## Revelation

Pluto as seen by New Horizons on 13 July 2015

Tomorrow is a rather big day for the exploration of the Solar System. A world that has long represented our ignorance about the outer solar system will come into splendid view, as revealed by the New Horizons spacecraft. Pluto, a dwarf planet discovered in 1930, has always been that question-mark at the end of the book about the Solar System.

I tend not to cover solar system exploration on this blog because it’s dedicated to extrasolar planet science, but I think there are some interesting parallels one can make between Pluto and extrasolar planets. Until recently, Pluto has just been an unresolved dot in the sky. Indirect methods had allowed for the mapping of crude surface features — we came to learn that Pluto has significant albedo variation across its surface, with patches of bright and dark, but other than that, we knew nothing about their composition or even what caused them. To a large extent, we still don’t. We now know (as of the past couple days) that Pluto’s north pole is covered in an ice cap dominated by nitrogen and methane, and that the dark regions are comparably methane-poor, but we don’t really understand a lot of what’s going on yet. But the advance in our knowledge of Pluto over the past month has been truly revolutionary.

In the not-too-distant future, indirect methods will begin to yield crude albedo maps of extrasolar planets. These maps may have a similar quality to those acquired for Pluto. It will be worth remembering, however, that there’s so much more about those planets that we won’t be able to see simply because we lack the ability to send a probe of some sort to those extreme distances.

New Horizons Map Comparison

The image above shows a comparison between our “best map” of Pluto before New Horizons, and what is currently our limit of knowledge (with full credit to Bjorn Johnson). Quite a difference a spacecraft visit makes! The top map is an average of five separate maps acquired by HST and ground photometry, so an important caveat here is that not only is the wavelength coverage different than New Horizons’ LORRI camera made to use the bottom map, but the mapping technique is different, too, and prone to different biases. That all being said, there is a decent amount of similarity between the two.

We may live to see an exoplanet’s surface resolved to an extent similar to Pluto’s before New Horizons, however it will be several generations before we are able to map an exoplanet with the same level of precision as the bottom map in the image above.

## 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

## 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.

## Tidal Theory

Tidal Wave (Source)

We’re used to thinking of various astronomical bodies having gravity and gravitationally interacting with each other, and it’s certainly true that they do, and that these interactions can be exploited to detect extrasolar planets through their pull on their parent stars (either by astrometry or Doppler spectroscopy). So far in our study of extrasolar planetary systems, we have treated astronomical bodies as point masses — that is, the entirety of their gravity is assumed to come from a single point, and that the entirety of their matter exists at that point. But this is, of course, an oversimplification. The consideration of astronomical bodies as extended objects introduces a range of new effects that are collectively called “tides.”

We may be familiar with the concept of the tides in the ocean, where the gravitational pull of the Moon on the oceans of Earth causes them to be raised twice a day, and so this is where we will start our consideration of tides. The gravity of Earth’s moon pulls on Earth, causing it to stretch into a more elliptical shape. A diagram illustrating this is shown below.

We see then that the rise over the side of Earth opposite the Moon is caused by the distortion of the shape of Earth. It’s important to understand that this is a very general phenomenon: It is not exclusive to the Earth-moon system, but any two-body system can be represened this way. Furthermore, while it is not shown in the above image for simplicity, the primary body (Earth in the case of the Earth-Moon system, the star in the case of a hot Jupiter system) will also raise a tide on the secondary mass.

The tidal deformation of the primary body is not an instantaneous process. There is inertia to overcome and so if the primary mass is rotating, the tidal bulge will be dragged away from the line connecting the two bodies. A diagram of this is shown below.

In the above image, we assume a prograde orbit for the satellite, and $r$ is a line joining the centre of the two bodies. Note that the rotation of the primary forces its long axis to lead ahead of the orbiting body. For a circular orbit, the long axis of the planet will lead ahead of $r$ by some constant angle, even while the planet rotates. This is why there are two lunar tides each day. Aside from causing boats to rise and descend in the water, relative to fixed points on land, these tidal effects have some important implications.

Firstly, it affects the orbits of the two bodies. In the case of the Earth-Moon system, the oceanic tidal bulge facing the Moon has a stronger gravitational attraction on the Moon than the tidal buge on the opposite hemisphere on Earth, simply because it is closer (and gravitational attraction scales inversely with $r^2$). Since the nearer tidal bulge is pushed ahead of the $r$ line, the net gravitational attraction of the Moon toward Earth is also displaced from $r$. Since the tidal bulge is pushed ahead of the Moon in its orbit, the gravitational attraction of the Moon toward Earth has a small component in the direction of its orbital motion. This causes the Moon to accelerate (it’s orbital velocity increases), which then causes its orbital altitude (the semi-major axis) to increase. Any readers casually aware of orbital mechanics is familiar with this effect. If you’re unfamiliar with orbital mechanics, a full description of it is beyond the scope of this writing, but I would encourage you to try Kerbal Space Program.

Conversely, if the tidal bulge lagged behind the $r$ line, the orbit of the Moon would decay. This can be done either by having the Moon in a retrograde orbit as is the case for Neptune’s moon Triton, or an orbit that is closer to the planet than the geosynchronous orbit radius, as is the case for Mars’ moon Phobos. Triton and Phobos are thus doomed.

Let’s define two factors (given by Dobbs-Dixon et al. 2004, Ferraz-Mello, et al. 2008), $\hat{s}$ and $\hat{p}$, as the strength of the stellar and planetary tides, respectively,

$\displaystyle \hat{s}\equiv\frac{9}{4}\frac{k_0}{Q_0}\frac{m_1}{m_0}R_0^5,\qquad\hat{p}\equiv\frac{9}{2}\frac{k_1}{Q_1}\frac{m_0}{m_1}R_1^5$

Where $k_0$ and $k_1$ is the Love number of the primary and secondary, respectively, $Q_0$ and $Q_1$ is the tidal dissipation factor of the primary and secondary (a quantity that encapsulates how a body responds to tidal deformation, but it is very hard to measure, and usually poorly known), respectively, $m_0$ and $m_1$ as the masses of the two bodies, and $R$ as their radii. The Love number quantifies the mass concentration of the body and will not be expanded on significantly here. The separation between the two bodies will change with a rate of

$\displaystyle \dot{a}=-\frac{4}{3}na^{-4}\hat{s}\left[(1+23e^2)+7e^2(\hat{p}/2\hat{s})\right]$

Where $a$ is the semi-major axis of the orbit, $e$ is the eccentricity of the orbit, and $n$ is the

Secondly, the tidal deformation of a body will be resisted by inertia, causing its rotation to slow (and its synchronous orbit radius to expand outward). This will continue until either the body’s spin period mathes its orbital period (as is the case for the Moon), or is some integer ratio of the orbital period other than 1:1 (as is the case for Mercury), as is often the case for bodies in eccentric orbits.

For fluid bodies (such as gas giant planets or stars) in circular orbits, the end result of tidal spindown will result in a perfectly synchronous rotation, via a process called tidal synchronisation. In eccentric orbits, the planet moves around the star at varying speeds, making a synchronous rotation impossible. For such planets, the end result of tidal spindown is “pseudosynchronisation,” with a planet in a pseudosynchronous rotation such that the rotation rate of the planet matches the angular velocity around the star at periapsis (where the planet moves fastest), but with the rotation of the planet being faster than the angular velocity elsewhere. In rigid bodies (such as the terrestrial planets), tidal synchronisation is achieved in the form of a spin-orbit resonance, such that the long axis of the body is aligned with $r$ at each periapsis. It doesn’t need to be the same hemisphere of the body, as we see in the case of Mercury, which presents alternating hemispheres toward the sun at each periapsis.

Thirdly, tidal deformation of a body in a non-synchronous, eccentric orbit will cause the orbital eccentricity to tend to zero. This process, called “tidal circularisation,” is responsible for the circular orbits we see for the hot Jupiter population. As a body swings around periapsis, the tidal deformation intensifies and inertia resists this motion. In addition to causing heating to the object, it also saps some of its orbital energy, causing it to slow down. As this happens at periapsis more strongly than anywhere else, the net result is that the eccentricity of the orbit tends to zero.

The rate of the change in the eccentricity is given by

$\displaystyle \dot{e}=-\frac{2}{3}nea^{-5}\hat{s}\left[9+7(\hat{p}/2\hat{s})\right]$

Tidal deformations are a significant source of heating of a form that is collectively called “tidal heating.” An extreme example of it is seen at Io, where Jupiter’s other large moons perturb Io, pumping up its eccentricity, which is subsequently damped by tidal interaction with Jupiter, causing heating, and some rather spectacular volcanism.

Understanding the influences of tides on planetary systems is a vital component of understanding their evolution, formation and habitability. Tidal interactions between moons and planets can completely melt those moons, and tidal interaction between planets can tidally heat those planets and potentially completely melt them, too. Tides can also dominate the final rotational behaviour of a planet, with effects for its climate that are still being examined.

## Slight Hiatus

Uranus

I’ve been rather distracted by various things so I haven’t had a lot of time to work on this site. Still, it is worth pointing out a fairly noteworthy find from Kepler, namely the discovery of a transiting ice giant planet with a 700-day period at Kepler-421. This planet is beyond the snowline in its system (the distance from a star where water can condense into icy planetesimals and contribute to the planet formation process) and could even have formed where we see it today. Rings and perhaps moons are a distinct possibility. With migration-dominated planetary systems being the easiest to detect, it is quite fortunate and reassuring to find a fairly “normal” planet.

## 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.

## New Toys

New Toys (source)

A major limitation to the discovery rate of extrasolar planets is the hardware available with which to detect them. Instruments like HST, Spitzer and Keck have revolutionized astronomy, and have provided a major source of extrasolar planet science — both in atmospheric characterisation and planet detection. These instruments are not dedicated to extrasolar planets, though, and time on the instruments must be shared with astronomers investigating cosmology, interstellar dust, galactic structure, and dark matter, to name a few. There is considerable interest in dedicated exoplanet science instruments – instruments that were designed to do extrasolar planet science, as opposed to instruments and spacecraft designed years ago that we have been fortunate enough to be able to torture exoplanet data out of.

One such dedicated observatory is the Automated Planet Finder, a robotic 2.4 metre telescope whose task is to search for extrasolar planets around nearby stars with a Doppler precision of ~1 m s-1. It observes ten starts a night, and will observe about a thousand stars in the solar neighbourhood. By now, the APF has been in service for a few months, but its data has already been crucial in confirming a new four-planet system of gas giants at HD 141399, and a Neptune-mass planet at GJ 687 (.pdf link) which is only 4.5 pc away.

Automated Planet Finder

The age of the dominance of Doppler spectroscopy in the discovery of new planets is clearly over. Doppler spectroscopy has fallen to second place behind transit detection, largely as a result of Kepler‘s superb performance and its discovery of over 3,000 planet candidates, as well as a change in strategy by ground-based Doppler surveys. The discovery of planets are no longer interesting for the most part, so important assets are being targeted away from large surveys of bright, metal-rich stars in the hopes of detecting intermediate- to long-period Jovian planets, and toward focoused observations of nearby stars in the hopes of detecting low-mass planets that more closely resemble our solar system. The interest now is in attaining higher precision spectrometers to observe nearby stars to try to detect very low mass planets. This takes a lot of time, and explains both why the nature of discoveries from Doppler spectroscopy has changed in recent years, as well as why the discovery cadence has trailed off. As we move away slightly from discovery for the sake of understanding the underlying planet population distribution, and toward looking for nearby targets for future follow-up, the motto is quality over quantity.

However, the outer regions of planetary systems have been largely unprobed, as Doppler surveys and transit surveys are both biased toward short-period planets. Microlensing results have shed some light on the planet population distribution at higher separation, comparable to the outer Solar System, and it seems that the abundance of gas giant planets picks up quite a bit, but this will need to be confirmed. Fortunately, new instruments are coming online that will help address these issues.

The Gemini Planet Imager on the Gemini Observatory is another new toy that has come to light. It will be used to conduct a 890 hour survey of ~600 stars from 2014 to 2016, and this January they posted first-light images, including one of β Pictoris b.

Beta Pictoris b

Another imaging instrument that is about to contribute to extrasolar planet science is SPHERE for ESO’s Very Large Telescope, capable of detecting giant planets with orbital radii >5 AU. It will observe nearby young star associations with ages of 10 – 100 Myr within 30 – 100 parsecs, as the planets of these stars will be both bright in infrared due to their age, and well separated from their star due to their proximity. All stars within 20 parsecs will also be observed, as well as stars with known long-period planets. First light is expected to be soon in 2014, perhaps as early as within the month.

A more long-term instrument of interest is ESO’s ESPRESSO spectrograph, also on the VLT. ESPRESSO will surpass the highly successful HARPS spectrograph. With a required accuracy of 10 cm s-1 (and a goal of a few cm s-1), It is expected to be sensitive to terrestrial and Earth-like planets around sun-like stars as faint as V ~ 9. First light is expected to occur in 2016.

With new planet transit search missions such as TESS and PLATO to identify transiting planets around bright stars, and improved spectrographs for mass and density determination of those planets, in addition to new direct imaging instruments and astrometry with the GAIA mission to probe the population statistics of the middle to outer regions of planetary systems, the post-Kepler era is promising to be quite an exciting, with a diverse array of complementary instruments working together to further illuminate the nature of planetary systems in the Galaxy.