A new Balloon-borne telescope to image exoplanets (2/11/2008)

Austin, Texas, (USA).- A new project called "PLANETSCOPE" was presented last month at the American Astronomical Society meeting in Austin, Texas. It will use a balloon-borne telescope floating in the stratosphere to try to obtain a direct view of planets in other solar systems. If successful, the project would be able to obtain images of alien worlds impossible to see from the ground, and all for a fraction of what it would cost to do the same task from space.

"It's one of those ideas that actually has a remote chance of making it off the drawing board" says team leader Wesley Traub, a senior research scientist at NASA's Jet Propulsion Lab in Pasadena, California, US.

Astronomers have identified near 270 exoplanets orbiting distant stars. They were detected by indirect means and are too faint and too close to the stars they orbit to be imaged directly. Previous measurements have shown that light from distant stars remains relatively undisturbed while passing through the stratosphere, where turbulence is low.

To answer whether the air currents generated by a rising balloon, which is warmer than its surroundings, would spoil the incoming light of an exoplanet, in mid-2007, the group piggy-backed a small instrument called "PlanetScope Precursor Experiment" aboard the Solar Bolometric Imager gondola, launched from Fort Sumner, New Mexico.

While one beam passed though the surrounding thin air on its way to a reflector that jutted out 1 metre from the balloon's gondola, the adjacent reference beam travelled through an evacuated tube to the same reflector.

After comparing the reflected light from both beams, Traub and his colleagues concluded that distortions to the beam caused by the movement of air around the gondola were not serious enough to disrupt the image of a distant planet.

Traub envisions a telescope with a 1-to-2 metre mirror that would perform well enough to image approximately 20 exoplanets that lie relatively far away from the glare of their host stars. It will use a coronagraph, which blocks the light of a target star while allowing the light from surrounding planets to reach the camera. The camera would image through a variety of colour filters, which would provide clues to the atmospheric composition of any planets it sees.

"We'll be seeking funding for a balloon-borne coronagraph in 2008" said Traub. "If we come up with a good design, maybe we'll have a chance at this." Traub estimates the project, would cost $10 million dollars, roughly 1% the cost of a space-based planet imager. "It's the kind of thing that needs to be tried, because it's much cheaper than going to space".