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Exoplanet radius density relation according to NASA exoplanet archive

Exoplanet radius density relation according to NASA exoplanet archive


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I downloaded the info from NASA exoplanet archive at: https://exoplanetarchive.ipac.caltech.edu/cgi-bin/TblView/nph-tblView?app=ExoTbls&config=compositepars

I named the file "NASA.Composite.Planet.Data.csv". I had to edit the file to remove the header which was preventing the program below from running. (I put the header at the top of the program below)

I then plotted it with Matplotlib using this python (anaconda) program: https://planet.wikia.com/wiki/User:Granpa/Python_program

I got this image: https://i.imgur.com/1057YvQ.png">


The straight lines you see in the diagram must be coming from extrapolated radius or mass values. Keep in mind that most planets don't have both values simultaneously measured, because their discovery technique is only sensitive to radius (transits) or mass (doppler).

So what people in those archives often do is assume some mass-radius relationship in order to fill those gaps. I'm 99.9% sure that this is where those perfectly straight lines come from. The other, scattered points are what @astrosnapper's answer refers to.


What you are seeing is a combination of several things as you move across the diagram from left to right with increasing planet radius. As this diagram from a talk by Cayman Unterborn at the recent "Know Thy Star - Know Thy Planet" conference shows, is a transition from rocky (like Mars, Venus, Earth; the 'Ma', 'V' and 'E' on the diagram) exoplanets with increasing density as ~1.5 Earth radii is reached to "Neptune-like" gas giants with increasing amount of a gas envelope, possibly with a denser core. (The size and type of Jupiter's core is not a totally settled question for example and so the wider range of exoplanets could have a wide variety of core properties). There is a lot more information in this blog post on 'Which small exoplanets are rocky' which talks about what we can learn about planets in the 0.5 to 4 Earth radius regime.

At higher radii (approximately 11 Earth radii/1 Jupiter radius) what you are seeing is the effect of electron-degeneracy pressure. This causes the radii of exoplanets with a mass of about Jupiter, brown dwarfs up to 80 times the mass of Jupiter and low mass M dwarf stars (with a mass of hundreds of Jupiters) all to have very similar radii but with widely varying densities. This causes the near-vertical line on the right hand side of your plot. This is also what makes it very difficult to know whether you have a star, brown dwarf or a planet if you only a measure of a transiting object's radius (from e.g. Kepler) and don't have a mass measurement (usually from radial velocities) to go with it.


Tight Measurement of Exoplanet Radius

Both the Kepler and Spitzer space telescopes had a role to play in recent work on the planet Kepler-93b, whose size is now known to an uncertainty of a mere 120 kilometers on either side of the planet. What we have here is the most precise measurement of an exoplanet radius yet, a helpful result in the continuing study of ‘super-Earths,’ a kind of world for which we have no analogue in our own Solar System. A third instrument also comes into play, for studies of the planet’s density derived from Keck Observatory data on its mass (about 3.8 times Earth’s mass) and the known radius indicate this is likely an world made of iron and rock.

And that is absolutely the only similarity between Kepler-93b and Earth, for at 0.053 AU, six times closer than Mercury to the Sun, the planet’s surface temperature is estimated to be in the range of 760 degrees Celsius. The planet is 1.481 times the width of Earth. The accuracy of the measurement is the story here, a result so precise that, in the words of Sarah Ballard (University of Washington), lead author of the paper on these findings, “it’s literally like being able to measure the height of a six-foot tall person to within three quarters of an inch — if that person were standing on Jupiter.”

Image: Using data from NASA’s Kepler and Spitzer Space Telescopes, scientists have made the most precise measurement ever of the size of a world outside our solar system, as illustrated in this artist’s conception. The diameter of the exoplanet, dubbed Kepler-93b, is now known with an uncertainty of just one percent. Credit: NASA/JPL-Caltech.

Just how the measurement was made is a story in itself. The Spitzer instrument provided data for seven transits of Kepler-93b between 2010 and 2011, three of them studied with a new observational technique called ‘peak up’ that halved the uncertainty of Spitzer’s own radius measurements. Kepler-93 thus served as a test subject for the new technique, which was developed in 2011 and allows tighter control over how light affects individual pixels in the observatory’s infrared camera. The paper examines all seven light curves in detail.

Meanwhile, we have the Kepler data, which provided light curves as well as the dimming of the star caused by seismic waves in motion in the interior. Now we’re in the realm of asteroseismology, which is a powerful way to probe the makeup of individual stars. Asteroseismic measurements over a long observational baseline can provide useful information about the density of the star (with a precision of 1 percent) as well as its age (within 10%). Such measurements require a long observational baseline at high cadence — cadence refers to the time between observations of the same target — as well has high photometric precision.

When we have both an asteroseismic density measurement of the exoplanet host star as well as a transit light curve, we can improve the precision of our radius measurements. Sara Seager (MIT) and colleagues examined host star densities in relation to planetary orbits and the radius of the star as early as 2003, and later work by a team led by Philip Nutzman (Harvard-Smithsonian CfA) used asteroseismology along with transit light curves to constrain the radius of HD 17156b, highlighting a method that has been found to be relevant to a wide number of recent studies.

The Kepler mission’s long baselines and unprecedented photometric precision make asteroseismic studies of exoplanet hosts possible on large scales… Kepler-93 is a rare example of a sub-solar mass main-sequence dwarf that is bright enough to yield high-quality data for asteroseismology. Intrinsically faint, cool dwarfs show weaker-amplitude oscillations than their more luminous cousins. These targets are scientifically valuable not only as exoplanet hosts, but also as test beds for stellar interior physics in the sub-solar mass regime.

The combination of the Kepler data and Spitzer’s new technique was powerful, and adds luster to the already rich history of Spitzer’s Infrared Array Camera (IRAC) in exoplanetary science. The instrument has been helpful in mapping planetary weather and characterizing super-Earth atmospheres, and has been a major tool in ruling out exoplanet false-positives, because an actual planet will present the same transit depth no matter the wavelength at which it is observed. After losing its coolant in 2009, the telescope, now dubbed ‘warm Spitzer,’ continues to provide key readings that are now enhanced with the development of the ‘peak up’ process.

Kepler-93 is a star of approximately 90 percent of the Sun’s mass and radius, located some 300 light years from Earth. With the Spitzer data corroborating the find and the use of asteroseismology to constrain the result, we wind up with an error bar that is just one percent of the radius of Kepler-93b. A planet thought to be 18,800 kilometers in diameter might be bigger or smaller than that by about 240 kilometers, but no more, an outstanding result for exoplanetary science and a confirmation of the power of asteroseismology in determining stellar radii.

The paper is Ballard et al., “Kepler-93b: A Terrestrial World Measured to within 120 km, and a Test Case for a New Spitzer Observing Mode,” The Astrophysical Journal Vol. 790, No. 1 (2014), 12 (abstract / preprint). A JPL news release is also available.

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Surface gravity is 1.73 gee, so tolerable. Pity about the distance, the temperature, and goodness knows what else.

This is an excellent example of the sort of synergism between measurements from various sources (Kepler, Spitzer, Keck-HIRES) but also various astronomical specialties (photometery, spectroscopy, asteroseismology) needed to derive the properties of super-Earth size extrasolar planets. Recent analyses of Kepler data and ground-based radial velocity measurements show that an important transition takes place at planet radii of about 1.5 (or so) times that of the Earth from planets with a predominantly rocky composition (i.e. terrestrial planets) to non-rocky (i.e. mini-Neptunes and gas dwarfs). This not only has implications on how planets form but just how big habitable planets can get. In fact, if recent work on the mass-radii function of planets is correct, most of the planets some people have argued are “potentially habitable” are not rocky planets never mind habitable ones. This is discussed in detail in the following essay:

More measurements like the one Paul describes here are going to be needed to pin down the characteristics of this important transition in planet composition.

Is the planet tidally locked at that distance? If so, could a temperate though windy belt straddling the terminator offer some harbour for life? I don’t know enough about the possibility of this ‘tidally-locked biosphere’ scenario, sorry if the question sounds absurd.
Mark.

One Planet, Two Stars: A System More Common Than Previously Thought

by SHANNON HALL on SEPTEMBER 4, 2014

There are few environments more hostile than a planet circling two stars. Powerful tidal forces from the stars can easily destroy the rocky building blocks of planets or grind a newly formed planet to dust. But astronomers have spotted a handful of these hostile worlds.

A new study is even suggesting that these extreme systems exist in abundance, with roughly half of all exoplanets orbiting binary stars.

NASA’s crippled Kepler space telescope is arguably the world’s most successful planet hunter, despite the sudden end to its main mission last May. For nearly four years, Kepler continuously monitored 150,000 stars searching for tiny dips in their light when planets crossed in front of them.

As of today, astronomers have confirmed nearly 1,500 exoplanets using Kepler data alone. But Kepler’s database is immense. And according to the exoplanet archive there are over 7,000 “Kepler Objects of Interest,” dubbed KOIs, that might also be exoplanets.

There are a seeming endless number of questions waiting to be answered. But one stands out: how many exoplanets circle two stars? Binary stars have long been known to be commonplace — about half of the stars in the Milky Way are thought to exist in binary systems.

A team of astronomers, led by Elliott Horch from Southern Connecticut State University, has shown that stars with exoplanets are just as likely to have a binary companion. In other words, 40 to 50 percent of the host stars are actually binary stars.

Wet exoplanet has clear skies

Neptune-sized orb is smallest alien world known to have water vapour.

The smallest exoplanet yet found to contain water is about the size of Neptune — and a rare glimpse at its atmosphere reveals clear conditions. The handful of other small planets whose atmospheres have been studied all have cloudy skies.

“It’s the smallest planet that we’ve seen anything in the atmosphere besides clouds,” says Jonathan Fraine, an astronomer at the University of Maryland in College Park. “The fact that it’s clear at all is significant.”

Fraine and his colleagues describe the atmosphere of the planet in the 25 September issue of Nature1. Known as HAT-P-11b, the body is about 38 parsecs (124 light years) away, in the Cygnus constellation.

Astronomers have been piecing together details on the atmospheres of several alien worlds, trying to find an Earth-like world with an Earth-like atmosphere. So far, however, clouds have generally obscured their view.

HAT-P-11b is different. Fraine’s team used the Hubble and Spitzer space telescopes to monitor the dimming of its star’s light as the planet passed in front of it, along with spectral details of the light during those transits. The astronomers could briefly glimpse its atmosphere twice, as the planet moved onto the disk of the star and then off it.

Using Lasers to Lock Down Exoplanet Hunting

Topics: Planetary Society Projects, explaining technology, extrasolar planets, Exoplanets Laser

The Planetary Society is launching a new collaboration with Yale exoplanet hunter Debra Fischer and her team, the Exoplanets Laser project. We will support the purchase of an advanced, ultra stable laser to be used in a complex system they are designing to push radial velocity exoplanet hunting to a whole whole new level – a level intended to facilitate the discovery of Earth sized planets around nearby stars. As Debra says:

“The search for exoplanets is motivated by the question of whether life exists elsewhere. This drives our interest in the detection of planets that are similar to our own world: rocky planets with the potential for liquid surface water and plate tectonics worlds that might harbor life that we can recognize.”

Clear Skies Above: Astronomers Detect Water Vapor On Cloud-free Atmosphere Of A Hot-Neptune

Sunny and hot all-year-round, with no clouds on the horizon. That’s not a weather forecast only for the Maldive Islands here on Earth, but also for exoplanet HAT-P-11b as well, according to the latest findings by an international team of astronomers. But don’t start packing for that holiday package just yet, for HAT-P-11b is a steamy Neptune-sized world located so close to its host star that average temperatures there reach a scortching 1,120 degrees Fahrenheit.

Exoplanet research has transitioned in recent years from the simple discovery of planets around other stars to that of their detailed characterization. Following the exciting findings of thousands of extrasolar worlds during the last two decades, which have established that planetary formation is a common occurrence in the galaxy, astronomers around the world are now striving to understand the overall evolution of these distant worlds, by studying their properties, bulk composition and internal structure. To that end, astronomers use one of the best tools at their disposal, which is called transmission spectroscopy.

More specifically, when an extrasolar planet happens to cross or transit the face of its star as seen by our line of sight here on Earth, it causes a small dip in the star’s brightness which is proportional to the size of the exoplanet itself. If that planet also happens to have an atmosphere, the latter will absorb some of the star’s light in certain wavelengths as it transits, resulting in a wavelength-dependent transit depth, better known as a transmission spectrum. By studying this spectrum of the combined star-planet light, astronomers can extract detailed information about the planet’s atmosphere, like its chemical composition, temperature, density, and overall dynamics.

If the results by Fraine’s team are any indication, then the first detailed atmospheric observations of a super-Earth located in a more life-friendly orbit around a distant star somewhere in the Milky Way galaxy, might indeed be a few years in the future.


Exoplanet: TRAPPIST - 1h

A planet that orbits a star outside the solar system is an exoplanet.

Imagine a place with not one, not two, but 7 Earth-sized planets orbiting a single star. TRAPPIST-1 is an Ultra-Cool Dwarf Star. This artist's concept shows what the planet might look like. To learn more about the how artists' took tiny bits of data and made a vivid picture, read here.

TRAPPIST-1 is a system about 40 light years away from Earth (12 parsecs) in the Constellation, Aquarius. The 7 Earth-sized Exoplanets are currently (March, 2017) labeled, "b, c, d, e, f, g, and h". In order based on proximity to their star ("b" is closet to TRAPPIST-1). The Largest planets, g and b are about 10% larger than the Earth. The Smallest planets, d and h are about 25% smaller than the Earth. According to NASA/Caltech's Exoplanet Archive, as of 2017 there are more than 3,450 confirmed exoplanets in the Milky Way Galaxy.

It is likely that most, if not all, of the exoplanets keep the same one side of their surface facing their star at all times. This phenomenon is called Tidal Locking. This is the same phenomenon we observe with our on Moon in Relation to the Earth. Only one side is ever facing us. On the exoplanets this would cause massive temperature differences across their surfaces. This also could, (given the right circumstances) mean there is a possibility of finding liquid water on any of these exoplanets. This is why each of these exoplanets on SOS appears to have one side in shadow. Unlike our moon, the entirety of these planets would likely not ever see the sun.

Though we do not believe any of these exoplanets have moons, as they are too close to their star, if you were to stand on the surface of one of these planets, you would clearly see the other planets in the system and some of them, at certain times would appear even larger than our moon appears to us. This is because TRAPPIST-1 is only a little larger than Jupiter, and its planets orbit only a little further away than that of Jupiter's moons.

Interplanetary Trips would be measured in Days in TRAPPIST-1, unlike in our solar system, where we have to measure them in Months and Years. It only takes 1.5 days for the innermost planet to orbit it's star.

The Discovery

In May 2016 astronomers using the Trappist Telescope (x2 Belgian optic robotic telescope) in the La Silla Observatory in Chile first identified 3 Earth-Sized planets. The Spitzer Space Telescope then followed-up on this discovery. Spitzer was uniquely up to the challenge as it is finely sensitive to the cool glow of the dwarf star. Spitzer confirmed 2 of the three planets and found the 3rd planet to actually be 3 different planets, it then went on to discover 2 more planets, leading to a total of 7 exoplanets. NASA published their findings in the journal, Nature on Feb. 23, 2017.

Spitzer studied the TRAPPIST-1 Star for over 21 days nearly continuously (500 Hours), pausing only to send data back to Earth. Spitzer looked for tiny dips in the star's brightness, as the circling planets passed in front of the star while in transit. Hubble then followed up on Spitzer's data to peer into the system to look for the chemical fingerprint of Hydrogen gas in the atmospheres of the planets. So far Hubble has not found evidence for hydrogen gas in the atmospheres, which is a good indicator that these planets are not gas planets, but terrestrial, rocky bodied planets.

Spitzer Space Telescope launched in 2003 was designed to last at least 2.5 years. 13 years (2016) later Spitzer has operated far beyond the scope of its original mission. Spitzer uses infrared vision (IR) to peer into the cosmos and see it in new ways. It is able to see through the dust in space. Because of this, it originally was used to peer through the dust and debris, deep into stellar nurseries where stars are born. It has been more recently used to map IR temperature of exoplanets, create a 360 degree panorama image of the Milky Way, find a new ring of Saturn hundreds of times larger than any other ring previously known. Spitzer is now expected to last past the launch of its successor, the James Webb Space Telescope, in 2018.

Already more than 130 million miles away, Spitzer faces communication challenges with NASA because of distance. In mid-2009 the telescope ran out of coolant, however the engineers' design still allows it to operate one of its three cameras. (Coolant is needed so the ship's own heat and IR do not interfere with the data collected by the cameras. For all three cameras to function as designed, the temp had to remain at just 5 degrees above absolute zero.) Unlike other orbital telescopes like Hubble, Spitzer was design to move away from the Earth slowly, in a Earth-Trailing orbit. (Because of the massive amount of IR light waves that the Earth would radiate onto the telescope, making it hard to see clearly through.)


Mind the Gap! Investigating a Potential Cause of the Exoplanet Radius Valley

Editor’s note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we occasionally repost astrobites content here at AAS Nova. We hope you enjoy this post from astrobites the original can be viewed at astrobites.org.

Title: Bridging the Planet Radius Valley: Stellar Clustering as a Key Driver for Turning Sub-Neptunes into Super-Earths
Authors: J. M. Diederik Kruijssen, Steven N. Longmore, & Mélanie Chevance
First Author’s Institution: Center of Astronomy, Heidelberg University, Germany
Status: Published in ApJL

Neptunes and Jupiters and Earths, Oh My!

Extrasolar planets, or exoplanets, have been theorized for centuries, and studied firsthand since the 1990s. Much of the common classification of exoplanets is based on analogs in our own solar system: hot Jupiters, super-Earths, and super-Jupiters, just to name a few. The authors of today’s paper focus on two types of exoplanets: super-Earths (planets with more mass than Earth but less mass than Neptune) and sub-Neptunes (planets of 1.7–3.9 times the size of the Earth, but with a composition similar to Neptune’s).

Figure 1: A histogram of planets with given radii from a sample of 900 Kepler systems. The decreased occurrence rate between 1.5 and 2.0 Earth radii is apparent. [Fulton et al. 2017]

Compiling the Sample

The authors analyze a sample of exoplanets from the NASA Exoplanet Archive with radii of 1–4 Earth radii and orbital periods of 1–100 days. These radii and periods are chosen so that they only analyze planets that have had these values directly measured rather than derived from mass–radius relationships. The density of stars around the planet’s host star is part of the archival data, and the sample is split into “field” and “overdensity” subgroups that consist of low stellar density and high stellar density host star regions, respectively. In this case, what constitutes low and high densities is determined by the probability of there being many stars within 40 pc of the system: field stars have an 84% probability that there aren’t many neighboring stars, and overdensity stars have an 84% probability that there are. Additionally, only systems with ages of 1–4.5 billion years are considered, since younger systems may not be stabilized and the overdense group is too small in older systems. Finally, they constrain the host star mass to 0.7–2.0 solar masses to limit the chance of observing effects that are actually caused by mass differences rather than stellar clustering. With these cuts, the authors are left with 8 field planets and 86 overdensity planets, for a total of 94.

Results

Figure 2: Left: The orbital periods and radii of the planets. The radius valley is marked with the black line, and its uncertainty is given by the grey stripe. Center: The planetary radii versus the density of their stellar fields, with the grey line representing a constant radius. Right: A histogram of how many planets have each radius. Note that the radius is plotted on a logarithmic scale in all three panels. [Kruijssen et al. 2020]

Simply plotting the densities and radii suggests that the authors’ idea holds up (Figure 2). In the middle panel, the gray line represents a constant radius within the radius valley. The fact that there are fewer planets around this line shows the radius valley exists, but how does that prove their idea? The field stars all lie above the radius valley, while a little more than half of the overdensity stars lie below the radius valley. If residing in a dense field can cause dynamic and radiative effects that decrease the planet’s radius, having more small planets in overdense regions is expected.

But what if it’s really the effect of some other properties of the systems? Comparing the planets’ host star masses, metallicities, and ages shows no clear differences that might suggest the trend is caused by one of those characteristics. This data is compiled in Table 1. But what about the distance from Earth to the system? The further from Earth a system is, the less likely we are to be able to observe smaller planets. Could that be a factor skewing the numbers, since that could mean we just aren’t seeing the smaller planets? On average, the field systems are closer to Earth, but all of their planetary radii lie above the valley. The authors therefore conclude that the distance is probably not a contributing factor either.

Table 1: Characteristics of the sample planets. The authors split the sample into three groups: field planets, overdensity planets with radii above the radius valley, and overdensity planets with radii below the radius valley. The median stellar masses, metallicities, ages, and distances from Earth for each group are given with their uncertainties. The authors conclude that these values are all close enough to suggest that they are not the cause of the radius valley. [Kruijssen et al. 2020]

But what about those other mechanisms we discussed earlier? The authors consider photoevaporation within the system, mass loss, and rocky formation alongside the potential effects of densely clustered stars near the system. They conclude that stellar clustering alone can’t be responsible for the trends seen in planetary radius, but alongside one of the other three theories, clustering is certainly a potential contributor to the radius valley. The clustering would, however, affect each of the three scenarios differently. For the rocky core mass loss scenario, it is unlikely that clustering has any direct effect, since that mechanism is purely internal to the planet. The likelihood of rocky planet formation, on the other hand, can be increased by clustering effects, since neighboring stars could cause photoevaporation within the protoplanetary disk. This would decrease the amount of gas in the disk, increase the dust-to-gas ratio — the ratio of solid particles to gaseous particles in the disk — and thus increase the likelihood of rocky formation. Additionally, clustering could cause more stellar encounters with the system, which in turn could change the orbits of the planets and the effects of photoevaporation inside the system.

In this paper, the authors conclude that, in addition to previous theories, the dynamic and photoevaporative effects of stars near planetary systems can contribute to the radius valley between super-Earth and sub-Neptune exoplanets. Although this doesn’t provide definite answers to why this valley exists, it provides another piece to the puzzle. Solving the mystery of this radius valley can give us more insight into planetary formation mechanisms in extrasolar systems.

Original astrobite edited by Mike Foley.

About the author, Ali Crisp:

I’m a third year grad student at Louisiana State University. I study hot Jupiter exoplanets in the Galactic Bulge. I am originally from Tennessee and attended undergrad at Christian Brothers University, where I studied physics and history. In my “free time,” I enjoy cooking, hiking, and photography.


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