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9.26 Future Detection of Extrasolar Planets

The search for extra solar planets is escalating. The number of known and candidate extra solar planets has grown to over 4800 since the first one was discovered in 1995. New techniques and new technology will advance this search considerably in the coming decades.The largest share of candidate planets is from the Kepler mission. The primary goal of this science project is to determine the fraction of Sun-like stars which have an Earth-like planet. That is, a planet of about 1 Earth mass orbiting at about 1 A.U., in the circumstellar habitable zone. Determining this number is one of the main goals of exoplanet science. Kepler has not yet finished its work, and so we only have estimates but these estimates indicate that Earth-like planets in the habitable zone are not uncommon. Future results from Kepler and other surveys will refine these estimates. An extrapolation of the statistics of extra solar planet discovery so far suggest about 20 billion Earth-like or super-Earth planets in the Milky Way — a truly staggering number! About 100 million of these should be in the habitable zones of their stars and have all the ingredients for biology. Astronomers are laying the ground work to answer one of the most profound questions we can ask about the universe: is there life beyond Earth?

Once a planet is identified, the next step is for astronomers to try and characterize it. This means determining how big it is, what it's made of, and what the surface and atmosphere are like. Radial velocity and transit detections give direct measurements of the planet's size; at least a minimum mass for radial velocity and the radius for transits. Having both numbers gives a mean density, which can potentially say if the planet is rocky, gassy, or rock and liquid combined. To know whether a planet could harbor life, we need to collect additional information, such as spectra of the reflected light from the planet. Gathering more information, though, requires collecting the feeble light from the planet itself. The article on direct imaging discussed the many challenges this poses, mainly due to blurring effects of the Earth's atmosphere.

Meanwhile, the surveys for extra solar planets will continue. Individual discoveries used to make news headlines; that no longer happens unless the planet is Earth-like or very bizarre. The field has moved into a statistical phase, which implies progress towards a deeper understanding how and where extra solar planets form. Although the radial velocity method seems to have been "eclipsed" by the phenomenal success of Kepler and the transit method, Doppler detection is still the only way to measure planet mass. The Doppler signal from a Jupiter orbiting a Sun is 13 meters per second, but an Earth exerts a much smaller tug. Earth is 320 times less massive than Jupiter and five times closer to the Sun so it would give a Doppler signal of 9 centimeters a second, or a very slow crawl! Although this level of signal may be detectable, it is at a level where it can be confused with turbulent gas motions in the atmosphere of the host star, so the radial velocity method may reach a limit around Earth mass.

Transit surveys also continue. Kepler is in its twilight phase, with reduced capability after the loss of its best pointing capability in 2013. But it has widened its search beyond the initial target area and it continues to find extrasolar planet, including three super-Earths in a single system. One of the limitations of following up Kepler targets is the faintness of the stars. They are typically hundreds of light years away and thousands of times fainter than the eye can see. The Transiting Exoplanet Survey Satellite (TESS) will complement Kepler by looking across the entire sky for transiting planets around closer and brighter stars. TESS is a NASA mission scheduled to launch in 2017.

The next generation of giant telescopes, such as the planned 24.5 meter Giant Magellan Telescope (GMT), will improve the sensitivity of direct imaging by nearly a factor of ten. This gain assumes the use of adaptive optics. Such giant telescopes will be able to probe much closer to stars, and will search stars much further away from the Earth. These efforts will likely work in concert with radial velocity and transit surveys, attempting to characterize planets already known to exist.

Even with giant telescopes, the Earth's own atmosphere will limit types of planets that can be detected. To finally take a picture of a planet like the Earth, we will have to move our efforts into space. Even in space, the challenges are huge. The Hubble Space Telescope (HST), despite its superior performance and incredible scientific accomplishments, is not sensitive enough to detect an Earth around a nearby star. Even the planned 6.5-meter James Webb Space Telescope (JWST) will not be able to directly detect an Earth. The biggest problem is the extreme brightness of the star compared to its nearby planet. The light from the star must be blocked. In addition, an Earth will be 1 billion times fainter than its star, so any small optical imperfections will prevent detection. To overcome these difficulties, NASA is currently investigating dedicated Earth-imaging spacecraft which will use sophisticated coronagraphs and active control of the on-board optics.

All signs point to point to planets being ubiquitous throughout our galaxy. This means that astronomers attempts to discover, characterize, and explain the results from their surveys are only beginning. It is an exciting and fascinating time. Someday soon, we may finally discover the first Earth twin, a planet like our own, a planet where humans could live.

Astropedia Image
Diagram showing how gravitational microlensing can be used to detect extra solar planets. Click here for original source URL.