August 2, 2004
(PLANETQUEST) -- The Milky Way is a big, diverse neighborhood. And if you're hoping to find Earthlike planets that may harbor life, you'll need to figure out a way to narrow the search.
The search for planets begins with stars, because the dusty discs around stars spawn young planets. Are stars of certain ages or in certain locations more likely to include planets like Earth within their realms? What about binary stars? Are they good places to look?
The Extrasolar Planet Interferometric Survey aims to help answer those questions. A project of NASA's Space Interferometry Mission, planned for launch in 2009, the survey will look closely at a hundred nearby stars and more broadly at several thousand distant stars to see if they have planets and if there are Earthlike planets among them.
"The result will be an inventory of planetary systems in different environments," said the survey's principal investigator, Dr. Michael Shao of NASA's Jet Propulsion Laboratory, Pasadena, Calif.
The survey will take advantage of Space Interferometry Mission's ability to measure the positions and distances of stars outside own solar system much more precisely than ever before. It will also be able to detect planets orbiting distant stars indirectly by measuring the change in a star's position, called an astrometric wobble, caused by the gravitational pull of an orbiting planet. The mission will provide enough information about these faraway planets to determine which might be similar to Earth in mass and temperature.
The search is on
For the first part of their survey, Shao and his colleagues will make multiple observations of about 100 nearby stars. Since another Space Interferometry Mission team, headed by Dr. Geoffrey Marcy of the University of California, Berkeley, will also be studying nearby stars in a different study, the researchers held a "draft pick" of the closest 200 stars.
"It turned out that we had slightly different criteria," Shao said. "Our team slightly favors luminous stars, because we're interested in stars where if there were Earthlike planets in the habitable zone, they would be easiest to detect." The habitable zone is that fortunate spot that Earth occupies in relation to the Sun -- close enough to be warm but not so far away that it is cold. "More luminous stars have a habitable zone further away from the star resulting in a larger astrometric wobble," he said.
For the second part of the survey, Shao's team will observe more than 2,000 stars of different types and different ages. They're also choosing stars in different locations in the galaxy, some in the Milky Way's spiral arms, between the arms, and outside the galaxy's disc.
"We want to know what types of stars are likely to produce planets, no planets, or multiple planet systems," Shao said. "Some stars are young and hot. Are they too young to have planets formed? We don't know enough yet about the planet formation process."
The survey team will use the Space Interferometry Mission's ability to study planets' orbits as well. "Multiple planet systems can get tricky. When you have two big Jupiter-like planets and they're not in the right orbit, they could perturb each other so much that one gets thrown out of the solar system. It can be a violent system for millions of years. We want to be able to figure out where orbits are stable," Shao said.
Orbital shape matters, too. "Planets in our solar system have circular obits, but the vast majority of planets found outside our solar system have elliptical orbits, " Shao said. "They may be very cold when far from the Sun and very hot when close to Sun. We're much more likely to find life where the temperature is more uniform."
Shao's survey will help pave the way for the planned Terrestrial Planet Finder Mission, which will be able to observe planets outside of our solar system directly for the first time ever. The mission is planned for a 2014 launch.
"We'll help create a target list for the Terrestrial Planet Finder in its quest to find habitable planets," Shao said. "We'll be able to suggest where to look, and because we'll be able to determine a planet's orbit, we'll know also know when to look -- when the planet will be most detectable."
Written by Rosmary Sullivant/PlanetQuest
