Provided below is a brief summary of current and near-future approved missions and facilities that are pushing the frontiers of the study of exoplanets and the search for potentially habitable worlds.
Hubble Space Telescope (HST)
The Hubble Space Telescope, launched in 1990, is a 2.4m telescope in low-Earth orbit. It is one of NASA’s four Great Observatories. HST’s high angular resolution allows some types of exoplanet studies to be pursued. For example, the spectrograph on HST called the Space Telescope Imaging Spectrograph was used to obtain the spectrum of the atmosphere of several gas giant planets orbiting very close to their host stars. Such exoplanets are dubbed “Hot Jupiters.” The HST spectrum was obtained using an indirect observation method called “transit spectroscopy.” HST is also able to obtain direct images of exoplanets – but only for very large planets (several times the size of Jupiter) that are orbiting very far away from their host stars. To make these observations, HST used the Advanced Camera for Surveys.
Spitzer Space Telescope
The Spitzer Space Telescope is an infrared observatory in an Earth drift-away orbit. This means the Spitzer telescope is orbiting the Sun in the same path as Earth, but is getting slowly farther away from Earth each day. Spitzer was launched in 2003 and, like HST, is one of NASA’s Great Observatories. Spitzer is able to conduct observations of exoplanets that eclipse, or are eclipsed by, their host stars. Using the subtle change in the brightness of the star as this occurs, astronomers estimate the size and the mean temperature of the exoplanet.
Kepler is a 0.95m telescope that is in an Earth drift-away orbit. Kepler was launched in 2009 and was designed to stare at single patch of sky containing over 100,000 stars. It takes many images of this region of the sky over the course of several years. It has a high-precision camera that can sense very small changes (1 part in 100,000) in the brightness of any one of these stars. In some cases, these variations in brightness are due to a planet eclipsing a star. By measuring the precise change in the brightness of the star during the eclipse, astronomers can use Kepler to determine the size of the planet. And by measuring the intervals between eclipses, astronomers can determine the planet’s orbital period. To date, Kepler has confirmed the discovery of nearly 1,000 new exoplanets. Most of these planets orbit stars that are about 2,000 light years from the Sun. As such, most of them are too far away to study in detail (like getting the spectrum of the atmospheres). However, detailed exoplanet studies is not what Kepler was designed to do. It was specifically designed to accurately determine the size distribution of exoplanets. It has successfully achieved this goal.
Transiting Exoplanet Survey Satellite (TESS)
The Transiting Exoplanet Survey Satellite is scheduled to be launched in 2017. Over the course of two years, TESS will search approximately 100,000 nearby stars for the presence of exoplanets using the transit technique. TESS will survey the entire sky by observing 26 separate sectors. It will take repeated observations of each sector (about one image every minute) over the course of 27 days. TESS will then move onto the next sector and will continue until it maps the full sky. TESS is the first all-sky exoplanet mission. While the method used by TESS to find exoplanets is very similar to that used by Kepler, the targets of the TESS mission are much closer than the stars studied by Kepler. The stars TESS will study are all within 200 light years of the Sun. This means that many of the exoplanets that TESS will discover will be suitable for more detailed follow-up observations with the James Webb Space Telescope, ground-based telescopes, and even more-advanced telescope of the future, such as ATLAST.
James Webb Space Telescope (JWST)
The James Webb Space Telescope is an observatory with a segmented 6.5m primary mirror. JWST, scheduled for launch in 2018, is a cryogenic telescope (operating temperature of 40K) that is optimized for acquiring data in the infrared region of the electromagnetic spectrum. It will be located at the Sun-Earth 2nd Lagrange point (L2) – a stable orbital location that is 1.5 million km from Earth. JWST will be capable of pursuing many astronomical research topics and studying exoplanets is one of its prime areas of investigation. JWST will use transit spectroscopy to study the properties of exoplanets orbiting nearby cool stars known as M-dwarfs. JWST will be able to get high-quality spectra of the atmospheres of a number of exoplanets around these cooler stars. While such stars are much cooler than the Sun, if they have exoplanets orbiting closer in they may have surface conditions that allow for the presence of liquid water – a key ingredient for life as we know it.
Several ground-based telescopes have newly installed coronagraphic instruments designed to detect and characterize exoplanets orbiting nearby stars. While none of these instruments will have the contrast necessary to study Earth-sized planets around Sun-like stars, they will provide fundamentally new observations of planets around cooler stars and larger gas giant planets around many different types of stars. They will also allow us to study the early phases of planetary formation through their ability to observe proto-planetary dust and debris disks around stars. In addition to direct imaging, ground-based telescopes are the go-to facilities for radial velocity studies. Such observations allow astronomers to detect the small wobble in a star's position due to the reflex motion from planets that are orbiting that star. Such radial velocity measurements allow the masses of exoplanets to be estimated. When combined with planet sizes that are measured from transit surveys (e.g., Kepler, TESS) astronomers can estimate the density of the planets. The density allows us to determine whether the planet is a rocky planet or a gas giant.