A proposed space mission could find hundreds of free-floating, Earth-size planets that were ejected from their solar systems in primordial tussles with their siblings. Based on a technique called microlensing, the mission could also find many planets orbiting their parent stars at the right distance to support life.
Astronomers have discovered more than 200 planets orbiting other stars. But the main technique used so far, which finds planets by the wobble they induce in their parent stars, is not sensitive enough to find planets as small as Earth.
One technique that has the required sensitivity to detect Earth-like planets is called microlensing. Using the technique, ground-based telescopes have so far detected four planets, each much more massive than Earth.
The technique takes advantage of the fact that a massive object's gravity can act like a lens, bending light rays that pass nearby. A background star can appear to brighten if an object – such as a star – passes between it and Earth, and if the intervening star hosts a planet, the planet can produce a second, smaller spike in brightness.
Such events are rare, so microlensing surveys monitor dense star fields towards the centre of the galaxy in the hopes of catching planets. But Earth's atmosphere makes many of the stars blur together, making it harder to detect slight brightness changes.
So a group of astronomers has put together a low-cost design for a space-based mission called the Microlensing Planet Finder. Led by David Bennett of the University of Notre Dame in Notre Dame, Indiana, US, the team hopes to garner funding from NASA to launch as early as 2012.
The mission would place a telescope with a 1.1-metre mirror in orbit around Earth. It would monitor about 100 million stars in the galactic bulge, the dense concentration of stars towards the galaxy's centre.
It would be able to detect planets with just one-tenth the mass of Earth, with orbits slightly wider than that of Mercury's around the Sun. This would include the so-called habitable zone where the temperature would be right for liquid water and, potentially, life.
During the four-year mission, it could turn up about 150 planets about as massive as Earth, if these are common around other stars, and an estimated 5000 or so Jupiter-mass planets, Bennett says.
"We could discover analogues of all the planets in our solar system except for Mercury," Bennett told New Scientist. A planet like Mercury would be too lightweight and too close to its parent star to be discernible in the microlensing event.
The microlensing mission could also detect planets not bound to any star. Many planets could get ejected early on in developing solar systems because of the gravitational influence of other planets, Bennett says.
"It could be that there are 100 other Mars-like objects that were originally in our solar system but were scattered out by encounters with other planets," he says, though he says a more likely number of such ejected objects would be about 10.
Bennett says the mission would cost less than $400 million, making it cheap enough to qualify for NASA's low-cost Discovery mission programme.
The concept lost out to other proposals in the last competition for Discovery funding in 2006, but the group plans to submit it in the next competition, which is expected to open in late 2008 or early 2009. If selected for funding at that point, it could launch as early as 2012, Bennett says.
NASA already has one mission in the works that could detect Earth-like planets. Called Kepler, it is set to launch in late 2008, and will watch for the dimming of stars as planets pass in front of them.
Kepler team member David Latham of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, US, says microlensing can provide a more thorough census of planets around other stars, especially for very low-mass planets. "I think it's something that is well worth doing," he told New Scientist.
But he points out that planets detected by Kepler can be observed repeatedly, and with different telescopes, potentially allowing details such as their atmospheric composition and weather to be discerned.
"That's not something you can do with the microlensing planets," he says, since the events rely on the chance alignment of two stars as one passes in front of the other.
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10:45 10 October 2008
