Tag Archives: Kepler-36

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

2012 Review

An Earth-mass planet orbiting Alpha Centauri B. Credit:ESO

2012 brought us yet another remarkable year of extrasolar planet science. While the planet catch for 2012 was a little less than last year’s, the quality and importance of planets revealed this year was amazing. By far the most major results have been the discovery of an ~Earth-mass planetary companion orbiting the secondary component of the nearest star system to our own, Alpha Centauri (see here), and evidence for a system of planets around the nearby star Tau Ceti (see here). I hesitate to draw conclusions from a small amount of data, but the discovery of a terrestrial planet at none other than our nearest neighbour seems to really emphasize the point that terrestrial planets are likely as common as dirt.

A nice system of planets was reported at Gliese 676A consisting of super-Earths and Jovian planets, HATnet and SuperWASP produced more hot Jupiters, and interestingly, a couple sub-Earths may have been found around the nearby star Gliese 436. Spitzer provided us with the first detection of thermal radiation from a super-Earth (see here). A pair of M giants also became the first known to have planets, with planets reported around HD 208527 and HD 220074.

Circumbinary planets were announced around RR Cae, NSVS 14256825, Kepler-34 and Kepler-35 and Kepler-38, which is notable as the first Neptune-sized circumbinary planet.

Kepler results picked up en masse this year. At first it started out nice and slow, with small groups of planets being announced in batches (See here, here, here and here), followed by dozens and dozens of planets.

Interesting Kepler results included Kepler-64, the first quadruple-star system with a planet. The planet is a circumbinary planet, no less. But easily the most important circumbinary planet find was Kepler-47, the first transiting multi-planet circumbinary system. Multi-planet circumbinary systems have been found before but this is the first to have multiple planets transiting. This allows not only for their existence to be much more certain (non-transiting circumbinary planets still suffer from the mass-inclination degeneracy), but allows us to test for coplanarity. The Kepler-47 system demonstrates conclusively that short-period binary stars can host full systems of planets. Another pair of planets with very close orbits to each other, yet very dissimilar densities were reported at Kepler-36. The orbits of the planets in the Kepler-30 system were shown to be well-aligned with their host star’s equator, showing us that systems of planets are, like ours, often neatly arranged and not chaotically scattered.

Good news and bad news about the Kepler spacecraft. The good news is that the mission is extended for another three years. The bad news is that unfortunately, a reaction wheel on the Kepler spacecraft failed, and the mission’s continued usefulness now rests on all of the other reaction wheels remaining operational.

Kepler also unveiled a system of three sub-Earth planets huddled around a dim red dwarf, Kepler-42, which is very similar to Barnard’s Star, as well as a possible small terrestrial planet being evaporated away due to the heat from its star (see here). One of these three planets is Mars-sized(!).

We gained more evidence that the Galaxy is just drowning in planets both from continued Kepler results, HARPS results, and from gravitational microlensing data. Kepler showed us that hot Jupiter systems are frequently lacking in additional planets.

Last but not least, habitable planet candidates were reported around Gliese 163 and HD 40307, with unconfirmed habitable planet candidates reported at Tau Ceti and Gliese 667 C – with two more planets possibly occupying the star’s habitable zone. If GJ 667 Ce is confirmed, then it would be the most promising habitable zone candidate to date, based on its low mass.

At the end of 2011, I gave some wild guesses as to how the extrasolar planet landscape would look like at the end of 2012. Here we are and how have those predictions held up?

The Extrasolar Planets Encyclopaedia lists 854 planets as of the time of this writing, however it is missing quite a few. My own count has us at 899 planets.

  • The discovery of a ring system around a transiting planet

There are hints of ring systems (or perhaps rather circumplanetary disk systems) around Fomalhaut b, β Pictoris b, and 1SWASP J140747.93-394542.6 b (see here) but none of these are confirmed. So I’m calling it a missed prediction.

  • More low-mass planets in the habitable zone from both radial velocity and transit

Two new habitable planet candidates from radial velocity, none from transit.

  • Confirmation of obvious extrasolar planet atmospheric variability (cloud rotations, etc).

I was counting on continued monitoring of the HR 8799 planets to search for atmospheric variability, but it simply didn’t happen (or rather, if it did happen, the results are still pending). So I’m calling this a miss.

2013 could be a very interesting year, especially for Kepler. It seems we are on the verge of finding a true Earth analogue. The detection rate of candidate habitable planets is picking up and we’re really starting to get a list of targets to follow-up in the next decade. Here’s some more brave guesses for the end of 2013:

  • 1200 Confirmed planets and planet candidates
  • A satellite of an extrasolar planet (an “exomoon”)
  • A confirmed ring system around an extrasolar planet
  • Phase curve mapping of a sub-Jovian planet

2012 Planets