Introducing the ‘Mega-Earth’

by Paul Gilster on June 3, 2014

Building public interest in deep space is a long-term goal for most of us in the interstellar community, and the release of the film Interstellar this fall may set off a new round of discussion among reviewers and movie fans alike. Also helpful is the DVD release of the Neil deGrasse Tyson Cosmos series, given Tyson’s performance and the stunning visuals that communicate the majesty and power of the universe around us.

But I think it’s encouraging that while these blockbuster media releases work their magic, what used to be staid scientific conferences frequented only by specialists are turning into media events of their own. The American Astronomical Society is currently meeting in Boston, with exoplanet papers that I’m already seeing discussed well outside the usual venues. The more we see the excitement and sheer scope of the exoplanet hunt communicated to the public, the more likely we’re building the kind of interest among young people that may one day turn into scientific careers and — dare I say it — funding for missions that will fly beyond our system’s edge.

Maybe Dimitar Sasselov (Harvard-Smithsonian Center for Astrophysics) had a bit of media buzz in mind when he described a rocky world known as Kepler-10c as “the Godzilla of Earths.” The finding is exciting enough that he can, in any case, be forgiven the hyperbole. Kepler-10c is located about 560 light years from us in the constellation Draco, orbiting its host star every 45 days. A second world in this system, the star-hugging Kepler-10b, was the first rocky planet detected by Kepler and confirmed by radial velocity follow-ups from Earth. With a diameter of about 29,000 kilometers, Kepler-10c looked to be another ‘mini-Neptune,’ a world with a thick atmosphere covering a much smaller rocky core.

The surprise came when a research team led by Sasselov’s colleague Xavier Dumusque at the CfA went to work with the HARPS-North instrument on the Telescopio Nazionale Galileo (Canary Islands) to measure the mass of the planet. The result: Kepler-10c weighs fully 17 times as much as the Earth, demonstrating it is made of rocks and other solids. A rocky planet on this scale is beyond the designation ‘super-Earth.’ This CfA news release calls it a ‘mega-Earth,’ a world that we would have expected to have become the core of a Neptune- or Jupiter-class gas giant.

PIA18018_ip

Image: The newly discovered “mega-Earth” Kepler-10c dominates the foreground in this artist’s conception. Its sibling, the lava world Kepler-10b, is in the background. Both orbit a sunlike star. Kepler-10c has a diameter of about 18,000 miles, 2.3 times as large as Earth, and weighs 17 times as much. Therefore it is all solids, although it may possess a thin atmosphere shown here as wispy clouds. Credit: David A. Aguilar (CfA).

The paper on this work notes that “Kepler-10c might be the first firm example of a population of solid planets with masses above 10 M.” And it goes on to discuss the work of Lars A. Buchhave, a CfA astronomer who likewise presented at the AAS meeting in Boston, whose work supports the idea that orbital period correlates with the size at which a planet makes the transition from rocky to gaseous:

If this model is correct, it implies that the core mass limit for gas accretion should increase with orbital period. A recent study from Buchhave et al. (2014, in press.), analyzing hundreds of Kepler candidates, seems to agree with this theoretical prediction. With a period of 45.29 days, a radius of 2.35 R, and a density higher than Earth, Kepler-10c would be at the limit of the transition from terrestrial to gaseous planets observed by Buchhave et al. (2014, in press.). Kepler-10c might be the first object confirming that longer period terrestrial planets can be more massive than ones with shorter periods.

The implication is that Kepler-10c is not likely to be the last ‘mega-Earth’ we find. And this is interesting:

We note that Kepler-131b (Porb = 16 days, 16.1 ± 3.5 M, 2.4 ± 0.2 R, Marcy et al. 2014) lies in the same location of the mass-radius diagram as Kepler-10c. However the mass determination of Kepler-131b is not as robust as for Kepler-10c and more data are needed to confirm the high density of this planet. Measuring precisely the mass of several other long-period Kepler candidates orbiting bright stars could test this speculation. This experiment has just been started as a new observational program on HARPS-N.

Observational errors dominated by photon noise are a problem with a star as faint as Kepler-10, and the paper adds that future transit searches focusing on bright stars will allow the high-quality radial velocity findings needed to measure planetary masses to greater precision. The TESS (Transiting Exoplanet Survey Satellite) mission should provide numerous candidates, as should the ESA’s PLATO mission (PLAnetary Transits and Oscillations of stars).

Meanwhile, we may be learning something new about the early universe, for the Kepler-10 system is some 11 billion years old. Huge, rocky planets could evidently form despite the fact that heavy elements were scarce in this era. Says Sasselov: “Finding Kepler-10c tells us that rocky planets could form much earlier than we thought. And if you can make rocks, you can make life.” Even stars as old as this one, then, shouldn’t be ruled out when we look for astrobiology candidates, a finding that expands the parameters of our search for life elsewhere in the universe.

The paper is Dumusque et al., “The Kepler-10 planetary system revisited by HARPS-N: A hot rocky world and a solid Neptune-mass planet,” accepted for publication in The Astrophysical Journal (preprint).

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{ 30 comments… read them below or add one }

ljk June 3, 2014 at 11:22

The quest for inhabited habitable planets

June 3, 2014

Which came first: life or habitability? Although this question seems at first sight contradictory, a new paper by Colombian researchers is bringing to the attention of astrobiologists a classical conundrum: Is life also required for habitability?

On Earth it is almost a matter of fact that in the same way as habitable conditions on our planet are mandatory for life, the existence of life could also be determinant at making our world permanently habitable.

And, if this is the case for Earth, it should be also for other inhabited habitable planets elsewhere. Consequently, if our goal is to find life in the Universe, we should not exclude life itself when predicting on which planets it could thrive.

“We should make things as simple as possible, but not simpler”. This is the famous Einstein quote that opens a new paper written by a group of Colombian scientists and accepted for publication in the journal Biogeosciences Discussion.

The paper entitled “The Habitable Zone of Inhabited Planets,” authored by Jorge I. Zuluaga, FACom researchers and others, cites the quote to highlight the fact that most of the models used today to predict the conditions under which a planet will be habitable are probably simpler than they should be.

According to the authors, life has been systematically excluded when calculating the plausible environmental conditions in habitable planets. The situation is paradoxical, since life is the ultimate goal of our search for habitable worlds in the Galaxy.

Full article here:

http://phys.org/news/2014-06-quest-inhabited-habitable-planets.html

DCM June 3, 2014 at 12:35

Could any inhabitants ever achieve escape velocity?
They must be comparatively small and not vary tall no matter how smart.

andy June 3, 2014 at 12:37

If you read the paper, they estimate that about 5-20% of the planet’s mass is in the form of “ices”, i.e. water, ammonia, methane, etc. This is not a terrestrial planet: even with 5% ices it is far more volatile-rich than any of the Sun’s rocky planets, despite apparently being quite hydrogen-poor compared to the ice giant planets. Incidentally, that lower limit of 5% corresponds to a mass similar to that of Venus in ices. I’d therefore expect a quite substantial atmosphere. Good luck seeing down to the rocky surface (if a well-defined surface even exists under the conditions in the interior of the planet) through all that – the artistic image strikes me as being highly misleading.

Michael June 3, 2014 at 12:38

Here is a bit more info on Kepler 10c, note the density is 5 g /cm^3 +/-5 so there is a great range of densities and therefore possibilities of the makeup of the planet. The density is a little low as self compression would have boosted it quite a bit more if it was a completely rocky world, so more likely a thick dense H/He atmosphere with water/ice/rock combination. The gravity on this world must be high, around equal to or greater than Jupiter’s at higher densities.

http://exoplanets.org/detail/Kepler-10_c

nullzero June 3, 2014 at 12:47

Is it in the star’s HZ? I hope it’s not tidally locked being this massive.
Do you think humans could survive on a body like this?

Andrew Palfreyman June 3, 2014 at 17:00

Average density = 1.4x Earth’s
Average surface gravity = 3.21 gee

Possible restriction on M-dwarf exoplanet habitability
http://www.nature.com/news/wind-may-deflate-search-for-habitable-planets-1.15335

Phil June 3, 2014 at 20:39

And if Kepler 10c isn’t odd and controversial enough, the zoo just keeps getting weirder and wilder. Todays ArXiv brings us 2 Super Earths around the old halo dwarf Kapteyn’s star, one in the Hz.

http://arxiv.org/pdf/1406.0818.pdf

Bound the be the subject of one of Paul’s posts soon :)

P

NS June 3, 2014 at 23:24

Albeit with no evidence, I’m skeptical about interstellar (or further) panspermia. But given what we know about how material can move between objects in the solar system, it seems possible that within a stellar system life could emerge in a single highly favorable location, and then spread throughout the system from there. Smaller habitable worlds (e.g. early Mars) would be good candidates since their low gravity would allow material from them to spread more easily. Under this scenario life could even end up in places (no water/air?) that superficially look uninhabitable.

Harry R Ray June 4, 2014 at 13:10

Geoffrey Marcy was very suprised that the radius was 2.3Re or a 17Me planet, because, according to him, theory pradicts that gravity would crush the radius down to 2Re. But, if the planet were composed of wolaitles with a low enough melting point, the COMBINATION of external AND internal heat could COMPLETELY LI@QUIFY THE SURFACE, making a detection of the SECONDARY ECLIPSE possible by JWST( and maybe even Spitzer,though I doubt it because it is so far away)!

ljk June 4, 2014 at 13:38

Strange alien planets are not the only new things being discovered and announced this week:

http://phys.org/news/2014-06-astronomers-thorne-zytkow-bizarre-hybrid-star.html

In summation, they are hybrids of red supergiant and neutron stars!

Michael June 4, 2014 at 14:20

@DCM June 3, 2014 at 12:35

‘Could any inhabitants ever achieve escape velocity?
They must be comparatively small and not vary tall no matter how smart.’

By my reckoning multi staging only and a small payload at best, however an Orion type vehicle, no problem.

@nullzero June 3, 2014 at 12:47

‘Is it in the star’s HZ? I hope it’s not tidally locked being this massive.
Do you think humans could survive on a body like this?’

At 0.2 Au it is not in the HZ and it is mostly likely tidally locked or in a resonance. See neat program below to work out rough HZ zones.

http://astro.unl.edu/naap/habitablezones/animations/stellarHabitableZone.html

As for the surviving on these worlds I tried walking around once with weights equal to my body weight ~80kg and it was doable but highly tiring. I could probably walk about 200 m before dropping dead! We could make ‘mass’ suits to test the effects on humans and other animals though.

ljk June 4, 2014 at 14:41

Seeing as we are discovering celestial objects that until very recently were either mere theories or not even supposed to exist, let us go one step further just for the intellectual exercise:

http://student.vub.ac.be/~clvidal/writings/Vidal-Starivore-Binary.pdf

For reference and comparison:

http://www.centauri-dreams.org/?p=23203

Norman Wells June 4, 2014 at 18:35

Let us be realistic and not let imagination carry us into the realms of fantasy . That is for the writers of Sci-Fi who never the less keep the dreams of Space travel alive ,while we struggle with the realities .Or the theorists whose satisfaction is gained by the study of the ‘what -ifs’ of science and technology.
If we are realistic , it will take most of this century and probably the next doing the R&D that will enable pioneering manned voyages of a very basic nature to Mars and the Asteroids , and the establishment of a manned base or bases on the Moon-if the funding involved can be found -and if the World’s major powers can be persuaded not to continue with the political and economic games which lead to Major wars. At present it seems more likely that we will slide into another World conflict which will result in a catastrophe from which it could take centuries to recover, and consequently set back our space endeavours for a very long time . Unless and until we can restrain ourselves from conflict and concentrate on collaboration we are not only going to worsen conditions on the only planet on which we are certain we can live,but leave dreams of Space Travel forever a distant reality .
I believe too that the majority of earth’s population will take a lot of convincing that there is any virtue in Space travel,aside from it being the subject for increasingly phantastic scenarios for their entertainment. Who,except the very dedicated , would wish to risk their lives in travel to distant locations where they might receive a very hostile welcome,and were unlikely to survive. And what evidence exists that commercial interests might reap rich rewards from valuable material discoveries or new markets on distant Planets. There exists an enormous amount of information about Space and its inhabitants [and it likely that they exist] to gather,and a lot of new scientic and technical discoveries to made and these activities may occupy scientists and engineers for centuries before serious attempts at mass travel to distant worlds or large scale migration and colonisation can take place. But still I feel certain that it will eventually happen .

ljk June 5, 2014 at 9:27

‘Mega-Earth’ Planet Discovered Orbiting Distant Sun-Like Star

By Paul Scott Anderson

Astronomers on Monday made a “big” announcement about exoplanets, and it is big – literally. Another new world has been discovered, which is quite routine now these days, but this one is different, and unexpected; a planet which is more than twice as large as Earth and about 17 times heavier, a sort of “mega-Earth” as some have referred to it. Nothing else like it has been seen before, until now.

Why is this so significant?

The planet, Kepler-10c, orbits a Sun-like star about 560 light-years away in the constellation Draco. It presents a conundrum for astronomers because it is a type of planet that was thought to be impossible to exist – any planets of that size and mass were presumed to always become a gas or ice giant like Uranus or Neptune, or even larger like Jupiter or Saturn. This world, however, appears to defy that expectation, and has remained a massive rocky planet, kind of like an Earth on steroids.

The new discovery was announced at a press conference during a meeting of the American Astronomical Society (AAS).

As astronomer Xavier Dumusque of the Harvard-Smithsonian Center for Astrophysics (CfA) notes, “We were very surprised when we realized what we had found.” Dumusque led the analysis which used data from the Kepler space telescope to make the discovery.

The new findings were made using the HARPS-North instrument on the Telescopio Nazionale Galileo in the Canary Islands. It was previously known that Kepler-10c had a diameter about 2.3 times that of Earth, which wasn’t too unusual in itself, but it’s mass was still a question. Now that question has been answered, but the result is still more questions as to how this planet formed.

Some exoplanets, like Kepler-62 depicted here, are “super-Earths”; Kepler-10c is more like a “mega-Earth.”

As for habitability, Kepler-10c is not a likely candidate, since it orbits very close to its star, in only 45 days. Much too hot, at least for life as we know it. But it’s existence also means that other ones like it are probably out there and that may be good news for finding life elsewhere:

“Finding Kepler-10c tells us that rocky planets could form much earlier than we thought. And if you can make rocks, you can make life,” said Dimitar Sasselov, another CfA researcher and director of the Harvard Origins of Life Initiative.

“This is the Godzilla of Earths!” he added. “But unlike the movie monster, Kepler-10c has positive implications for life.”

Full article here:

http://www.americaspace.com/?p=61824

Godzilla? Really?

Michael Fidler June 5, 2014 at 10:21

How about a double planet? Look at how many stars are close binaries, look at the Earth and Venus, two of the densest planets in our solar system and come as close as 24 million miles. Two 8.5 mass planets would make a little more sense and would make for a much more interesting details, no tidal lock to the star but to each other, etc… Could we tell with the data already available, changes in Kepler light curves maybe? Let chaos reign on yin and yang.

Brian Swiderski June 6, 2014 at 0:59

Could such a world be the exposed core of what had previously been a Neptune-class planet that had most of its atmosphere stripped by some process?

Michael June 7, 2014 at 9:30

@Michael Fidler June 5, 2014 at 10:21

@Brian Swiderski June 6, 2014 at 0:59

‘Could such a world be the exposed core of what had previously been a Neptune-class planet that had most of its atmosphere stripped by some process?’

There is the possibility of two gas dwarfs colliding early on in the solar systems formation. As the two young hot planets collided the gas and water/steam mantle would have extended towards each other and collided at very high velocity, in effect substantial quantities of the H/He and water mantle could have been blown off leaving the rocky core.

DCM June 7, 2014 at 14:03

Dreams of space travel are more realistic than dreams of a world without conflict. It was conflict among themselves and against Moslems that got Europeans to try to circumnavigate the Earth to gain more wealth and it was conflict between the US and USSR that got us to the moon.
Of course we should work toward cooperation but work toward escaping our planet while doing so. Large empires of various sort don’t really do much besides stifle creativity and efforts to advance.

R Kelley June 7, 2014 at 15:41

With respect to the new Mega-Earth:
(1) There has been a debate about whether the compact solar systems that seem to be the most common type of solar system in the galaxy formed by (a) the migration of already formed planets from beyond the snow line versus (b) the migration of planetesmials from beyond the snow line followed by in-situ formation of the planets once the planetesmials were close to the central star versus (c) no migration at all but the planets form from a giant nebula (much more massive than the minimum mass solar nebula) around the central star.
(2) It would appear that the new Mega-Earth would argue for the scenario in (1)(b) above. If (1)(a) were true, it would seem that the Mega-Earth would have acquired a gaseous envelope during the early migration process from the then intact solar nebula (and would have not have later lost such gaseous envelope). (1)(c) has been effectively attacked in articles and seems very unlikely.
(3) Of course, the real question (for those of us interested in the probability of earth-like planets around solar-type stars) is what effect the migration of such planetesmials would have on the already existing planetesmials in the star’s habitable zone. Would such migration of planetesmials (a) destroy or scatter the already existing planetesmials in the star’s habitable zone, (b) provide volatiles and enhance the size of the pre-existing planetesmials in the star’s habitable zone, or (c) basically leave the pre-existing planetesmials unscathed.
(4) The migration of massive amounts of materials from beyond the snow line to very close to the central star ( or onto the central star) would likely account for the enhanced amount of refractory materials in so many solar type stars discussed in a recent article. The presence of a Jupiter type planet (apparently present in only about 17% of stars) or a fairly close steller companion may prevent such migration onto the central star in other cases.

Would love to hear your thoughts and speculations on the above.

Rob Henry June 7, 2014 at 16:44

Michael, surely two planets that consist largely of gas colliding very early on will be in coplanar orbits of low eccentricity. Such conditions only provide just enough energy to strip those gasses, yet most energy should be lost by radiation. Furthermore, all water, methane and ammonia would be transformed to plasma in the collision, and their hydrogen striped leaving no ice. So it seems that, if this is a valid formation scheme, I am missing some factor from it.

Michael June 8, 2014 at 9:06

@Rob Henry June 7, 2014 at 16:44

‘Michael, surely two planets that consist largely of gas colliding very early on will be in coplanar orbits of low eccentricity.’

Gas dwarf in my book is not a very good analogy, they behave more as liquids.

A rear head on collision such as the one suspected of creating our moon (L1 or L2 fall point) is possible, the impact energy of two very young hot worlds say the size of around Neptune also having a high closing velocity >>10km/s would be enormous. During the impact the material would be thrown out at the impact terminator (two water bombs coming together) weakening the gravitational pull of the collating mass allowing much more gases such as hydrogen/Helium and (oxygen from water) to escape along that path. Remember the planet is very hot and even hotter deeper down >9500 K so releasing the pressure by the distortion of the planets would cause almost instant ‘ionisation’ in the vacuum of space, mono hydrogen at 9500K is traveling very fast.

I find it very difficult to understand for the planet to have collected so much rock as a percentage, surely as the gas fell towards the star it would have held on to a lot of that gas been as massive as it is.

Rob Henry June 8, 2014 at 16:42

@Michael yes, such collisions produce more than enough heat to begin to drive of an atmosphere through hydrodynamic flow, but the process should run out of energy with a mass fraction loss 1% tops. Were you hypothesising that the stored heat in the cores were already high enough to take that process much further, but was trapped until the collision allowed mixing?

You allude to the Earth -Theia collision. As collisions go that was unusually efficient at producing mass loss due to its high collision angle. Perhaps that is what I am missing, need to reduce angular momentum. The immediately post collision object might have an equatorial rotation near orbital velocity. That would make the calculations rather complex, but could even this push that mass loss above 10%? Maybe, but losing half or so feels unlikely to me.

Michael June 8, 2014 at 23:43

Sorry Paul that should have been L4 and L5 points not L1 and L2.

Michael June 10, 2014 at 9:11

@Rob Henry June 8, 2014 at 16:42

‘…such collisions produce more than enough heat to begin to drive of an atmosphere through hydrodynamic flow, but the process should run out of energy with a mass fraction loss 1% tops.’

The amount of energy I get available from a L4 or L5 fall would move less than 0.2% of the mass of the planet to escape velocity, best case (rough work)

‘Were you hypothesising that the stored heat in the cores were already high enough to take that process much further, but was trapped until the collision allowed mixing?’

Yes, the impact would cause the outer H/He/water/ice mantle to shear outwards perpendicular to the impact in a hydrodynamic flow pattern much more than the cores (which are more rigid), in essence like a shaped charge. Now the further the hot mantle moves away from the planet the weaker gravity is and now it is exposed to space. So we have weaker gravity, high momentum, super hot gases >10000K and a vacuum for it to expand into all contributing to mass escaping.

‘You allude to the Earth -Theia collision. As collisions go that was unusually efficient at producing mass loss due to its high collision angle.’

The moon/earth impact (due to angle) I believe was efficient at putting mass into orbit, not at removing it to escape velocity.

‘The immediately post collision object might have an equatorial rotation near orbital velocity’

It is a head-on collision, little or no rotational involvement.

Rob Henry June 10, 2014 at 17:30

“The moon/earth impact (due to angle) I believe was efficient at putting mass into orbit, not at removing it to escape velocity”

That is why the calculations would be complex for gas. If a large fraction of molecules moving in the direction of rotation can reach orbital velocity, I imagine the gas torus orbiting around its equator must get rather dense. Such a system would be very complex, with ridiculously strong winds, and high transfers of thermal energy between planet and torus. The only simplifying assumption I can think of would be that all gas is lost from the torus.

Michael June 11, 2014 at 13:08

@Rob Henry

‘That is why the calculations would be complex for gas.’

The planets would be mostly of liquid, here is a little more on the subject but relating to Jupiter masses. I have not gone into much depth as I would have liked though, if you spot something please point it out.

http://www.geo.brown.edu/classes/geol1950g/Dokuchaev.Eroshenko.pdf

‘For the Jupiter, orbiting a Sun-like star, the corresponding orbital radius for planet destruction is… ~0.25 AU.’

Kepler-10c orbits at around 0.24 AU but its sun has a little less mass and so affects it. It appears the closer you go towards the star the more efficient the mass removal process is, Mercury may be our solar systems example.

Notice fig 2, as the depth of penetration increases the mass loss fraction gets a lot higher! and I like the bit about,

‘In addition, without the pressure support, the metallic hydrogen transforms into the dielectric phase with a possible energy release 290MJ/kg.’

~60 times as much energy as dynamite on a mass basis!

Michael June 12, 2014 at 10:27

An interesting point in this article is this,

http://www.geo.brown.edu/classes/geol1950g/Dokuchaev.Eroshenko.pdf

‘At the same time, the planets loss some part of their gas envelopes during mutual collisions.’

Now if there is a rate of collision per year surely there will be more shallow angle impacts than mergers and complete destructions. Could these shallow angle impacts be partly responsible for the highly eccentric orbits of some gas giants as the impacts would tend to throw them apart?

ljk June 27, 2014 at 10:29

GJ 832c: Habitable Super-Earth or Super-Venus?

By Andrew Lepage

June 27, 2014

I readily admit that one of my pet peeves going back almost 20 years to the discovery of the first extrasolar planets has been overblown claims about the potential habitability of some of these discoveries. This has been one of the motivations for my ongoing series of Habitable Planet Reality Check posts in recent months on this web site (for the full background story, see Habitable Planet Reality Check: Kepler 186f).

This past week, a team led by Robert Wittenmyer (UNSW Australia/ University of Southern Queensland) announced the discovery of a potential super-Earth orbiting on the inside edge of the habitable zone of the nearby red dwarf star, GJ 832. And in a refreshingly honest change of pace, instead of claiming that this new discovery might be a habitable planet (as far too many other teams have done with similar discoveries), this team makes the statement right up front that they do not believe that the planet they found, GJ 832c, is likely to be habitable and is more likely to be a uninhabitable “super-Venus”.

This intellectual honesty about the potential nature of this newly-discovered planet probably explains why there had been little attention paid to it during most of the last week in the various astronomical news outlets. That changed on June 25 with the wildly optimistic press release by Abel Mendez Torres of the Planetary Habitability Laboratory at the University of Puerto Rico at Arecibo which, despite the actual claims made by Wittenmyer and his team in their discovery paper, rates GJ 832c at 0.81 on their Earth Similarity Index (ESI).

The press release goes on to state that GJ 832c is one of the best Earth-like candidates found so far and certainly the closest (which once again makes me question the value of PHL’s rating system but that is a topic for another post). But while the discovery of GJ 832c did not initially attracted the attention of many people, it did get mine and deserves a closer look as a counterexample to some of the more dubious claims made about the potential habitability of extrasolar planet discoveries.

Full article here:

http://www.drewexmachina.com/2014/06/27/gj-832c-habitable-super-earth-or-super-venus/

ffp July 8, 2014 at 12:32

Now there are some even more massive solid exoplanets that are about the same size as Kepler-10c around 2 Earth-radii but they are denser than iron planets and have masses between 30-100 Earth masses.

Kepler-52b, Kepler-52c and Kepler-57b are likely to have originally been gas giants that have evaporated because they are so close to their star and have left behind just their solid rocky cores which were compressed to super-dense levels when they were enveloped by gas, but now they still remain very compressed because the cores can take billions of years to re-expand after the outer layers have been lost.

More info at http://meetingorganizer.copernicus.org/EPSC2013/EPSC2013-986-1.pdf

http://rsta.royalsocietypublishing.org/content/372/2014/20130164

ljk July 11, 2014 at 14:13

The Habitable Zone of Inhabited Planets

Posted by Jaime Green

2014/07/07 20:14 UTC

Topics: life, extrasolar planets

To at least some extent, the search for extrasolar planets is the search for extrasolar life. We don’t get excited about Earth-like planets because we’re worried our planet is lonely. We’re the ones looking to be less alone. Public interest piques at the mention of Earth-like discoveries: rocky worlds, roughly Earth’s size, in their star’s habitable zone. It seems simple. But of course, it’s not. Habitability is a very complicated thing, and if we want to search for real candidates for habitability, we have to delve more deeply into what habitability means—deeper than the headlines about the latest Earth-like find, for sure.

A team of Colombian researchers are arguing for a new refinement to the idea of the habitable zone that takes the presence of life itself into account. The habitable zone, or HZ, is the basic metric of habitability. It is the range of distances at which a planet can orbit its star and be at the right temperature to have liquid water on its surface.

Full article here:

http://www.planetary.org/blogs/guest-blogs/2014/0701-the-habitable-zone-of-inhabited-planets.html

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