Tag Archives: Kepler-78

Super-Earths and Mini-Neptunes

super-earths

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.

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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.

M-R_Diagram

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.

kepler-chart

New Kepler habitable planet candidates

2013 Review

HD 106906

HD 106906 b, a directly imaged planet announced in 2013

First, foremost, and perhaps most painfully: α Centauri Bb may not really exist. What we thought was the Keplerian signal of an Earth-mass planet at our nearest neighbouring system may actually be noise in the data. While a bit painful, this is how science works – claims are rigorously tested and beaten tirelessly until they either continue to stand on the merit of the evidence, or they are refuted and disproved. This is how we keep the muck out of our pool of knowledge. Stay tuned… this could take a while to fully resolve.

The year began with direct imaging news: A new HST detection of Fomalhaut b (see here), suggesting the “planet” orbit is either not coplanar with the system disk or crosses the ring orbit and has a much lower mass than initially suspected. The imaged planet around β Pic b has also been independently confirmed. A circumbinary planet at 2MASS J01033563-5515561 became the first to be directly imaged. A planet with a mass of ~4 MJ became the lowest-mass planet directly imaged at HD 95086 (with the caveat that it isn’t clear what the nature of Fomalhaut b is). A planet perhaps of similar mass was later reported at GJ 504 (59 Vir).

Habitable zone discoveries started with the first known transiting Jupiter-sized planet in the habitable zone, PH2 b. Then things got very interesting with the simultaneous announcements of a super-Earth straddling the inner edge of the habitable zone of Kepler-69, and two habitable planet candidates at Kepler-62, which was covered here. HARPS found a nice system of planets around the M dwarf GJ 163. One of the planets is somewhat near the habitable zone, but it is my position that this planet does not deserve the attention worthy of a habitable planet candidate, with the planet receiving 40% more irradiation than Earth, and with the host star being an M-type dwarf, the atmosphere will not provide as much scattering of irradiation as Earth’s (Rayleigh scattering is increasingly efficient with decreasing wavelength), causing the surface of the planet to actually receive more than 40% more irradiation than Earth. Despite this, HARPS did provide us with another potentially exciting habitability result, with no less than three super-Earths in the habitable zone of GJ 667 C, with evidence for at least six, perhaps seven total planets there, however a reanalysis of the RV data seems to suggest that these new planets do not exist. Stay tuned…

Other noteworthy announcements included DW Lyn b, a giant planet orbiting a pulsating subdwarf B-type star. A hot Jupiter was also found orbiting a late-K/early-M dwarf by SuperWASP – a particularly rare find. A pair of super-Earths were found in a 2:3 resonance at HD 41248. A giant planet was found in a close orbit around a red giant branch star. Evidence of a second planet accompanying a newly discovered debris disk was presented for κ CrB. A super-Earth around HD 97658 was reported to be transiting (as was suspected two years ago but later dismissed due to a non-detection). The pair of planets at HIP 11952 ended up not existing – an error in compensating for the radial velocity of the observing site relative to the star.

Kepler results continued to stream in, starting with a rather interesting three-planet system at Kepler-68, with a mini-Neptune closest to the star, then an Earth-sized planet just outward of that, and a Jovian planet in a long-period orbit. It was shown that systems of multiple, low-mass planets uncovered by Kepler, like our own solar system, have orbits that are well-aligned with their host star’s equator (see here and here). Kepler results also uncovered a system with a pair of planets in a 2:1 resonance producing very strong transit timing and transit duration variations. A hot Jupiter at Kepler-76 provided strong evidence of super-rotation in the atmosphere via its secondary eclipse visible light photometry. Of particular note is the announcement of a planet smaller than Ganymede(!) at Kepler-37. A new population of small, rocky worlds in extremely short orbits was uncovered by Kepler, specifically Kepler-78 b wih its 8.5 hour orbit and KOI-1843.03 with its 4.2 hour orbit(!). Furthermore, Kepler unveiled the first transiting planets in an open cluster, NGC 6811.

Of particular note is the discovery of a transiting hot Jupiter orbiting a young, oblate, gravity-darkened T Tauri star. This remarkable system seems to imply that the formation mechanism behind hot Jupiters is fairly fast.

Exoplanet catalogues for WASP and Kepler saw their first triple digit identifiers, with WASP reaching WASP-100 and several Kepler planets being assigned triple digit Kepler-ID’s as well (e.g., Kepler-114, Kepler-128, Kepler-177, …).

While Kepler suffered another reaction wheel failure, effectively ending its primary mission, the year ended on a positive note with the launch of Gaia, which will likely find as many planets as Kepler, but in more intermediate period orbits and closer to the solar system.