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Future Planet Searches: Kepler & Darwin

In the previous lecture, we discovered how to determine, based on remote observations, whether or not life exists on an extrasolar planet. In this lecture, we will examine two upcoming space missions which will play a part in putting this knowledge into practice.

The Kepler Mission

In December 2001, the Kepler mission was selected as one of NASA's new Discovery Programme missions (recent successes in the Discovery Programme include Mars Pathfinder). The principal scientific goal of the Kepler mission is to search for extrasolar planets situated within or near to the habitable zone of their parent star. As well as enabling the selection of candidate planets for subsequent searches for life, the results from Kepler will further our understanding of planetary formation.

The Kepler spacecraft

The Kepler spacecraft

Kepler is scheduled for launch in October 2006, when it will be put into an Earth-trailing heliocentric orbit (i.e., orbiting the Sun rather than the Earth). Its primary instrument is a photometer, made from a mosaic of 42 CCDs (an electronic version of photographic film) situated in the focal plane of the 0.95m reflecting telescope which makes up most of Kepler's bulk. This mosaic enables Kepler to view over 100 square degrees of the sky at any one time, a huge amount by astronomical standards.

Kepler's photometer

Kepler's photometer

Using the photometer, Kepler will measure the brightness of approximately 100,000 solar-type stars, situated within a field-of-view along the the Orion arm of the Milky Way. This field of view will remain unchanged throughout the entire 4-year mission; therefore, with brightness measurement taken every 15 minutes, each of the 100,000 stars will be observed nearly 150,000 times!

Kepler's target area

Kepler's target area

The aim of the photometry is to detect the minute dimming in a star's brightness, which occurs during the transit of a planet across the star's disk. The photometer onboard Kepler will be sensitive enough to detect an Earth-sized planet orbiting at 1 AU around a solar-type star of visual magnitude mv=12.

The ability of Kepler to detect planets obviously depends on the size of the planet; larger planets block out more starlight during transit, and are therefore easier to detect. Based on simulations and our present knowledge of solar system formation, Kepler is expected to detect the following numbers of terrestrial-sized planets:

  • About 50 planets, if most have a radius R ~ 1.0 Re
  • About 185 planets, if most have a radius R ~ 1.3 Re
  • About 640 planets, if most have a radius R ~ 2.2 Re

The fact that Kepler will find transiting planets is an added bonus; as was discussed in the previous lecture, it is much easier to analyze the atmospheric composition of a planet which undergoes transits, than to do the same for one which doesn't. However, it should be stressed that Kepler will not have any capability to perform atmosphere analyses; it is targeted purely at the detection of extrasolar planets, with a particular emphasis on terrestrial ones.

The Darwin Mission

As part of their Horizons 2000 programme, the Research and Scientific Support Department of the European Space Agency is currently studying a proposed mission to look for extrasolar life. This mission's proper name is the Infra-Red Space Interferometer, but it is commonly known as Darwin (or IRSI-Darwin). If the mission is approved and built, Darwin is due to be launched in 2014 or later.

Artist's impression of Darwin

Artist's impression of Darwin

Darwin is essentially an infra-red telescope, designed to obtain spectra of extrasolar planets and look for absorption lines of atmospheric compounds associated with life: water, carbon dioxide and ozone (see previous lecture). This is a difficult task, since the light from planets is swamped by the light from their parent star, which in the infra-red is typically 10 million times brighter.

To deal with the contrast problem, Darwin has been designed to work as a nulling interferometer, made from six separate telescopes flying in formation. Each 1.5-m diameter telescope observes the same region of the sky, and beams its focused light to a hub situated at the centre of the formation. Within this hub, the light from each of the six telescopes is combined in such a way that the light from one part of the field of view interferes with itself, and is canceled out.

Schematic of one of the six Darwin telescopes

Schematic of one of the six Darwin telescopes

Schematic of the Darwin hub

Schematic of the Darwin hub

Using this technique, the light from a given star can be canceled (or 'nulled'), leaving only the residual light from those extrasolar planets which orbit the star. This residual light will be analyzed by a spectrograph inside Darwins hub, and the resulting data will be beamed to a master satellite'. This master satellite, which flies a couple of hundred metres behind the telescope/hub formation, will handle all inbound and outbound communication with the Earth.

Principle of an interferometer

Principle of an interferometer

The master satellite will also monitor the relative positions of the Darwin formation. For the the nulling interferometry to work, the six telescopes plus hub must be positioned to better than 20 billionths of a metre. To achieve this, a technique similar to that used by the Global Positioning System (GPS) will be used. By sending regular radio pulses to each spacecraft, and listening for the timing of replies, the master satellite can detect when a spacecraft is drifting out of position, and order the spacecraft to make appropriate corrections.

Darwin at the L2 point

Darwin at the L2 point

The Darwin spacecraft will sit 1.5 million kilometres from the Earth, at the gravitationally-stable L2 point. This point is far beyond any interference caused by the Earth, and offers an uninterrupted view of the sky.

Darwin is still very much on the drawing board, and it is not yet completely certain whether it will be funded and built. However, progress is being made toward testing the various technologies which will be required for this very ambitious mission to work. In particular, the European Southern Observatory (a collaboration between many European countries) is in the process of commissioning the VLTI, an optical interferometer based on the four 8-metre telescopes of the Very Large Telescope (VLT) situated at Paranal, Chile.

The four 8m telescopes of the VLTI array

The four 8m telescopes of the VLTI array

The VLTI will be the first-ever large-telescope implementation of an optical nulling interferometer. Many of the techniques and technologies used in its development will be invaluable when it comes to building Darwin. Unfortunately, the VLTI will not be suitable for searching for life itself: the Earth's atmosphere blocks out the mid infra-red wavelengths of interest in searches for life.

Artist's impression of SMART-2

Artist's impression of SMART-2

Also to help develop the technology to be used by Darwin, ESA is currently working on the SMART-2 (Small Missions for Advanced Research in Technology) mission, which is due for launch in 2006. This mission will consist of two spacecraft flying in formation, allowing tests of Darwin's positioning technology to be made. SMART-2 will also be acting as a testbed for the LISA (Laser Interferometry Space Antenna), a proposed ESA mission intended to look for the gravitational waves predicted by Einstein's General Theory of Relativity.


Updated 2009-10-14 05:25:34