Using computer models, the researchers studied clues from the orbits of the planet, its system, its host star and apparent companion stars Alpha Centauri A and B -- plus what is known of stellar evolution to begin evaluating Proxima b's chances.

Relatively little is known about Proxima:

• It's at least as massive as Earth and may be several times more massive, and its "year" -- the time it takes to orbit its star -- is only 11 days

• Its star is only 12 percent as massive as our sun and much dimmer (so its habitable zone, allowing liquid water on the surface, is much closer in) and the planet is 25 times closer in than Earth is to our sun

• The star may form a third part of the Alpha Centauri binary star system, separated by a distance of 15,000 "astronomical units," which could affect the planet's orbit and history

• The new data hint at the existence of a second planet in the system with an orbital period near 200 days, but this has not been proven

Perhaps the biggest obstacle to life on the planet, Barnes writes, is the brightness of its host star. Proxima Centauri, a red dwarf star, is comparatively dim, but wasn't always so.

"Proxima's brightness evolution has been slow and complicated," Barnes writes. "Stellar evolution models all predict that for the first one billion years Proxima slowly dimmed to its current brightness, which implies that for about the first quarter of a billion years, planet b's surface would have been too hot for Earth-like conditions."

Barnes notes that he and UW graduate student Rodrigo Luger recently showed that had modern Earth been in such a situation, "it would have become a Venus-like world, in a runaway greenhouse state that can destroy all of the planet's primordial water," thus extinguishing any chance for life.

Next come a host of questions about the planet's makeup, location and history, and the team's work toward discerning answers.

• Is the planet "rocky" like Earth? Most orbits simulated by the planetary lab suggest it could be -- and thus can host water in liquid form, a prerequisite for life

• Where did it form, and was there water? Whether it formed in place or farther from its star, where ice is more likely, VPL researchers believe it is "entirely possible" Proxima b could be water-rich, though they are not certain.

• Did it start out as a hydrogen-enveloped Neptune-like planet and then lose its hydrogen to become Earth-like? VPL research shows this is indeed possible, and could be a viable pathway to habitability

• Proxima Centauri flares more often than our sun might such flares have long-since burned away atmospheric ozone that might protect the surface and any life? This is possible, though a strong magnetic field, as Earth has, could protect the surface.

Also, any life under even a few meters of liquid water would be protected from radiation.

Another concern is that the planet might be tidally locked, meaning one side permanently faces its star, as the moon does Earth. Astronomers long thought this to mean a world could not support life, but now believe planetwide atmospheric winds would transport heat around the planet.

"These questions are central to unlocking Proxima's potential habitability and determining if our nearest galactic neighbor is an inhospitable wasteland, an inhabited planet, or a future home for humanity," Barnes writes.

Planetary laboratory researchers also are developing techniques to determine whether Proxima b's atmosphere is amenable to life.

"Nearly all the components of an atmosphere imprint their presence in a spectrum (of light)," Barnes writes. "So with our knowledge of the possible histories of this planet, we can begin to develop instruments and plan observations that pinpoint the critical differences."

READ NEXT: NASA Just Confirmed the Discovery of 9 Potentially Habitable Exoplanets

At high enough pressures, he notes, oxygen molecules can momentarily bind to each other to produce an observable feature in the light spectrum.

"Crucially, the pressures required to be detectable are large enough to discriminate between a planet with too much oxygen, and one with just the right amount for life.

As we learn more about the planet and the system, we can build a library of possible spectra from which to quantitatively determine how likely it is that life exists on planet b."

Our own sun is expected to burn out in about 4 billion years, but Proxima Centauri has a much better forecast, perhaps burning for 4 trillion years longer.

"If Proxima b is habitable, then it might be an ideal place to move. Perhaps we have just discovered a future home for humanity. But in order to know for sure, we must make more observations, run many more computer simulations and, hopefully, send probes to perform the first direct reconnaissance of an exoplanet," Barnes writes. "The challenges are huge, but Proxima b offers a bounty of possibilities that fills me with wonder."

Proxima Centauri b may be the first exoplanet to be directly characterized by powerful ground- and space-based telescopes planned for the future, and its atmosphere spectroscopically probed for active biology. The research was funded by the NASA Astrobiology Institute. "Whether habitable or not," Barnes concludes, "Proxima Centauri b offers a new glimpse into how the planets and life fit into our universe."

This article has been republished from materials provided by University of Washington. The original item was written by Peter Kelley. Note: material may have been edited for length and content. For further information, please contact the cited source.

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There is still much that is unknown about Kepler-1649c, including its atmosphere, which could affect the planet's temperature. Current calculations of the planet's size have significant margins of error, as do all values in astronomy when studying objects so far away. But based on what is known, Kepler-1649c is especially intriguing for scientists looking for worlds with potentially habitable conditions.

There are other exoplanets estimated to be closer to Earth in size, such as TRAPPIST-1f and, by some calculations, Teegarden c. Others may be closer to Earth in temperature, such as TRAPPIST-1d and TOI 700d. But there is no other exoplanet that is considered to be closer to Earth in both of these values that also lies in the habitable zone of its system.

"Out of all the mislabeled planets we've recovered, this one's particularly exciting &ndash not just because it's in the habitable zone and Earth-size, but because of how it might interact with this neighboring planet," said Andrew Vanderburg, a researcher at the University of Texas at Austin and first author on the paper released today in The Astrophysical Journal Letters. "If we hadn't looked over the algorithm's work by hand, we would have missed it."

Kepler-1649c orbits its small red dwarf star so closely that a year on Kepler-1649c is equivalent to only 19.5 Earth days. The system has another rocky planet of about the same size, but it orbits the star at about half the distance of Kepler-1649c, similar to how Venus orbits our Sun at about half the distance that Earth does. Red dwarf stars are among the most common in the galaxy, meaning planets like this one could be more common that we previously thought.

National Science Foundation - Where Discoveries Begin

Discovery suggests our galaxy may be teeming with potentially habitable planets

This artist's conception shows the inner four planets of the Gliese 581 system and their host star.

September 29, 2010

View a webcast with Steven Vogt and Paul Butler, leaders of a team that discovered the first potentially habitable exoplanet.

This material is available primarily for archival purposes. Telephone numbers or other contact information may be out of date please see current contact information at media contacts.

A team of planet hunters led by astronomers at the University of California, Santa Cruz (UC Santa Cruz), and the Carnegie Institution of Washington, and supported by the National Science Foundation (NSF) and NASA, has announced the discovery of an Earth-sized planet (three times the mass of Earth) orbiting a nearby star at a distance that places it squarely in the middle of the star's "habitable zone," where liquid water could exist on the planet's surface. If confirmed, this would be the most Earth-like exoplanet yet discovered and the first strong case for a potentially habitable one.

"This is clearly one of the most exciting areas of science these days" said Ed Seidel, assistant director for NSF's Mathematical and Physical Sciences directorate. "If we do discover life outside our planet, it would perhaps be the most significant discovery of all time."

To astronomers, a "potentially habitable" planet is one that could sustain life, not necessarily one that humans would consider a nice place to live. Habitability depends on many factors, but liquid water and an atmosphere are among the most important.

"Our findings offer a very compelling case for a potentially habitable planet," said Steven Vogt, professor of astronomy and astrophysics at UC Santa Cruz. "The fact that we were able to detect this planet so quickly and so nearby tells us that planets like this must be really common."

"With modern techniques, it is now possible to actually search for worlds that might be able to support life as we understand it," added Seidel. "Just a few years back I wouldn't have thought this could have advanced so fast."

This discovery was the result of over a decade of observations on the W. M. Keck Observatory in Hawaii. "Advanced techniques combined with old-fashioned ground-based telescopes continue to lead the exoplanet revolution," said Paul Butler of the Carnegie Institution. "Our ability to find potentially habitable worlds is now limited only by our telescope time."

"One of the three main science objectives of the Astronomy and Astrophysics Decadal Survey released last month is labeled 'New Worlds: Seeking nearby habitable planets,'" added NSF Astronomy Division Director Jim Ulvestad. "It is very gratifying to see that long-term scientific investments by NSF and NASA toward meeting this objective are paying off, and we expect continued discoveries in this area as nearby stars are monitored for longer periods."

Vogt and Butler lead the Lick-Carnegie Exoplanet Survey. The team's new findings are reported in a paper to be published in the Astrophysical Journal and posted online today at It is also linked to this release. Coauthors include associate research scientist Eugenio Rivera of UC Santa Cruz associate astronomer Nader Haghighipour of the University of Hawaii-Manoa and research scientists Gregory Henry and Michael Williamson of Tennessee State University.

The paper reports the discovery of two new planets around the nearby red dwarf star Gliese 581. This brings the total number of known planets around this star to six, the most yet discovered in a planetary system other than our own solar system. Like our solar system, the planets around Gliese 581 have nearly circular orbits.

The most interesting of the two new planets is Gliese 581g, with a mass three to four times that of the Earth and an orbital period of just under 37 days. Its mass indicates that it is probably a rocky planet with a definite surface, and that it has enough gravity to hold on to an atmosphere, according to Vogt.

Gliese 581, located 20 light years away from Earth in the constellation Libra, has a somewhat checkered history of habitable-planet claims. Two previously detected planets in the system lie at the edges of the habitable zone, one on the hot side (planet c) and one on the cold side (planet d). While some astronomers still think planet d may be habitable if it has a thick atmosphere with a strong greenhouse effect to warm it up, others are skeptical. The newly discovered planet g, however, lies right in the middle of the habitable zone.

"We had planets on both sides of the habitable zone--one too hot and one too cold--and now we have one in the middle that's just right," Vogt said.

The planet is tidally locked to the star, meaning that one side is always facing the star and basking in perpetual daylight, while the side facing away from the star is in perpetual darkness. One effect of this is to stabilize the planet's surface climates, according to Vogt. The most habitable zone on the planet's surface would be the line between shadow and light (known as the "terminator"), with surface temperatures decreasing toward the dark side and increasing toward the light side.

"Any emerging life forms would have a wide range of stable climates to choose from and to evolve around, depending on their longitude," Vogt said.

The researchers estimate that the average surface temperature of the planet is between -24 and 10 degrees Fahrenheit (-31 to -12 degrees Celsius). Actual temperatures would range from blazing hot on the side facing the star to freezing cold on the dark side.

If Gliese 581g has a rocky composition similar to the Earth's, its diameter would be about 1.2 to 1.4 times that of the Earth. The surface gravity would be about the same or slightly higher than Earth's, so that a person could easily walk upright on the planet, Vogt said.

The new findings are based on 11 years of observations of Gliese 581 using the HIRES spectrometer (designed by Vogt) on the Keck I Telescope at the W. M. Keck Observatory in Hawaii. The spectrometer allows precise measurements of a star's radial velocity (its motion along the line of sight from Earth), which can reveal the presence of planets. The gravitational tug of an orbiting planet causes periodic changes in the radial velocity of the host star. Multiple planets induce complex wobbles in the star's motion, and astronomers use sophisticated analyses to detect planets and determine their orbits and masses.

"It's really hard to detect a planet like this," Vogt said. "Every time we measure the radial velocity, that's an evening on the telescope, and it took more than 200 observations with a precision of about 1.6 meters per second to detect this planet."

To get that many radial velocity measurements (238 in total), Vogt's team combined their HIRES observations with published data from another group led by the Geneva Observatory (HARPS, the High Accuracy Radial velocity Planetary Search project).

In addition to the radial velocity observations, coauthors Henry and Williamson made precise night-to-night brightness measurements of the star with one of Tennessee State University's robotic telescopes. "Our brightness measurements verify that the radial velocity variations are caused by the new orbiting planet and not by any process within the star itself," Henry said.

The researchers also explored the implications of this discovery with respect to the number of stars that are likely to have at least one potentially habitable planet. Given the relatively small number of stars that have been carefully monitored by planet hunters, this discovery has come surprisingly soon.

"If these are rare, we shouldn't have found one so quickly and so nearby," Vogt said. "The number of systems with potentially habitable planets is probably on the order of 10 or 20 percent, and when you multiply that by the hundreds of billions of stars in the Milky Way, that's a large number. There could be tens of billions of these systems in our galaxy."

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Steven Vogt and Paul Butler lead a team that discovered the first potentially habitable exoplanet.
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The planetary orbits of the Gliese 581 system compared to those in our own solar system.
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An interior view of the Keck I Telescope at the W. M. Keck Observatory in Hawaii.
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Media Contacts
Lisa-Joy Zgorski, NSF, (703) 292-8311, email: [email protected]
Tim Stephens, UC, Santa Cruz, (831) 459-2495, email: [email protected]

Principal Investigators
Steve Vogt, UC, Santa Cruz, email: [email protected]

Paul Butler, Carnegie Institution of Washington, (202) 248-1938, email: [email protected]

Related Websites
UC, Santa Cruz News:
Carnegie Institution of Science:
Ground-based Astronomy at NSF:
The Lick-Carnegie Exoplanet Survey (PDF):

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Steven Vogt and Paul Butler lead a team that discovered the first potentially habitable exoplanet.
Credit and Larger Version

The planetary orbits of the Gliese 581 system compared to those in our own solar system.
Credit and Larger Version

An interior view of the Keck I Telescope at the W. M. Keck Observatory in Hawaii.
Credit and Larger Version

Earth-Mass Exoplanet Found in Habitable Zone around Proxima Centauri

A planet described as the ‘single most promising exoplanet discovered to date in terms of habitability’ and a ‘natural location where our civilization could aspire to move to’, has been found orbiting the closest star to our Sun.

This artist’s impression shows Proxima b orbiting Proxima Centauri, which at only 4.23 light-years is the closest star to our Solar System. The double star Alpha Centauri AB also appears in the image between the exoplanet and Proxima itself. Image credit: M. Kornmesser / ESO.

The discovery, made by a team led by Dr. Guillem Anglada-Escudé, from Queen Mary University of London, of a planet orbiting Proxima Centauri has been published in the journal Nature.

The rocky planet, named Proxima b, is slightly larger than Earth and sits within its star’s habitable zone, where liquid water could theoretically exist on the surface.

“Many exoplanets have been found and many more will be found, but searching for the closest potential Earth-analogue and succeeding has been the experience of a lifetime for all of us,” Dr. Anglada-Escudé said.

“The search for life on Proxima b comes next…”

This artist’s impression shows a view of the surface of Proxima b orbiting Proxima Centauri. Alpha Centauri AB also appears in the image to the upper-right of Proxima itself. Proxima b is a little more massive than the Earth and orbits in the habitable zone around its star, where the temperature is suitable for liquid water to exist on its surface. Image credit: M. Kornmesser / ESO.

The discovery was warmly welcomed by scientists involved in the Breakthrough Starshot project which earlier this year unveiled plans to send light-propelled nanocrafts 40 trillion km to the Alpha Centauri system, where Proxima Centauri and its orbiting planet reside.

On hearing the news Breakthrough Starshot’s chairman Professor Avi Loeb from Harvard University described Proxima b as “an obvious target for a flyby mission,” which could “take color images of the planet and infer whether it is green and harboring life, blue with water oceans on its surface, or just brown, dry rock.”

Prof. Loeb added that because the lifetime of the star is several trillion years, “a habitable rocky planet around Proxima would be the most natural location to where our civilization could aspire to move after the Sun will die, five billions years from now.”

Astronomer Royal Lord Martin Rees who sits on the project’s Advisory Board said: “it is excellent news to have strong evidence for a planet around such a nearby star, on which perhaps life could have emerged.”

This infographic compares the orbit of Proxima b with the same region of our Solar System. Proxima Centauri is smaller and cooler than the Sun and the planet orbits much closer to its star than Mercury. As a result it lies well within the habitable zone, where liquid water can exist on the planet’s surface. Image credit: M. Kornmesser / G. Coleman / ESO.

The discovery was made with the ESO 3.6-m telescope at La Silla in Chile (alongside other telescopes around the world), as part of the Pale Red Dot campaign to find a tail tell wobble of our second nearest star that could indicate the gravitational pull of an orbiting planet.

The wobble they discovered shows Proxima Centauri approaching the Earth at about 5 km per hour – normal human walking pace – and then receding at the same speed.

This regular pattern repeats every 11.2 days, and when compared against natural fluctuations in the star’s brightness indicated the presence of a planet slightly more massive than the Earth, orbiting its star at only 5% of the distance from our planet to the Sun.

Despite this close orbit, the fact that Proxima Centuri is far fainter than our own star keeps Proxima b in the crucial habitable zone.

Professor Ed Turner, from Princeton University, another member of the Breakthrough Starshot project advisory board, described the newly discovered planet as “the single most promising exoplanet discovered to date in terms of habitability.”

“That this planet is orbiting the nearest star to the Sun is an extraordinary and wonderful piece of good luck. It could hardly be more exciting from the perspectives of both astrobiology and interstellar flight,” he added.

G. Anglada-Escudé et al. A terrestrial planet candidate in a temperate orbit around Proxima Centauri. Nature, published online August 25, 2016

Two Super Earths

Planet GJ 887b and GJ887c may be 'rocky worlds' similar to Earth's surface but the team predicts that they are tidally locked, meaning that only one side is always facing its star, making that side constantly daytime and the opposite side nighttime.

Dr. John Barnes, an astrophysicist at the Open University, said, 'Close orbiting planets like these have a high chance of being tidally locked to their host star. This means that as the planet orbits its star, the same hemisphere always faces the star.'

Using the Doppler wobble method find the exoplanets and measure their gravitational relationship with GJ 887, the start would wobble slightly as the pair would orbit. They also observed that the wobble affects the star's light as well.

GJ 887c's proximity to its parent star gives it an estimated temperature of 158 degrees Fahrenheit on the surface as it takes almost 22 days to circle the star. GJ 887b, which is closer to its host star with a 9.3-day orbit, is most likely to have higher surface temperatures. Since only half of each planet is facing the star, ' it could be much cooler on some parts of the planets than others,' explained Dr. Barnes.

TESS helps astronomers pinpoint possibly habitable exoplanet

An artist’s impression of the GJ 357 system, featuring three known planets. One of them, if it has a dense atmosphere, could be habitable. Image: Carl Sagan Institute/Jack Madden

Astronomers have found two additional planets orbiting a star 31 light years away where the Transiting Exoplanet Survey Satellite – TESS – earlier found an exoplanet 22 percent larger than Earth but orbiting 11 times closer to the star than Mercury orbits the Sun.

TESS discovered the previously known world, a “hot Earth” catalogued as GJ 357b, by measuring the slight dimming of the parent star when the exoplanet crossed in front as viewed from the spacecraft. The observations indicated GJ 357b completes one orbit every 3.9 days and has an equilibrium temperature or nearly 500 degrees.

To confirm the TESS observations, astronomers used Earth-based instruments to take a closer look. By measuring a subtle back-and-forth movement of the star, they were able to discern the gravitational influences of two additional planets. A key instrument in the discovery was the Planet Finder Spectrograph used with one of the Magellan telescopes at Carnegie’s Las Campanas Observatory in Chile.

“Because it is a nearby M dwarf, which we know often host small planets, the PFS team started monitoring this star in 2016,” said Carnegie astronomer Johanna Teske. “As soon as we saw that TESS had indeed detected a small transiting planet, we accelerated our PFS observing campaign.”

One of the two newly discovered planets, GJ 357c, is at least 3.4 times more massive than Earth and orbits the star every 9.1 days, giving it an equilibrium temperature of around 127 Celsius (260 Fahrenheit).

The third planet, GJ 357d, is 6.1 times more massive than Earth, completes one orbit every 55.7 days and has an equilibrium temperature around -53 Celsius (-64 Fahrenheit). Additional study is needed to determine if an atmosphere might be present but if so, enough heat could be trapped to permit liquid water to exist in a theoretically habitable environment.

TESS’s mission is to identify terrestrial worlds orbiting stars relatively close to the sun. Follow-up studies by more powerful instruments, like the upcoming James Webb Space Telescope and next-generation “extremely large” telescopes, are needed to determine an exoplanet’s habitability.

In the meantime, TESS, in concert with current ground-based instruments, is identifying a growing list of targets for future research. Like the new worlds discovered orbiting GJ 357.

“This planetary system helps demonstrate how crucial tools such as the PFS are for TESS’ success,” said Carnegie’s Sharon Wang.