The Search for Distant Earths
|The detection method known as the Doppler Shift is a ground-based method for searching for extrasolar planets.
Credit: California & Carnegie Planet Search
Imagine trying to see an ant crawling across the headlight of a car miles away. That is the magnitude of the task facing astronomers as they search for small, rocky Earth-like planets around other stars – planets that might harbor life.
"The question is, ‘Are Earths frequent or rare in our galaxy?’" says Bill Borucki, an astrophysicist at NASA’s Ames Research Center in Mountain View, CA. "If they are frequent, the galaxy is probably full of life. ‘Star Trek’ will happen. If there are no other Earth-like planets, there can be no ‘Star Trek.’ There is no place to go."
During the next 15 years or so, American and European scientists hope to launch more than half a dozen missions to search our corner of the Milky Way galaxy for terrestrial planets.
So far, Earth-based telescopes have found more than 80 planets orbiting relatively nearby stars. Almost all of them, however, are believed to be giant gaseous planets like Jupiter, a few hundred times more massive than Earth, without a solid surface or liquid water. Some are smaller planets located near pulsar stars with radiation that would make life impossible, Borucki says.
The traditional ground-based method of looking for extrasolar planets uses the Doppler shift a change in color of a star’s light to measures changes in the star’s velocity caused by the gravitational pull of an orbiting planet. The technique can detect only Jupiter-sized planets.
Finding Earth-sized planets may well require placing telescopes in space, because the Earth’s atmosphere blurs and distorts the light from the star and planet, and because daylight, weather and Earth’s rotation prevent continuous observations needed for some planet-hunting methods.
The United States and Europe plan six space-borne missions in which planet hunting is a major goal. They include France’s small-scale COROT, NASA’s more-ambitious Kepler mission, the European Space Agency’s (ESA) Eddington and NASA’s Space Interferometry Mission (SIM). These four missions would set the stage for two more-expensive and sophisticated missions, NASA’s Terrestrial Planet Finder (TPF) and ESA’s Darwin.
The French COROT mission, approved and due for launch in late 2004 or 2005, will study asteroseismology, or oscillations within stars, and likely will be the first orbiting telescope to search for extrasolar planets.
It will look at 50,000 to 60,000 stars and should find a few dozen terrestrial planets and several hundred close-in gas-giant planets during a two- to three-year mission, says Pierre Barge, an astronomer at the Laboratory of Astrophysics in Marseille and leader of COROT’s exoplanets group.
COROT – for Convection, Rotation and Planetary Transits – is a mission of CNES, the French National Center for Space Studies, in partnership with ESA, Italy, Belgium and Germany.
When searching for extrasolar planets, COROT’s 27-centimeter (10.6-inch) telescope will use a method called photometry, in which sensitive light detectors look for a slight drop in a star’s brightness as a small planet "transits" the star (crosses the face of the star as viewed from COROT).
"It will be on the edge of possibility as to whether COROT can detect a planet the same size as Earth," says Alan Penny, a space scientist at Britain’s Rutherford Appleton Laboratory.
Meanwhile, as part of its Discovery Program, NASA has approved a more powerful planet-hunting photometry telescope, named Kepler. The Kepler mission is scheduled for launch into solar orbit in October 2006.
|Kepler with distant solar system.
Credit: David Koch
Kepler will simultaneously observe 100,000 stars in our galactic "neighborhood," looking for Earth-sized or larger planets within the "habitable zone" around each star – the not-too-hot, not-too-cold zone where liquid water might exist on a planet.
To highlight the difficulty of detecting an Earth-sized planet orbiting a distant star, Borucki, Kepler’s principal investigator, points out it would take 10,000 Earths to cover the Sun’s disk.
"It’s really very much like if you looked at a big highway from miles away and saw all these cars and headlights coming toward you at night, and looked to see if there was an ant crawling across one of the headlights," Borucki says.
One NASA estimate says Kepler should discover 50 terrestrial planets if most of those found are about Earth’s size, 185 planets if most are 30 percent larger than Earth and 640 if most are 2.2 times Earth’s size. In addition, Kepler is expected to find almost 900 giant planets close to their stars and about 30 giants orbiting at Jupiter-like distances from their parent stars.
Because most of the gas giant planets found so far orbit much closer to their stars than Jupiter does to the Sun, Borucki believes that during the four- to six-year mission, Kepler will find a large proportion of planets quite close to stars. If that proves true, he says, "We expect to find thousands of planets."
Kepler will also catalog the brightness, temperature, stellar type and other properties of stars orbited by habitable planets, helping to provide targets for later missions.
In 2008 or later, the European Space Agency hopes to launch the Eddington mission (named for the late British astronomer Arthur Eddington). Eddington primarily would study stars’ interior structures and the processes that govern how stars evolve, but it would spend three years scanning 500,000 stars for planets, including terrestrial planets in habitable zones.
Like Kepler and COROT, Eddington would use a camera with a large optical telescope, in this case a wide-field 1.2-meter (47-inch) one, to detect planetary transits using photometry.
ESA now lists Eddington as an unfunded "reserve" mission. Penny says Eddington’s future will be decided within a year or so.
After Kepler, NASA is considering a 2009 launch for SIM, the Space Interferometry Mission. SIM’s primary mission will be to measure distances to stars with 100 times greater precision than now is possible. This will improve estimates of the size of the universe and help astronomers determine the true brightness of stars, and thus learn more about their chemical composition and evolution.
|SIM, scheduled for launch in 2009, will determine the positions and distances of stars several hundred times more accurately than any previous program.
Credit: NASA / JPL
SIM also will look for Earth-sized planets in the habitable zones around some 200 stars. SIM will be an interferometer, which means it will combine interacting light waves from its three component telescopes. This interaction, called interference, makes the individual telescopes, which are separated from each other on the spacecraft, act as though they were a single, larger telescope with greater light-gathering ability.
SIM, in solar orbit, won’t actually see Earth-sized planets. Instead, it will use astrometry, measuring the angle between two stars (as viewed from the spacecraft, which will form the third point of a triangle). By repeatedly measuring the angle between a target star and each of several more distant background stars, SIM will be able to determine whether the target star wobbles periodically because of gravity from orbiting planets, including planets as small as Earth.
NASA says SIM might detect Earth-sized planets around the nearest stars, identifying potential targets for the later Terrestrial Planet Finder, and Jupiter-size planets at greater distances.
Also due for launch in 2009 is the almost $1 billion NASA-ESA Next Generation Space Telescope, or NGST, a near-infrared telescope that will succeed the Hubble Space Telescope. Planet hunting will be "a minor part of its job, " says Penny. Like Hubble, NGST will be a general-purpose telescope with an emphasis on cosmology. But it will investigate stars with dusty disks – the early stage of planet formation – and may also be able to study Jupiter-size planets, Penny says.
Kepler and SIM will serve as precursors if all goes according to plan to a tentatively scheduled 2014 launch for NASA’s Terrestrial Planet Finder (TPF), while in Europe, COROT and Eddington will pave the way for ESA’s Darwin, also to be sent into solar orbit in 2014 or later.
Both instruments will be designed to search for Earth-like planets and to examine their atmospheres for evidence of gasses water vapor, oxygen, ozone and methane are leading candidates that could indicate the presence of life.
University of Arizona astronomer Nick Woolf says NASA plans for TPF to look for Earth-like planets around 150 stars within about 50 light years (about 473 trillion kilometers, or 294 trillion miles). The basic design of the telescope is still under discussion.
Nor has NASA decided whether TPF will make observations at visible or infrared wavelengths. Woolf states that, with both infrared and visible observations, it would be possible to determine the size, surface temperature and atmospheric pressure of a distant planet like Earth, the abundances of many different gases in its atmosphere, and the presence or absence of Earth-like land vegetation. Telescope design considerations, however, limit observation to either visible or infrared, but not both.
Darwin, like NASA’s SIM, will be an interferometer, comprised of a flotilla of four 1.5-meter (5-foot) solar-orbiting infrared telescopes. Darwin will look at 1,000 of the closest stars with 10 to 100 times the NGST’s ability to see details.
Woolf and Penny say needed technology still must be developed before either TPF or Darwin is feasible and they question whether either mission can be mounted for what historically has been the standard cost of a big orbiting telescope: about $1 billion.
Woolf, a member of the TPF science working group, wonders whether it’s even a good idea to build a large, expensive planet-hunting mission. He would prefer to see "something much smaller, cheaper and as soon as possible."