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Exotic exoplanet types


MichaelPoole
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What struck me when reading about the "super-Earth" type of exoplanet is that many people seem to assume these planets are either like terrestrials in our Solar system, or mini-Neptunes/Gas Dwarfs. Reading more about this struck me as I realized an "in-between" exoplanet type probably exists and many of the superterrestrials discovered to this date. These two papers https://arxiv.org/abs/1606.08088 https://arxiv.org/abs/1311.0329 are rather enlightening on this matter. For example of what struck me:

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On the other hand, if we assume a more typical low-mass planet with a 5 M Earth-like core, then to be 2.0 R it would need 0.5% of its mass in a H/He envelope. This may not sound like much, but it corresponds to 20 kbars of hydrogen and helium, 20× higher than the pressure at the bottom of the Marianias Trench. Moreover, the temperature at the bottom of such an envelope would be 3000 K, even for ages of several Gyr. We believe that such a planet is more properly classified as a sub-Neptune. As a result, 2.0 R is more of a quite hard upper limit for the size of a envelope-free super-Earth and most of the planets between 1.75 and 2.0 R are likely to be H/He rich sub-Neptunes.

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 Assessing the impact of thermodynamics for the case of Kepler 11b is more difficult, because the expected steam envelope is not modeled. If the gas pressure and temperature at the bottom of the envelope were, respectively ≈ 1 GPa and ≈ 1000 K, then Rc would be 5% larger than the value listed in Table 4 

Also, on Wiki 

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Whether or not the primitive nebula-captured H/He envelope of a super-Earth is entirely lost after formation also depends on the orbital distance. For example, formation and evolution calculations of the Kepler-11 planetary system show that the two innermost planets Kepler-11b and c, whose calculated mass is ≈2 M and between ≈5 and 6 M respectively (which are within measurement errors), are extremely vulnerable to envelope loss.[68] In particular, the complete removal of the primordial H/He envelope by energetic stellar photons appears almost inevitable in the case of Kepler-11b, regardless of its formation hypothesis.

This basically suggests that there is a class of planet that:

- has a solid or molten lava surface

- at the same time, has an atmosphere that is H/He rich, and while FAR lighter than Neptune (for this reason I would say calling them sub-Neptunes is not really accurate as the pressure and temperature, as high as they are, are orders of magnitude lower than those in the water mantle of Neptune), still much denser, hot and crushing than Venus (pressure on the surface of Venus is 9 MPa, the pressure on a hypothetical 5 ME/2 Earth radius planet is 2 GPa or 222x as much as on Venus, one of the theoretised pressures for Kepler 11b is 1 GPa). Now, these planets have some unimaginable surface conditions, but unlike Neptune, you can still say there is a surface there, and to compare, the pressure at the top of the water mantle of Neptune is 200 GPa or 100-200x as much as at the surface of these planets (the temperature at the top of Neptune's mantle is 3000 K). In case of Kepler 11 b, this atmosphere is likely to be steam/supercritical water (a sort of an inbetween phase between liquid and gas).

So, basically, there are probably many planets that are not really terrestrials as we know them from our Solar system, but not ice dwarfs like Neptune let alone gas giants like Jupiter. Of course, if we ever get there, exploring them would be the ultimate challenge of building landers, but the chemistry and processes (as they might feature processes we know from Neptune along with terrestrial geological phenomena like volcanoes) might be very fascinating. 55 Cancri is also sending some rather interesting signs that it is probably something we have not seen yet:

http://www.space.com/32416-super-earth-55-cancri-e-super-hot-weather.html

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To help solve the mysteries of 55 Cancri e, astronomers used NASA's Spitzer Space Telescope to monitor infrared emissions from the exoplanet for 75 hours total during the summer of 2013. The resulting thermal map revealed a strong difference in temperature between the planet's dayside and nightside.

55 Cancri e is tidally locked, meaning it always keeps the same face pointed at its star. On the dayside, temperatures on 55 Cancri e can reach about 4,400 degrees Fahrenheit (2,427 degrees Celsius). On the nightside, temperatures can dip to about 2,025 degrees F (1,107 degrees C).

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The nightside's relatively cooler temperatures suggest that 55 Cancri e does not possess a thick atmosphere that could carry heat from the dayside to the night side, Demory said. It also suggests this planet is not covered with a large envelope of water, ruling out the possibility that supercritical fluids envelop 55 Cancri e, Demory added.

About halfway between the dayside and the nightside, the researchers discovered that 55 Cancri e possesses a hotspot. They suggest this hotspot might be due to lava flows, and because the planet is hot, this lava may flow better than it does on Earth, behaving more like water does at room temperature and less like solid rock.

http://news.nationalgeographic.com/2015/05/150506-volcano-planet-space-cancri-astronomy/

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Volcanism, however, could also dim the planet, and the astronomers consider that to be more likely. “We know it happens all over our own solar system,” says Madhusudhan. “We see evidence for it on Venus, Mars, [Jupiter’s moon] Io, all kinds of solid bodies. That’s the natural explanation.”

The idea is that powerful eruptions would spew massive plumes of gas and dust into the planet’s atmosphere, masking the hot surface and making the planet appear cooler. The eruptions would have to be more powerful than anything ever seen, but there's a chance that's possible. 55 Cancri e orbits so close to its parent star, with a “year” lasting just 18 hours, that it it could be flexing under tidal forces generated by the star’s gravity. The heat from that flexing could keep the planet’s interior molten, which might drive the eruptions.

Yet, despite the temperature differences suggesting a thin atmosphere, one was indeed discovered, and it is a hydrogen/helium one with a mix of... probably hydrogen cyanide:

https://www.spacetelescope.org/news/heic1603/

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Using observations made with the Wide Field Camera 3 (WFC3) on board the NASA/ESA Hubble Space Telescope, the scientists were able to analyse the atmosphere of this exoplanet. This makes it the first detection of gases in the atmosphere of a super-Earth. The results allowed the team to examine the atmosphere of 55 Cancri e in detail and revealed the presence of hydrogen and helium, but no water vapour. These results were only made possible by exploiting a newly-developed processing technique.


“This is a very exciting result because it’s the first time that we have been able to find the spectral fingerprints that show the gases present in the atmosphere of a super-Earth,” explains Angelos Tsiaras, a PhD student at UCL, who developed the analysis technique along with his colleagues Ingo Waldmann and Marco Rocchetto. “The observations of 55 Cancri e’s atmosphere suggest that the planet has managed to cling on to a significant amount of hydrogen and helium from the nebula from which it originally formed.”

So here is some conflicting evidence. On one hand, the large temperature variations and possible evidence of volcanic ash blocking emissions suggests a relatively thin atmosphere. On the other hand, spectral evidence suggests a hydrogen/helium atmosphere. On one hand, the planet is 8.63x as massive as Earth, so it could have gathered a H2/He envelope. On the other hand, it is on an extreme torch orbit, worse than Kepler 10b and Corot-7b, which have practically no atmosphere, according to transit data. Yet this one has, yet no H2O was detected (while for a Neptune like planet it is the major component), drastic temperature variations, possible volcanism... yet it apparently retained some light gases. The radius is 2x of Earth, mass 8.63x of Earth, so it is much denser that the 5 ME/2 Earth radius hypothetical "borderline" planet. That would suggest it has a hydrogen atmosphere, but one with a lower surface pressure than 2 GPa. Basically, what I am saying is that some exoplanets might have hydrogen/helium atmospheres that have high pressures, but have not retained the extensive hydrogen envelopes like Neptune or Jupiter. I think planets like this might be frequent in torch orbits, as the gravity holds SOME of the light gases, but not everything. The result might be an unholy Venus/Neptune hybrid with features common to both ice giants and terrestrials.

But this does not end, apparently, some planets managed to grow to Neptune like masses while being so dense that they are clearly fully terrestrial and free of any H/He envelope:

https://www.eurekalert.org/pub_releases/2014-06/hcfa-afa053014.php

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Astronomers announced today that they have discovered a new type of planet - a rocky world weighing 17 times as much as Earth. Theorists believed such a world couldn't form because anything so hefty would grab hydrogen gas as it grew and become a Jupiter-like gas giant. This planet, though, is all solids and much bigger than previously discovered "super-Earths," making it a "mega-Earth."

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Kepler-10c was known to have a diameter of about 18,000 miles, 2.3 times as large as Earth. This suggested it fell into a category of planets known as mini-Neptunes, which have thick, gaseous envelopes.

The team used the HARPS-North instrument on the Telescopio Nazionale Galileo (TNG) in the Canary Islands to measure the mass of Kepler-10c. They found that it weighed 17 times as much as Earth - far more than expected. This showed that Kepler-10c must have a dense composition of rocks and other solids.

"Kepler-10c didn't lose its atmosphere over time. It's massive enough to have held onto one if it ever had it," explains Dumusque. "It must have formed the way we see it now."

The planet likely has a superheated ocean of water, but no helium or hydrogen:

https://en.wikipedia.org/wiki/Kepler-10c

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Kepler-10c has a mass of 15–19 Earth masses. With a radius only 2.35 (2.31 to 2.44) times that of Earth (and so a volume 12–15 times that of Earth), and a density higher than Earth's (6–8 g cm−3), it is unlikely to contain significant amounts of hydrogen and helium gas. Outgassed or accreted hydrogen-rich atmospheres would have been lost over the 10.6 billion-year lifetime of the Kepler-10 system. Instead, the composition is likely to be mainly rocky, with a water fraction of 5–20% by mass. The bulk of this water is likely to be in the form of high-pressure "hot ice" phases.

However, there are also apparently planets as light as Earth with an extensive light gas envelope:

http://www.nature.com/news/earth-mass-exoplanet-is-no-earth-twin-1.14477

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Astronomers have discovered an extrasolar planet with the same mass as Earth, but the resemblance ends there. Not only is the planet too warm for liquid water to exist on its surface, but it also has a radius 60% larger than Earth, suggesting a vast, puffy atmosphere of hydrogen and helium.

https://en.wikipedia.org/wiki/Gas_dwarf

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The smallest known extrasolar planet that is likely a "gas planet" is Kepler-138d, which has the same mass as Earth but is 60% larger and therefore has a density that indicates a thick gas envelope.

So, apparently, planets can be Neptune sized and terrestrial, Earth sized and with a huge gas envelope, or anything in-between. This is also why I don't really like when people throw terms like "super-Mars", "super-Earth", "super-Venus", or "super-Mercury". I think there is a very big factor that determines what a planet is like and that is - how exactly did it form and in what conditions. A planet is more that its orbital parametres.

Sorry if this got too long. Just had an urge to air my thoughts and stimulate a discussion. I personally feel that the obsession of astronomy about finding "Earth-like" planets limits our horizons and knowledge. I am fascinated by bizzare planets, even those that probably have no life (through the soup like atmospheres of "borderline" planets might have surprises waiting for us...), and think apart from the joy of knowledge and aesthetics (when we eventually manage to photograph them) they might eventually offer a lot to humanity.

EDIT - In addition, planets like the ones with a big, but not quite ice giant atmosphere (like the "sub-Neptunes" mentioned in the paper) might offer life bearing conditions if they are rogue planets:

https://en.wikipedia.org/wiki/Rogue_planet#Retention_of_heat_in_interstellar_space

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It is calculated that, for an Earth-sized object with a kilobar atmospheric pressure of hydrogen, in which a convective gas adiabat has formed, geothermal energy from residual core radioisotope decay will maintain a surface temperature above the melting point of water.[15] Thus, it is proposed that interstellar planetary bodies with extensive liquid-water oceans may exist. It is further suggested that these planets are likely to remain geologically active for long periods, providing there is a geodynamo-created protective magnetosphere, with possible sea floor volcanism providing an energy source for life.[15] Thus humans could theoretically live on a planet without a sun, although food sources would be limited. The author admits these bodies would be difficult to detect due to the intrinsically weak thermal microwave radiation emissions emanating from the lower reaches of the atmosphere, although later research suggests that reflected solar radiation and far-IR thermal emissions may be detectable if such an object were to pass within 1000 AU of Earth.

 

Edited by MichaelPoole
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Wonders of astronomy :) Two decades ago we didn't know about any planets beyond our system. And now we are finding more and more exoplanets - and only some of them conform to our ideas of how planets should look and behave. Luckily, we are building couple of really BIG telescopes - both in space and on Earth. Soon we'll see more :D

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