An Earth-Sized Habitable Exoplanet Candidate

Kepler-186f

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

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.

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

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

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.

Orbit Projections in the Sky

Projection

Projection (Source)

Whether you are determining the barycentric motion of a star with µas precision to determine the orbit and mass of an unseen planetary companion, or directly observing planetary companions orbiting dozens of AU from their stars over the course of some decent fraction of a decade to pin down their orbits, you are basically trying to determine the motion of an object in the sky. While the orbit of the object is three-dimensional, the path it takes in the sky is not (at least not for monitoring nearby objects, especially in the solar system). An integral part of binary star astrometry is therefore projecting a 3D orbit onto a 2D plane, and fitting data to that observed 2D orbit. Since a star plus planet system is physically analogous to a binary star with a high mass ratio, the mathematics ends up being the same.

With our focus thus far in this blog on Doppler spectroscopy and transit photometry, the orbital parameters we have concerned ourselves with are the semi-major axis, a, the eccentricity of the orbit e, the inclination of the orbit, i, and the longitude of periapsis ω, which define the size of the orbit, the deviation from circular, the angle between the orbit plane and the plane of the sky, and the angle of the periapsis of the orbit from the observer, respectively. For the longitude of periapsis, ω=0° is defined in such a way that the line connecting the star to the periapsis of the orbit is perpendicular to the line of sight — the Earth-star-planet angle is a right angle. ω=90° after a rotation of the orbit 90° in the orbit plane in such a way to where if the inclination of the orbit is 90°, the periapsis would be between Earth and the star, and the transit midpoint would occur at periapsis.

Longitude of Periapsis

Changing the longitude of periapsis rotates the orbit in the orbit plane

We have virtually ignored the ascending node, Ω, which defines the rotation of the orbit plane around the line of sight. For some inclination near 90°, when Ω=0°, the planet orbits “up-and-down” and when Ω=90°, the planet orbits “left-and-right.” Note that this does not necessarily mean a polar or equatorial orbit around the star, as in this discussion, we are agnostic about the orientation of the stellar rotation axis. The ascending node of a planetary orbit has been mostly ignored in this blog because it does not actually affect either the Doppler behaviour of the star, or for the most part the shape of the transit light curve. Technically, with all else held constant, varying Ω for a transiting planet will affect the projected angle between the star spin axis and the planet orbital axis, λ, which is detectable as the Rossiter-McLaughlin effect with Doppler spectroscopy (See here). However, since we do not know the orientation of the stellar spin axis in space, we aren’t able to fit Ω as an adjustable parameter to the data.

Ascending Node

Changing the ascending node rotates the orbit around the line of sight

Plotting 3D orbits as they appear on the (locally) two-dimensional plane of the sky to determine it’s orbit is the astrometric equivalent of calculating the radial velocity behaviour of a star. We input orbital parameters into a model and try to fit the data to that model to assess how well it approximates reality.

First, we will need to define a couple of functions that address the orbital motion of the object in the x and y dimensions in the sky, let’s call these functions f_x(t) and f_y(t).

\displaystyle f_x(t)=\cos E - e \\ f_y(t)=\sqrt{1-e^2} \sin E

where e is the eccentricity of the orbit, t is the time of calculation, and E is the eccentric anomaly as derived and calculated in the same way as in the Doppler spectroscopy method discussed here.

Now we will need to bring in transformations to describe the deprojection of the 3D ellipse into a 2D plane. These take the form of the Theile-Innes constants:

\displaystyle A=a(\cos\omega\cos\Omega-\sin\omega\sin\Omega\cos i)\\ B=a(\cos\omega\sin\Omega+\sin\omega\cos\Omega\cos i)\\ F=a(-\sin\omega\cos\Omega-\cos\omega\sin\Omega\cos i)\\ G=a(-\sin\omega\sin\Omega+\cos\omega\cos\Omega\cos i)

Where a is the semi-major axis of the orbit, ω is the longitude of periapsis, Ω is the ascending node and i is the inclination of the orbit. We now combine the two to get the position of the object in the x- and y- axes as a function of time:

\displaystyle x(t)=B f_x(t)+Gf_y(t)\\y(t)=Af_x(t)+Ff_y(t)

We are now in a position to evaluate the goodness-of-fit of a given model to data using essentially the same statistical tools and techniques described in this post. The observed-minus-calculated for an astrometric position is in this case of course determined with Pythagorean Theorem, O-C=\sqrt{\Delta x^2 + \Delta y^2}. As an example, here are the positions of a star in a 15 year orbit around the supermassive black hole in the centre of the Galaxy.

Star orbiting SgrA*

Star orbiting SgrA* (source)

2013 Review

HD 106906

HD 106906 b, a directly imaged planet announced in 2013

First, foremost, and perhaps most painfully: α Centauri Bb may not really exist. What we thought was the Keplerian signal of an Earth-mass planet at our nearest neighbouring system may actually be noise in the data. While a bit painful, this is how science works – claims are rigorously tested and beaten tirelessly until they either continue to stand on the merit of the evidence, or they are refuted and disproved. This is how we keep the muck out of our pool of knowledge. Stay tuned… this could take a while to fully resolve.

The year began with direct imaging news: A new HST detection of Fomalhaut b (see here), suggesting the “planet” orbit is either not coplanar with the system disk or crosses the ring orbit and has a much lower mass than initially suspected. The imaged planet around β Pic b has also been independently confirmed. A circumbinary planet at 2MASS J01033563-5515561 became the first to be directly imaged. A planet with a mass of ~4 MJ became the lowest-mass planet directly imaged at HD 95086 (with the caveat that it isn’t clear what the nature of Fomalhaut b is). A planet perhaps of similar mass was later reported at GJ 504 (59 Vir).

Habitable zone discoveries started with the first known transiting Jupiter-sized planet in the habitable zone, PH2 b. Then things got very interesting with the simultaneous announcements of a super-Earth straddling the inner edge of the habitable zone of Kepler-69, and two habitable planet candidates at Kepler-62, which was covered here. HARPS found a nice system of planets around the M dwarf GJ 163. One of the planets is somewhat near the habitable zone, but it is my position that this planet does not deserve the attention worthy of a habitable planet candidate, with the planet receiving 40% more irradiation than Earth, and with the host star being an M-type dwarf, the atmosphere will not provide as much scattering of irradiation as Earth’s (Rayleigh scattering is increasingly efficient with decreasing wavelength), causing the surface of the planet to actually receive more than 40% more irradiation than Earth. Despite this, HARPS did provide us with another potentially exciting habitability result, with no less than three super-Earths in the habitable zone of GJ 667 C, with evidence for at least six, perhaps seven total planets there, however a reanalysis of the RV data seems to suggest that these new planets do not exist. Stay tuned…

Other noteworthy announcements included DW Lyn b, a giant planet orbiting a pulsating subdwarf B-type star. A hot Jupiter was also found orbiting a late-K/early-M dwarf by SuperWASP – a particularly rare find. A pair of super-Earths were found in a 2:3 resonance at HD 41248. A giant planet was found in a close orbit around a red giant branch star. Evidence of a second planet accompanying a newly discovered debris disk was presented for κ CrB. A super-Earth around HD 97658 was reported to be transiting (as was suspected two years ago but later dismissed due to a non-detection). The pair of planets at HIP 11952 ended up not existing – an error in compensating for the radial velocity of the observing site relative to the star.

Kepler results continued to stream in, starting with a rather interesting three-planet system at Kepler-68, with a mini-Neptune closest to the star, then an Earth-sized planet just outward of that, and a Jovian planet in a long-period orbit. It was shown that systems of multiple, low-mass planets uncovered by Kepler, like our own solar system, have orbits that are well-aligned with their host star’s equator (see here and here). Kepler results also uncovered a system with a pair of planets in a 2:1 resonance producing very strong transit timing and transit duration variations. A hot Jupiter at Kepler-76 provided strong evidence of super-rotation in the atmosphere via its secondary eclipse visible light photometry. Of particular note is the announcement of a planet smaller than Ganymede(!) at Kepler-37. A new population of small, rocky worlds in extremely short orbits was uncovered by Kepler, specifically Kepler-78 b wih its 8.5 hour orbit and KOI-1843.03 with its 4.2 hour orbit(!). Furthermore, Kepler unveiled the first transiting planets in an open cluster, NGC 6811.

Of particular note is the discovery of a transiting hot Jupiter orbiting a young, oblate, gravity-darkened T Tauri star. This remarkable system seems to imply that the formation mechanism behind hot Jupiters is fairly fast.

Exoplanet catalogues for WASP and Kepler saw their first triple digit identifiers, with WASP reaching WASP-100 and several Kepler planets being assigned triple digit Kepler-ID’s as well (e.g., Kepler-114, Kepler-128, Kepler-177, …).

While Kepler suffered another reaction wheel failure, effectively ending its primary mission, the year ended on a positive note with the launch of Gaia, which will likely find as many planets as Kepler, but in more intermediate period orbits and closer to the solar system.

Gaia Launches Successfully

Gaia launch

Gaia launch

At 6:12 am local time on 19 December, 2013, a Soyuz rocket carrying ESA’s Gaia spacecraft launched from French Guiana. Gaia was successfully sent into a solar orbit, at the Earth-Sun L2 point.

And so begins a five year mission to map the Galaxy. It’s hard to know for sure what discoveries Gaia will make, but it is clear that we will learn a great deal about the Galaxy.

Mapping Extrasolar Planets III. Visible Light Surface Features from Kepler

Kepler-7b

Kepler-7b

In our previous looks at mapping extrasolar planets, we have focused on spatial variations in infrared brightness, and thus temperature, on the visible surface of extrasolar gas giant planets. The reason for this bias toward longer wavelengths is two-fold: 1) At the time, Spitzer phase curve photometry has been the dominant means of deriving crude longitudinal maps of extrasolar planets. 2) The ratio between the flux from the star and the planet is an order of magnitude less in infrared than it is in visible light – also the reason that the successes from direct imaging of extrasolar planets have been almost exclusively in infrared (the nature of the object imaged at Fomalhaut by HST in visible is unknown). Short-period extrasolar planets are thus prime candidates for secondary eclipse observations in the infrared. The following table lists several prominent planets and the eclipse depths as measured in four infrared channels by Spitzer, known as the IRAC channels (Infrared Array Camera) which have been a cornerstone of the building up of the foundation of our understanding about extrasolar planets in the past decade.

Spitzer IRAC Eclipse Depths
Planet 3.6µm 4.5µm 5.8µm 8.0µm
HAT-P-7 Ab 0.098% 0.159% 0.245% 0.225%
Kepler-5 b 0.103% 0.107%
Kepler-6 b 0.069% 0.151%
HD 189733 b 0.256% 0.214% 0.310% 0.391%
HD 209458 b 0.094% 0.213% 0.301% 0.240%

Of course the eclipse depth of a planet depends on its intrinsic brightness at the observed wavelength, the brightness of the star itself, and the radius ratio between the two, and given the comparitive brightness and size of a star, it is not hard to see why the drop in flux from the system during the planet’s eclipse behind the star would be so miniscule, < 1%.

The difficulty involved is exaggerated in visible wavelengths, where the flux is dominated less by thermal emission of the two bodies and more by intrinsic processes within the star and how reflective the planet is of the star’s light. Here are some secondary eclipse depth measurements from Kepler, which observes in a filter that is approximately visible light (0.400 – 0.865µm).

Kepler Eclipse Depths
Planet ΔF
HAT-P-7 Ab 0.0069%
Kepler-5 b 0.0021%
Kepler-6 b 0.0022%
Kepler-7 b 0.0042%
Kepler-12 b 0.0031%

Notice that the eclipse depths in visible light are much lower than the eclipse measurements in infrared. For planets with both Spitzer (infrared) and Kepler (visible light) eclipse depth measurements, the contrast is clear: the star-planet brightness ratio is less in infrared than it is in visible light and therefore hot Jupiters are essentially relatively brighter in infrared than in visible.

This does not exclude extrasolar planets from visible light mapping using the phase curve analysis and eclipse scanning techniques described before, it only makes it harder. Optical phase curves are weaker than infrared phase curves, so either a bigger telescope is needed to observe with enough photometric precision to resolve the phase curve in one orbit, or several orbits must be observed with existing instrumentation and allow the data to build up until a phase curve can be resolved.

Fortunately, Kepler has offered quasi-continuous coverage of the transits of many hot Jupiters over the course of four years, and in some cases, it is enough to confidently detect a phase curve. One such planet is Kepler-7 b.

Kepler-7b Phase_Curve

Kepler-7b Phase_Curve

In this image, the green curve corresponds to the expected phase curve if the planet reflected light in a geometrically symmetric way. The red and blue curves are fitted to the data and incorporate a longitudinal offset (see Demory et al for details). The primary transit is on the left side, and the planet passes behind the star on the left side (the secondary eclipse). The depth of the primary transit is so deep on this scale that it is off the image. Notice that the phase curve is not perfectly phased to the orbit of the planet – the secondary eclipse does not occur at the peak apparent brightness. This would be comparable to the Moon being brightest not when it is full, but rather at a gibbous phase. Keep in mind that this is an optical phase curve, the brightness variations on the planet’s dayside hemisphere suggested by the phase curve corresponds to actual features you would be able to see with the human eye (and maybe a welding helmet).

The Kepler-7 b phase curve shows us that the brightest part of the planet’s dayside atmosphere is on the westward side of the dayside hemisphere. Because this phase curve is an accumulation of 14 quarters of Kepler data, it is best thought of as an “average” phase curve over the course of 3.5 years, and therefore the longitudinally resolved visible light map of the planet is an average of the planet’s surface brightness over 3.5 years. Since the planet does not contribute to the phase curve of the system during secondary eclipse, the scatter of the data during that secondary eclipse is a good representative of the overall data scatter. It may come from instrumental noise or stellar noise, but whatever its origin, the fact that it is not obviously different from the scatter in the phase curve when the planet’s brightness is contributing implies that the surface features resolved here are both stable and long-lived.

Kepler-7b Visible Map

Kepler-7b longitudinal brightness distribution

What could be the cause of this bright area on the planet? Demory et al explain:

Kepler-7b may be relatively more likely to show the effects of cloud opacity than other hot Jupiters. The planet’s incident flux level is such that model profiles cross silicate condensation curves in the upper, observable atmosphere, making these clouds a possible explanation. The same would not be true for warmer planets (where temperatures would be too hot for dayside clouds) or for cooler planets (where silicates would only be present in the deep, unobservable atmosphere). Furthermore, the planet’s very low surface gravity may play an important role in hampering sedimentation of particles out of the atmosphere.

Now how do we know that this bright spot is not simply due to thermal emission? Some hot Jupiters are sufficiently hot that the glow from their heat in visible light can affect, or even dominate their eclipse depths. The obvious answer would be to check Kepler-7 b’s eclipse depths in the infrared, and this is a job for Spitzer. Spitzer observed the secondary eclipse of Kepler-7 b in both 3.6 µm and 4.5 µm, and in both wavelengths, the eclipse of the planet behind the star was not confidently detected. This means that the planet isn’t just not hot enough to produce the brightness asymmetry in the Kepler phase curve, but it’s eclipse couldn’t even be confidently detected in infrared. This firmly rules out thermal emission as the source of the optical phase curve asymmetry.

This represents the first time that visible light “surface” features have been identified on an extrasolar planet. This is but a baby-step forward in our ability to map extrasolar planets, but it is a milestone nonetheless. Kepler data may be able to detect phase curve asymmetries in other hot Jupiter systems (or if we are lucky, smaller worlds!), and this can significantly contribute to our understanding of the atmospheres of these planets.

Mapping the Galaxy

The Milky Way Galaxy

The Milky Way Galaxy (Source)

Measuring the distance to a star makes use of astrometry – the careful monitoring of a position of a star over time. As Earth orbits the sun, it has a maximum displacement from any given position along its orbit of about 2 AU (i.e., being on the other side of the orbit). By observing the angular change in the apparent position of a star 2 AU apart, simple trigonometry can allow you to calculate the distance to the star.

In the middle of the last century (not terribly long ago from a historical perspective), we knew the distances to very few stars and knew their positions with much poorer accuracy. The FK4 catalogue catalogued the position of stars in the year 1950 with a precision of position of about 0.04 arcsec in the northern hemisphere, and a dismal 0.08 arcsec precision in the southern hemisphere. It was suggested that using a network of astrolabes over ten years could reduce the errors to about 0.03 arcsec, only marginally better. Major obstacles to the advance of stellar cartography was the typical issues that plague amateur astronomers now — atmospheric distortion of stellar images, instrumental instability, and inability for a ground-based observatory to view the entire sky.

In 1966, Pierre Lacroute came up with an idea (that he himself called “weird”) of performing the necessary measurements from a spacecraft, orbiting Earth outside the atmosphere. The idea was presented in 1967 to the IAU where it received a great deal of interest, but the technological capacity at the time (and available rocketry in France) was not accommodating to the idea. The satellite, a 140 kg spacecraft designed to observe 700 stars all over the sky with a precision of 0.01 arcsec, had stability requirements that could not be met by the Diamant rocket used by France at the time.

The idea of a spacecraft to catalogue the distances and positions of a large number of stars evolved over time and was revised and improved for the next decade, while the rest of astrophysics advanced and continued running into the problem of distance scales being poorly known.

“The determination of the extragalactic distance scale, like so many problems that occupy astronomers attention, is essentially an impossible task. The methods, the data, and the understanding are all too fragmentary at this time to allow a reliable result to be obtained. It would probably be a wise thing to stop trying for the time being and to concentrate on better establishing such things as the distance scale in our Galaxy.” — Hodge (1981)

Support for a space-based astrometry mission continued to grow and recognising that France alone did not have the resources necessary to complete the task, the European Space Agency planned and devised a new spacecraft, Hipparcos, to catalogue the positions of 100,000 stars and to determine their positions with an accuracy of 0.001 arcsec (1 milliarcsec).

Hipparcos

Hipparcos

Hipparcos was launched on August 8, 1989 on a 3.5 year mission. It determined the positions of stars, monitored the position over the course of a half year to determine the parallax and thus distance to the star, monitored the position over the course of the entire mission to determine the proper motion of the star in space, measured the spectrum of stars to determine their composition, and performed radial velocity measurements on these stars to determine their motion toward or away from Earth. In total, 118,200 stars were observed with high precision observations (published in 1997), with another 2.5 million stars observed with lower precision (published in 2000).

Hipparcos data has practically revolutionised astronomy. With the knowledge of the positions and motions of over a hundred thousand stars in hand, we’ve been able to understand the structure and dynamics of nearby clusters, understand the local structure of the Galaxy, understand the orbits and true orientations of binary star systems, and more. Even an extrasolar planet transit was observed (though it was not known until the planet was discovered later).

This brings us to today. This Hipparcos catalogue remains as the best available source of uniform parallaxes and positions. It is time, however, to take another step forward, with greater precision, a larger sample, and newer science. The successor to Hipparcos is called GAIA – Global Astrometric Interferometer for Astrophysics – however it will not use interferometry due to a design change.

Gaia will essentially do exactly what Hipparcos did, but better. Whereas Hipparcos only measured a hundred thousand stars down to brightnesses of V = 9, Gaia will observe over a billion stars with brightnesses down to V = 20. Gaia will measure the angular position of all stars of magnitude 5.7 – 20. For stars brighter than V = 10, it will determine the position with a precision of 7 µas (microarcseconds), a precision of 12 – 25 µas down to V = 15, and 100 – 300 µas down to V = 20. It will acquire their spectrum (from 320 – 1000 nm) to determine their temperature, age, mass, and composition. It will also measure the radial velocity of stars with a precision of 1 km s-1 for V = 11.5, and 30 km s-1 for V = 17.5. Tangential velocities for 40 million stars will be measured with a precision better than 0.5 km s-1.

Gaia

Gaia (Source: ESA)

While the stellar astrophysics enabled by Gaia will be revolutionary in its own right, the unprecedented astrometric precision also makes the mission interesting from an extrasolar planet perspective. Hipparcos was not able to discover any planets on its own, but it was marginally helpful for extrasolar planet science. Planets detected with radial velocity have unknown true masses. The greater the true mass of the planet, the greater the astrometric amplitude of the barycentric motion of the star is (see this post where astrometry is discussed in the context of planet detection). Planets of especially high true masses would therefore have a chance of having their star’s barycentric motion detectable to Hipparcos. Otherwise, Hipparcos data could be used to set upper limits to the true mass of the planet, by knowing that it’s astrometric effect must be sufficiently low so as to not have been detected by Hipparcos (an upper limit to the astrometric amplitude and thus the planetary mass).

The astrometric precision and vast number of targets available to Gaia will allow for the detection of a large number of planets. Astrometry is, of course, less biased toward high values of the planetary orbital inclination, and will permit us to know the true mass of the planet and orientation of the orbit in 3D space. Still, several complications are expected to arise based on nearly two decades of radial velocity experience.

Just like with radial velocity (and, actually, science in general), models will need to be fitted to data points to yield high-quality fits, however as Doppler spectroscopy has shown us, planetary systems can often feature several components all contributing to the barycentric velocity profile of the star, complicating radial velocity fitting in the same way it can be expected to complicate astrometric fitting. Radial velocity surveys can often produce more than one model that fit the data nicely, where both models may disagree on certain aspects of the orbit, or even number of planets. Astrometry is likely to be prone to the same problems. In the case of astrometry, it may even be harder because of the greater number of free parameters – ascending node, inclination, etc, issues that need to be modelled for an astrometric fit that could usually be ignored for a radial velocity fit.

These challenges can be addressed and handled, and the Gaia data will be wonderfully productive to extrasolar planet science. It is hard to know how many planets we can expect Gaia to discover, because statistics for planets in intermediate-period orbits are still unconstrained, but with the accuracy and large number of stars Gaia will observe, it is likely that Gaia will discover thousands of giant planets. It will be sensitive to Jupiter analogues out to 200 parsecs.

Gaia Results

Gaia Results (Source: Sozzetti (2010)

What about transiting planets? A transit of HD 209458 b was squeezed out of Hipparcos data, which was not at all optimised for transiting planet science. Can Gaia be expected to detect transiting planets? As far as photometric precision, Gaia is expected to achieve 1 mmag precision for most objects Gaia will observe, down to V ~ 15, and 10 mmag precision at the worst case of V ~ 20. For most hot Jupiter systems, mmag precision is indeed sufficient for transit detections. The next major issue is cadence.

Focused transit searches tend to be high-cadence, narrow field observations, whereas Gaia is an all-sky, low cadence observatory. On average, each star will be observed by Gaia 70 times, giving us 70 measurements for a light curve of any given star with a baseline of five years. While 70 measurements spread out over five years seems dismal (and let’s not sugar-coat the issue — for a transit search, it is dismal, but Gaia is not designed to be a transit search mission), but for a planet in a short period orbit, perhaps three or four measurements may occur while the planet is transiting. Obviously, the longer the orbital period, the less a fraction of the planet’s orbital period is spent in-transit, and the fewer transits will be observed by Gaia. Since only 70 measurements will be taken, Gaia is severely biased toward short-period transiting planets.

Early studies suggested wildly fantastic transiting planet yields. Høg (2002) estimated over a half million hot Jupiters and thousands of planets in longer periods would be found, based on the (unrealistic) assumption that a transit could be identified based on a single data point and other oversimplifications. Robichon (2002) suggested that Gaia will detect 4,000 – 40,000 transiting hot Jupiters under the assumption that each star would receive an average of 130 measurements, however the currently planned Gaia mission has instead 70 measurements per star.

Dzigan & Zucker suggest that Gaia could potentially detect sub-Jupiter-sized planets around smaller stars, and that a ground-based follow-up campaign can easily observe hints of transiting planets that show up in Gaia data. They also suggest that a few hundred to a few thousand hot Jupiters could be found in Gaia photometry.

While Gaia will perform km s-1 radial velocity measurements on millions of stars, this precision level is simply not sufficient to detect even hot Jupiters. It will, however, be able to tell if a transiting planet candidate is a brown dwarf instead, or an eclipsing binary star, allowing for one method of ruling out false positives. Interestingly, the astrometric fit to the orbit of a planet will have the inclination of the planetary orbit sufficiently well-characterised that a list of planets that are likely to transit can be compiled and followed-up with ground-based radial velocity and photometry. These long-period transiting planets will certainly prove valuable – they will be likely to host detectable rings and moons.

ESA will launch Gaia on a Soyuz ST-B rocket in November of this year. It will take five years after a commissioning phase for the total extrasolar planets science results to become known. It will be very exciting to see what giant planets exist in the solar neighbourhood. They will attract interest in follow-up observations to discover smaller, inner worlds that may exist. Gaia has the potential for flagging the first solar system analogues in the solar neighbourhood for dedicated study.