Tag Archives: HIP 116454

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

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2014 Review

Comets

Comets orbiting β Pic (Credit: ESO)

Results from Kepler data continued to stream in. We were graced with the discovery of a transiting Uranus-sized planet in a 704-day orbit. This is the first time a transiting planet has been discovered beyond its system’s ice line, and this may even be where the planet formed, instead of the typical case for transiting planets where we see them were they are today because of extensive inward migration in their past. Kepler’s 10th transiting circumbinary planet was reported, Kepler’s second multi-planet system orbiting a sdB star was announced and Kepler’s second disintegrating short-period planet was identified.
Kepler short-cadence data may have allowed for the detection of a non-spherical (oblate, like Saturn) exoplanet for the first time, by looking at photometry for Kepler-39 b. While the planet itself is unremarkable, a transiting sub-Neptune around HIP 116454 marks an exciting development: The first planet discovered by Kepler during it’s new “K2” mission.

The dramatic turn of events in the investigation of the planetary system at GJ 581 seems to have finally come to a conclusion. In 2007 a media frenzy accompanied the discovery of two new planets found to accompany the known 5-day Neptune. The innermost of the two new planets was a 5 Earth-mass planet in a 12-day orbit, and a 7 Earth-mass planet in a 80-day orbit. The 12-day planet was hailed as the first habitable planet candidate, despite getting more stellar insolation than Venus. In 2009, an Earth-mass planet at 3-days was found, and the new dataset brought the orbit of the 80-day planet closer to the star, down to ~60 days. Since most people had come to their senses regarding the habitability of the 12-day planet, the 60-day planet became the flag-bearer for habitability in the GJ 581 system. Everything changed in late 2010 when a 3 Earth-mass planet in the habitable zone was announced by the HARPS team. This was truly the most habitable exoplanet candidate known to date… if it were real. Multiple studies drawn out over the following several years debated back and forth how many planets exist around GJ 581. Three? Four? Five? Six? It depended on how you merged your datasets, how you handled noise in your data, and so on. This year, the issue seems to have been resolved by carefully examining the H-alpha lines of the star’s spectrum and finding variation in them that had affected the stellar radial velocity measurements. When corrected for, suddenly all of the habitable zone planets vanished. As of the end of 2014, there are no habitable planet candidates orbiting the star that only a few years ago was unanymously acknowledged to be the most promising extrasolar planetary system for habitability. Interestingly, a similar story may be unfolding at GJ 667, where some planet signals have turned up missing in other studies of the RV data. The morale of this story? Finding low-mass planets is hard.

HARPS continued to give us new planets, but the pace seemed somewhat slow. Some interesting results are the presence of planets in the cluster M67 (link). Planets were found orbiting the stellar companion to XO-2, as well, making the system an example of a binary system with planets orbiting both components. Interestingly, both host at least two planets, and one of each of them are a hot Jupiter. The hot Jupiter around the other star doesn’t transit, suggesting some misalignment between the two planetary systems. Other HARPS results included the identification of two families of comets around β Pictoris, with the identification of hundreds of individual comets through via transmission spectroscopy through their tails.

The news media nearly peed their pants with excitement over the discovery of a planet that, most optimistically, isn’t habitable now, nor has it ever been, in the habitable zone of the very nearby Kapteyn’s Star. Similar unwarranted attention was given to a mini-Neptune discovered in the habitable zone of GJ 832. While the excitement that planets like these get is unwarranted, it is at least gratifying to see the general public so interested in planets that vaguely resemble something habitable-ish. I just hope that when we do find more planets that are more like Earth, the interest hasn’t faded away already. HIRES also found a super-Earth orbiting the very nearby star GJ 15.

We saw first results from the Automated Planet Finder, a radial velocity based system to look for planets in the solar neighbourhood. The project gave us a nice four-planet system orbiting HD 141399 and aided in the discovery of a Neptune-mass planet at GJ 687. We also got to see first results from ESO’s SPHERE instrument.

A number of naked-eye stars were found to have planets, as well, such as planets around β Cnc, μ Leo, β UMi and one around σ Per.

On the direct imaging, we got a new planet imaged orbiting GU Psc with a mass ratio of ~30 and a separation of ~2000 AU… which seems more like a low-mass binary star than a true planetary system. Also, while not planets, ALMA observations of HL Tau have revealed increible detail showing planet formation carving out gaps in the circumstellar disk.

Microlensing gave us a hand-full of planets this year, among the most interesting is OGLE-2013-BLG-0341L Bb, a terrestrial-mass planet orbiting the secondary component of a binary system with a projected separation of 15 AU. For the microlensing event OGLE-2014-BLG-0124L, the event was observed both from the ground and from Spitzer, making the first joint ground+space detection of an exoplanet microlensing event. Observing the event from two different perspectives allowed for the distance to the lens to be accurately measured.

In early year-review posts, the hints of a planetary ring system around a planet orbiting 1SWASP J140747.93-394542.6 has been discussed. We now find that radial velocity observations of the star have place limits to the mass of the orbiting body, restricting it to a planet or brown dwarf. The rings do exist, and they extend far enough away from the planet to expect them to form moons.

2014 has been an interesting year. A thousand planets have been reported this year, mostly because of Kepler’s enormous contributions. It’s hard to know what 2015 will bring. RV surveys are continuing, and there are still thousands of Kepler candidates awaiting confirmation.