## 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.

## Mapping the 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 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 (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 (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.

## Transiting Exoplanet Survey Satellite

TESS Spacecraft

The Kepler spacecraft is a wonderful asset. Despite its ailing health, it has managed to provide us with 3,000 candidate planets which permit a statistical look at the census of planets in the local Galaxy. We’ve learned planets are actually pretty common – they’re the rule rather than the exception. But for all of its contributions to science, and they have been numerous, the candidate planets Kepler has found tend to orbit stars that are far away. This is because the spacecraft stares at the same patch of sky continuously. If you take any patch of the sky, there will be far more distant stars than near stars in that patch because the volume of space within that patch increases with distance. So while the Kepler results are splendid for estimating the frequency of planets, they are not particularly useful for providing us with targets for follow-up study.

What we need is a mission to identify transiting planets around nearby and/or bright stars. It seems NASA has answered the call for such a mission and has, as of today (April 5, 2013), selected the TESS mission for development and launch in 2017.

The Transiting Exoplanet Survey Satellite (TESS) was conceived to address this problem. It will observe an area of sky 400 times that of Kepler at a time, studying two million stars with brightnesses V < 12, as well as the closest 1,000 M dwarf stars — essentially all red dwarfs within 30 pc. The mission will last two years and will uncover perhaps 2,000 planet candidates, a few hundred of which could be Earth-sized. The number of discoveries could be as much as from Kepler, but around nearby and bright stars where these planets would actually be feasible to study and examine for the possibility of life with transmission spectroscopy. Ground-based spectrographs would have a much easier time confirming the planets with Doppler spectroscopy.

It’s important to note that TESS will not stare at the same part of the sky for an extended time to detect Earth-analogues as Kepler does. TESS will move from one part of sky to another, observing each part for only a few months, so the orbital periods of discovered planets will be on the order of weeks – perfect for habitable planets around M dwarfs, but less so for habitable planets around G or K dwarfs. TESS’s wide, shallow approach to finding planets nicely complements Kepler‘s narrow, deep search for planets.

The study of transiting planets around M dwarfs is a worthwhile pursuit. M Dwarfs emit most of their energy in the infrared, where absorption lines of water and carbon dioxide reside and are prominent. The planets will necessarily be in shorter period orbits. This is hugely exciting for extrasolar planet science. It’s quite possible that the first extrasolar biosphere, and perhaps even the first extrasolar planet that is visited by a human spacecraft in the distant future, will be discovered by TESS. It is my opinion that this spacecraft is much more exciting than even Kepler.

## 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…