## 2014 Review

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.

## Habitable Value

Planets at Kapteyn’s Star (source)

Since the last post on this blog, there have been two additional habitable planet candidates announced. First, a two-planet system orbiting the very nearby, very old red dwarf Kapteyn’s Star was reported by Anglada-Escudé et al. The inner planet, the habitable zone world, at 4.8+0.9-1.0 ME is probably a mini-Neptune or micro-Jovian planet, based on its mass — the overwhelming majority of planets of this mass whose radii are known are clearly low-density worlds. The outer world is a cold super-Earth, and probably the same type of planet. Kapteyn’s Star is a member of the Galactic halo, and is quite ancient at ~11 Gyr old. The apparent fact that the Universe was assembling habitable planets when it was less than 3 Gyr old may have interesting implications for the Fermi Paradox, but I won’t go into that here.

Next is Gliese 832 c. In 2008, Bailey et al. reported the presence of a Jupiter-analogue orbiting the nearby red dwarf system GJ 832, and then last week, we learned of second planet, a super-Earth type planet straddling the inner edge of the habitable zone, reported by Wittenmyer, et al. It is almost certain that this planet is not habitable, certainly not to life “as we know it.” The planet’s mass comes in at 5.4±1.0 ME, and therefore likely a mini-Netune / micro-Jovian, much like Kapteyn’s Star b.

Then wandering through the news as I do on a daily basis, I found this

Note the description of the planet as “among the most habitable,” with artist images depicting oceans, lush green land, and so on, despite the description of the planet in the discovery paper as

However, given the large mass of the planet, it seems likely that it would possess a massive atmosphere, which may well render the planet inhospitable. Indeed, it is perhaps more likely that GJ 832c is a “super-Venus,” featuring significant greenhouse forcing.

And this was being generous! I personally thought the discovery of planets at Kapteyn’s Star was much more interesting than the discovery of GJ 832 c, but apparently news cycles have a different standard than I do as to what amounts to an interesting world. That standard, with respect to exoplanet discoveries, is the Earth Similarity Index (ESI) that the Planetary Habitability Laboratory uses to evaluate a planet’s habitability. A quick look at their site shows that, sure enough, GJ 832 c is the third most highly ranked exoplanet.

This is not the first time I have complained about the PHL. But this time I will instead work on providing an alternative method of evaluating a planet’s habitability. A child could look at the above diagram and tell you Kepler-186f was the most “Earth-like” of those planets based on their appearance, but to be rigorous and useful, we need a system to quantify a planet’s habitability. Let’s first look at how the ESI is determined.

$\displaystyle ESI=\prod_{i=1}^n\left(1-\left|\frac{x_i-x_{i_0}}{x_i+x_{i_0}}\right|\right)^\frac{w_i}{n}$

Where $x_i$ is the n-th property of the planet — in this case, either radius, density, escape velocity or surface temperature — $x_{i_0}$ is the value of this property for Earth, and $w_i$ is the weight exponent of a property. For the parameters usually used by the ESI, these values are

 Property Reference Weight Exponent Radius 1 $R_\oplus$ 0.57 Density 1 $\rho_\oplus$ 1.07 Escape Velocity 1 $V_{e_\oplus}$ 0.70 Surface Temperature 288 K 5.58

The formula I will use to evaluate the habitability of an exoplanet will be rather anthropocentric – for all I know, solid, hot super-Earth-type planets like Kepler-10 b may be the most frequently inhabited planets in the Galaxy, but all I know of is Earth-life, and so this formula will be centered around finding Earth-like life. It will effectively be based on Guassian distributions, and will take the form

$\displaystyle H = \prod_{i=1}^4 \frac{1}{\sigma\sqrt{2\pi}}\exp\left(-\frac{(x_i-\mu)^2}{2\sigma^2}\right)$

Here, μ acts as a reference value much as in the ESI formula, σ describes the broadening of the distribution and will effectively be used to determine the tolerance of variation on a particular parameter, and $x_i$ is the parameter we look at. As the product sign suggests, we calculate this for each of four parameters and multiply the results. Here, the four parameters are the stellar temperature, planet mass, planet radius, and planetary insolation.

For the stellar temperature, I chose σ=0.001 and μ=5500, which is some 277 K cooler than our sun. It seems that early K dwarfs are probably a sort of “sweet spot” for planet habitability. As such, if you found an Earth-analogue around an early K dwarf, it would rank higher on this scale than Earth itself. For the planetary mass and radii, I chose μ=1.0 for obvious reasons, and chose σ=5 and σ=0.75, respectively — punishing radius pretty heavily. Lastly, I chose insolation values of μ=1 and σ=1. All values of σ are in terms of that of Earth. Lastly, the values were normalised to make 1 the highest achievable value.

Unsurprisingly, the Solar System is the clear winner, followed by Kepler-186 f, which I made a big deal about earlier this year. The GJ 581 system, which was celebrated as hosting the first habitable planet candidates in the latter years of the last decade, doesn’t even make it up to 10-5, nor does GJ 832 c.

 Planet H Earth 0.96635 Venus 0.61220 Kepler-186 f 0.18525 Kepler-62 f 0.09104 Mars 0.04304 Kepler-62 e 0.00530 Kepler-283 c 0.00005 Kepler-296 Af 0.00003

I would say this set-up makes a lot more sense than the one the PHL is using. Anything below 0.1 is probably not worth a raised eyebrow these days.