Conjoined space telescopes could see alien worlds

10:09 01 February 2008
NewScientist.com news service
David Shiga

A pair of infrared telescopes attached together in space could determine the chemical makeup of alien planet atmospheres at a fraction of the cost of NASA's Terrestrial Planet Finder (TPF), which has been delayed indefinitely, a new study says.

TPF would have used a technique called interferometry, combining the light from several space telescopes flying in formation to spot Earth-like planets circling other stars. But the mission's likely cost of several billion dollars, combined with daunting technological challenges, led NASA to postpone it indefinitely in 2006.

"It is pretty clear in most people's minds that TPF isn't going to happen in 20 years or more," William Danchi of NASA's Goddard Space Flight Center in Greenbelt, Maryland, US, told New Scientist.

Danchi has been studying a simpler and cheaper alternative to TPF called the Fourier Kelvin Stellar Interferometer (FKSI), a concept he and his colleagues first proposed in 2003. TPF's postponement has given new impetus to the idea and Danchi's team recently posted a paper online pointing out the merits of this and similar concepts in a category they call the Small Prototype Planet Finding Interferometer (SPPFI).

A key challenge for TPF was the need to fly several telescopes in very precise formation. An SPPFI would involve just two telescopes attached together by a truss, eliminating this problem.

Habitable zone
Such a mission could see planets inside the so-called habitable zone of stars, the region where the temperature is right for liquid water to exist. It would not be able to see planets as small as Earth, however, since it would use mirrors 1 metre across – half the size of those envisioned for TPF, reducing its light-collecting ability, and its telescopes would be fixed much closer together, reducing its resolution. The best SPFFI could hope for would be to see so-called super-Earths, which are a few times our planet's size, Danchi says.

The mission could also reveal a wealth of information about many of the 200 or so known exoplanets, most of which have only been detected indirectly by the gravitational tug they exert on their parent stars.

By measuring the spectrum of infrared light coming from these planets, the mission could determine the chemical makeup of their atmospheres as well as their temperature and size.

"We have a bucket-load of planets that we know about, but we don't know a lot about them," Danchi says. "A modest system can characterise a fraction of those exoplanets and also discover new ones."

Danchi's team estimates the mission could be built and launched for between $600 and $800 million. And he says NASA has been talking about the possibility of sponsoring intermediate-cost missions in about this range.

Uphill battle
Sara Seager of MIT in Cambridge, Massachusetts, US, who has previously worked with Danchi on the FKSI idea, says that eliminating the formation flying aspect makes interferometry easier, but adds that space-based interferometry will have to compete for funding against other ideas. "It's a really exciting concept, but $600 million for this means you have to take $600 million away from something else," she told New Scientist.

Indeed, such a mission may face an uphill battle for funding. Another technique called astrometry, which monitors the position of stars for slight wobbles due to the gravitational pull of planets, has been given higher priority for the near term in preliminary reports by the Exoplanet Task Force, a panel of scientists advising NASA and the US National Science Foundation on funding for exoplanet research.

Astrometry has the potential to detect Earth-mass planets in the near term, and determine how common they are, according to the panel's preliminary report, something a mission like FKSI is not capable of.

A proposed mission called SIM PlanetQuest also calls for a pair of telescopes attached together in space, but unlike SPPFI, it would be used to detect planets indirectly via astrometry, rather than by measuring light from the planets.