Tag Archives: Kepler-10

Super-Earths and Mini-Neptunes


Low-Mass Habitable Zone Planets (artist images)

Our Solar System did not prepare us for what we would discover orbiting other stars. Instead, it told us that planets fall into neat categories: Gas giants made mostly of hydrogen and helium (of which Jupiter and Saturn are the archetypes), ice giants made mostly of water (for which Uranus and Neptune are representatives), and solid terrestrial planets with comparatively thin atmospheres — that would be the planets of the inner solar system and the one right under your feet). Since the discovery of thousands of planets orbiting other stars, and the measurement of their masses and densities, it has become clear that not all planets fit into this paradigm. Significantly, unless rocky worlds have an optimistically high abundance, what may be the most abundant type of planet in the Galaxy is a sort of mix between low-density, volatile-rich Neptune-like planets and rocky terrestrial planets. The Solar System features no such planet — after Earth, the next most massive planet is Uranus at ~14.5 times as massive. A casual look at the entirety of discovered transiting planet candidates discovered by Kepler reveals the magnitude of this problem.


While Kepler is no longer observing its original field, the massive amount of data can still be combed through to reveal new planet candidates. Here, previously discovered planet candidates are blue dots, and newly announced planet candidates are yellow. A few things are noteworthy. Firstly, the overwhelming majority of the newly discovered planet candidates have reasonably long orbital periods. This can be expected as shorter period planets have been detectable in the existing data for longer, and have had time to be spotted already. Secondly, and not really the point of this post… they’re still finding warm Jupiters in the data? Wow! What’s up with that? I would have thought those would have been found long ago.

With the obvious caveat that lower regions of that diagram feature harder to detect planets leading to that part being less populated than would be the case if all planets were detected, it would appear that there is a continuous abundance of planets from Earth-sized to Neptune-sized. While radius and mass may only be loosely related, it may also be that there is a continuous abundance of planets from Earth-mass to Neptune-mass, as well. Not having an example of such an intermediate planet in the Solar System, we really don’t know what to expect for what these planets are composed of. As such we began to call them (sometimes interchangeably) super-Earths or Mini-Neptunes. Are they enormous balls of rock with Earth-like composition extending up toward maybe 10 Me? Are they dominated by mass by a rocky core with a thick but comparatively low-mass hydrogen envelope? Do they have some fraction of rock, water and gas? Are they mostly entirely water with a minimal gas envelope? Answering this question would require some constraints on the masses of these planets, as it would allow one to know their density.

The first data point was CoRoT-7 b, the first transiting super-Earth — discovered before Kepler. The host star is very active, leading to a lot of disagreement in the literature about its mass, but further work seems to have settled on a rocky composition for the planet with ~5 Me. Great! Next data point was the transiting super-Earth orbiting GJ 1214, a ~6.5 Me planet with a much lower density, which is too low to be explained by even a pure water composition. This is decidedly not Earth-like. Additional measurements by highly precise spectrometers (namely HARPS and SOPHIE) of Kepler discovered planets have allowed for more data to be filled in, and an interesting trend can be seen.


Mass-Radius Diagram of Extrasolar Planets with RV-Measured Masses

Interestingly, planets less than ~1.6 Earth-radii seem to have not only solid, but Earth-like compositions. It’s worth noting that only planets where the mass measurement is acquired through Doppler spectroscopy are shown here. Planets like the Kepler-11 family where the masses have been derived by transit timing variations are not shown. If these planets are added, the adherence to the Earth-like composition is much less strict. This may imply that planets which have masses measurable by detectable transit timing variations have had a different formation history and therefore a much lower density. Further data will be very useful in addressing this issue.

On a somewhat unrelated topic, several new habitable planet candidates have been validated by ruling out astrophysical false positives. Among them is Kepler-442 b, which appears to me to be a more promising habitable planet candidate than even Kepler-186 f. Some newly discovered but not yet validated habitable planet candidates have been found as well, including one that appears to be a near Earth-twin.


New Kepler habitable planet candidates

2011 Review

Arguably the most important discovery of 2011: Earth-sized exoplanets

2011 was a banner year for extrasolar planet science with the Kepler results really beginning to come in. Among the more interesting:

    Kepler results:

  • Kepler-10: Kepler’s first rocky super-Earth, with a transiting Neptune further out.
  • Kepler-11: A system of six transiting super-Earths with anomalously low density.
  • Kepler-14: A massive hot Jupiter in a binary system.
  • Kepler-16: The first transiting circumbinary planet around an eclipsing binary star.
  • Kepler-18: A system of three planets: A super-Earth and two inflated Neptunes in a 2:1 resonance. Very similar to Kepler-9 but scaled down in masses.
  • Kepler-19: A transiting sub-Jovian planet and the first case of the discovery of a second planet through transit timing variations in the transiting planet.
  • Kepler-20: A system of five planets, two of which are ~Earth-sized.
  • Kepler-21: A transiting super-Earth around a bright (V = 8.27) star.
  • Kepler-22: A transiting “mini-Neptune” in the habitable zone, and the first transiting planet in the habitable zone of any star.
  • KOI-423: First transiting planet around a subgiant star.
  • KOI-730: A remarkable system of four planets in a 1:2:4:8 resonance.
  • KOI-55: What appears to be two remnant cores of gas giants engulfed by their parent star during its red giant phase.
    HARPS results:

  • 82 Eri: Three low mass planets only a few times the mass of Earth.
  • HD 136352: Three super-Earth/sub-Jovian planets.
  • HD 39194: Three super-Earths.
  • HD 134606: Three Neptunes.
  • HD 215152: Two lower-mass super-Earths.
  • Gliese 667 C: A second planet of a few Earth-masses in the habitable zone.
  • HD 85512: A super-Earth on the inner edge of the habitable zone.

On the orbital dynamics front, in January, it was found that the HD 37124 system, which hosts three intermediate period gas giant planets of roughly equal mass, may have a 2:1 resonance for the orbit of two of its planets. Furthermore, the planet candidate orbiting Rho Coronae Borealis, one of the first planet candidates, was proven to have a true mass far outside the planetary regime. The recovery of the planets of HR 8799 in old HST data has permitted the architecture of the system to be much more constrained.

A planet around a naked-eye giant star Alpha Arietis was reported in April. Later, in August, it was revealed that a new planetary mass object has been found orbiting a pulsar. On the subject of post-main sequence stars, a candidate planet was imaged orbiting a white dwarf. On the other side of the main sequence, a planet had been found in the late stages of formation at LkCa 15.

Gravitational microlensing provided us some constraints on the abundance of rogue giant planets as well as another cold super-Earth. It turns out that rogue giant planets may be twice as frequent as main sequence stars.

Several planets around eclipsing binaries were found this year, including planets at UZ For, HU Aqr, Kepler-16, and NY Vir.

Without a doubt, one of the most exciting stories of 2011 is the discovery that one of the first known super-Earths, the innermost planet at 55 Cancri, transits its star. This is the brightest star known to have a transiting planet, and it will prove very useful for the study of these kinds of planets. While it was initially thought that the planet was rocky and iron-rich, later observations suggest that the planet must have a significant envelope of volatiles.

In summary, 2011 has been an astounding year. The focus has shifted away from gas giant planets to sub-Jovian planets — Neptunes, mini-Neptunes and super-Earths. Below is a graph that shows this year’s total planet catch compared to previous years. One thing is clear: Not only is the galaxy full of planets, but we can look forward to seeing a huge number more in the near future.

Now for some fun, some predictions for what we might have by the end of 2012:

  • 1,000 Planets on the Extrasolar Planets Encyclopaedia
  • The discovery of a ring system around a transiting planet
  • More low-mass planets in the habitable zone from both radial velocity and transit
  • Confirmation of obvious extrasolar planet atmospheric variability (cloud rotations, etc).