## 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

## An Earth-Sized Habitable Exoplanet Candidate

Kepler-186f (Artist rendering)

A Major milestone was announced Thursday when NASA unveiled Kepler-186 f, a new habitable planet candidate that is, without a doubt, the most Earth-like extrasolar planet known. This planet could legitimately be habitable. Kepler-186 f is one of five known planets orbiting an M1V star with half the radius and mass of Sol. The planet itself is only 1.11 ± 0.14 RE, which suggests that the planet is probably not a mini-Neptune, however we don’t know its mass. If it is composed entirely of volatiles, it has a mass of 0.32 ME, or on the other extreme, if the planet has a pure iron composition, then it has a mass of 3.77 ME. An Earth-like composition places the mass of the planet at 1.44 ME. The planet is probably on the denser end of this, as a low-density planet of this size would probably not have survived the high-XUV stage of the M dwarf’s youth. The other four planets in the system are all less than 1.4 RE, and are likely terrestrial themselves.

The planet gets ~32% of the insolation from its star that Earth gets from ours, which seems a bit on the low end, but there are a couple factors to keep in mind. Because M dwarfs are redder, and because atmospheres scatter blue light, an Earth-like atmosphere for Kepler-186 f would scatter a lower fraction of starlight than Earth’s atmosphere. The insolation from the M dwarf, by virtue of its redder spectrum, gives more heating to a planet than for the same insolation from a G type star. Furthermore, for an Earth-like composition, the planet has a slightly higher mass and could therefore attract a thicker atmosphere, providing more greenhouse heating.

The Planetary Habitability Laboratory lists Kepler-186 f at a rather dismal 17th out of 21. It’s curious they would rank it so pitifully. All of the habitable planet candidates listed as being more Earth-like, with perhaps the exception of Kepler-64 f, are likely low-density mini-Neptunes. Clearly the PHL habitability ranking algorithm needs to be revised. Furthermore, some of the planets listed aren’t even known to exist. Gliese 581 g itself has been effectively disproven.

PHL

It’s still possible to imagine a feasibly attainable next step: an Earth-sized planet in the habitable zone of a sun-like star — a true Earth analogue. But this is definitely a remarkable discovery and one that probably won’t seem to be outside the realm of habitability when those future discoveries do come about.

At the risk of sounding pessimistic, this may be the only known terrestrial habitable exoplanet candidate we know of. Kepler-64 f could still be a mini-Neptune. Gone are the days where a 5 Earth-mass planet receiving similar insolation as Venus can stir the imagination with the prospects of luscious fields of green, with kittens playfully swatting at butterflies. As the time approaches where we begin to focus our attention on characterising the atmospheres of habitable planet candidates and searching their spectra for biospheres, we will have to prioritise the planets we look at. The overwhelming majority of the planets currently making people’s “habitable planet candidate” list simply aren’t going to be on the receiving end of that kind of attention. The discovery of planets like Kepler-186 f in the solar neighbourhood is the major next step for searching for an extrasolar biosphere.

## Staying Relevant

Mildly out-of-date computer.

It has been nearly 20 years since the discovery of the planet orbiting 51 Pegasi. What followed over the rest of the late 90s were the landmark discoveries of the first eccentric giant planets at 16 Cygni B, and 70 Vir, and the first two-planet system at 47 Ursae Majoris. As new discoveries are made that push the boundary of what is known, prior ones fade into distant memory.

The public interest in these objects also varies with time. It seems odd to think it today, but in the early 1800s, 61 Cygni was wildly more popular than Alpha Centauri. This was merely because at the time, only the former’s distance had been measured, but there does seem to be a correlation between the public interest in an object and its scientific importance. Consider for example three landmark discoveries, the first planet orbiting a sun-like star, the first confirmed brown dwarf, and the first known transiting planet (with stellar hosts 51 Pegasi, Gliese 229 and HD 209458, respectively).

Trends of interest in three landmark discoveries

51 Pegasi becomes wildly famous, and rightfully so being the first of its kind known. Even today most people with a casual interest in astronomy know why 51 Pegasi is important. Gliese 229 has never really reached the prestige of 51 Pegasi — brown dwarfs just aren’t as exciting, and as time went on, interest faded. What started out as just another hot Jupiter became the most important when it was found to transit, and interest in it has continuously increased over the timeframe allowable to me by Google Ngrams.

As time went on, new planets stopped grabbing people’s attention unless they were set apart by some level of spectacularity. From memory alone, what do you know about the planet HD 290327 b? If you’re like me, absolutely nothing. Still, over time new planets and planetary systems were announced that were genuinely interesting. At the turn of the century, the first super-Earths at Gliese 876 and 55 Cancri held our attention for a while, followed by our first transiting Neptune-mass planet at Gliese 436. HD 69830 and HD 40307 gave us our first multi-planet systems made up of sub-Jovians in the mid-to-late 2000s. CoRoT broke ground with the first transiting super-Earth at the end of the decade and a multi-planet system was imaged at HR 8799.

Throughout this evolution of the kinds of things that have kept our attention, it is truly remarkable to pause and realise how numb we seem to have become to some discoveries. The discovery of Earth-sized planets now occurrs so often that it does not even raise an eyebrow anymore. The time between when a type of discovery goes from immensely exciting to just-another-day-at-arXiv seems to be only on the order of a couple years or so. It almost appears that there seems to be a sort of Moore’s Law at hand for extrasolar planet discoveries as there is with computers.

Earlier this month, the Kepler team made public about 700 new planets. Keep in mind we only just recently achieved a total of a thousand known planets. Now we’re knocking on the door of two thousand known planets. These planets are all in multi-planet systems, which is the foundation of the statistical argument used to validate their existence — a single transiting planet candidate can be any number of false positives, but having multiple candidates in a system is much harder to emulate by a non-planetary phenomenon. Many of the planets are Earth-sized and super-Earth sized, with considerable gains in transiting Neptune-sized planets.

New Kepler Planets

To further drive home the point, among the new Kepler planets are four new habitable planet candidates (at Kepler-174, Kepler-296, Kepler-298 and Kepler-309). At least that’s what they’re being called — it is my assertion that their radii are much more consistent with being low-mass, low-density “mini-Neptunes” or “micro-Jovians.” The combined interest in these four new habitable zone planets is less than half the public interest in Kepler-22 b, for example.

Much closer to home, RV studies on M dwarf stars have yielded eight new planets in the solar neighbourhood, and constrained the frequency of planets around M dwarf stars.

According to our results, M dwarfs are hosts to an abundance of low-mass planets and the occurrence rate of planets less massive than 10 M⊕ is of the order of one planet per star, possibly even greater. …

They, too, report new habitable planet candidates, but their minimum masses are, again, consistent more with being more closely reminiscent of Neptune than Earth. Regardless, it is my opinion that this is actually more interesting than the 700 new planets from Kepler. By now, we know that planets are common. The Galaxy is drowning in planets and while new planets are great for population statistics, individual planet discoveries don’t count for anywhere near what they used to. We are moving from an era of having the attention and focus on planet detection and discovery to an era of planet characterisation. We’re hungry for planets that are actually accessible to HST, Spitzer, Keck and soon(-ish) JWST for transmission spectroscopy and eclipse photometry. New planet discoveries in the solar neighbourhood count for far more than Kepler planets because the nearby planets are the ones that we have a shot at studying in-detail from direct imaging in the near future.

They also report the existence of a Neptune-mass planet in a fairly circular, 400-day orbit around Gliese 229, bringing perhaps a little more relevance and attention to a star that saw its moment of fame twenty years ago.

## The Kepler Spacecraft

The Kepler spacecraft attached to its booster stage on a Delta II rocket

It’s the biggest thing in extrasolar planets right now so I figured it deserves an obligatory post. I would like to give more attention to instrumentation, techniques, and technology in these entries, and a post on the Kepler spacecraft seems a wonderful way to start.

The spacecraft was launched into a Heliocentric orbit on March 7, 2009 on top of a Delta II rocket. Compared to some other spacecrafts, Kepler is rather simple in design and purpose. It’s essentially a dedicated photometer attached to a 0.95 metre telescope operating in visible light (more specifically, from 400 – 865 nm wavelengths). It observes nearly 150,000 main sequence stars continuously, using the transit method to discover extrasolar planet candidates (see here for a description of how planets are found this way). It has uncovered thousands of planet candidates using this method. Light enters into the front of the telescope, bounces off the primary mirror at the back, and is focoused onto the CCDs in the middle of the telescope body. These CCDs measure the brightness of each star every 29.4 minutes for most of the stars, but some special target stars get high-cadence observations, with measurements being taken once every minute.

Kepler's CCD

The CCD (imaged above) is what does all the magick. One of the squares malfunctioned and no longer works, but beside that and some trouble with the spacecraft going into safe mode and resetting early on in the mission, everything continues to go well as of this writing.

What the spacecraft ends up seeing is the following:

The Kepler Field of View

Kepler sees this, all day, every day. Except for the seasonal 90° roll to keep the solar arrays aimed at the sun, this field of view does not change. The image appears to be mostly hazy, but upon closer inspection, it’s actually comprised of an obscene quantity of stars (see the full image here, but careful for it is a large image).

Despite the simplicity in its design and purpose, Kepler is revolutionising the field of astrophysics and extrasolar planets. Contributions of Kepler to astronomy include studying active galactic nuclei, finding additional Hot Jupiters, performed asteroseismology on red giant stars, studied the central stars of planetary nebulae, discovered more eclipsing binary stars, finding the first transiting giant planet around a limb-darkened star and constraining its spin-orbit alignment, performed asteroseismology on white dwarfs, performed asteroseismology on sun-like stars, studying red giant granulation, finding a class of bloated white dwarfs, measuring the frequency of terrestrial planets around sun-like stars, permitting the public to discover their own planet candidates, discovering a non-transiting planet through transit timing variations for the first time, improving our knowledge of RR Lyr stars, studying stars that tidally affect each other, studying stars in open clusters, measuring giant planet reflectivity, studying sdB star pulsation behaviour, studying B-type stars and roAp stars, and work is progressing toward other fields, including discovering extrasolar moons.

Kepler's Most Crowned Achievements: Low-Mass Planets

The mission was designed to last 3.5 years. The need for at least three years is due to the requirement to detect three transits of a planet to confirm its candidacy. One transit reveals the planet’s existence. The second transit constrains its orbital period. The third transit confirms the orbital period. While this need for a third transit might seem redundant, consider the case of two similarly-sized planets transiting a single star. One can see how one transit of both planets may be confused for two transits of one planet. A planet at 1 AU from a solar-like star will have a period of about 1 year (like, for example, Earth). Therefore, we expect all extrasolar Earth clones to transit roughly once a year. And therefore three years of observations are required to confirm the planet as a candidate.

However Kepler found that sun-like stars are more photometrically variable than expected. It turns out our star is a bit quieter for its type. What this means is that there is more noise in the data, and the transits do not stand out as much. multiple transits must now be stacked to get more data to confirm the planet. The punch line is that for Kepler to get a complete measurement for the frequency of Earth-sized planets in the habitable zones of solar-type stars (so called$\eta_{\oplus}$or “eta Earth”), Kepler‘s mission must get extended to six years. Kepler has the fuel onboard to do this, but the funding has not been secured. The topic is a discussion for later…