Upping the chances of finding planets.

A new generation of telescopes and the upgrading of existing instruments will soon enable astronomers to peer deeper into space and further back into time. But the improved optics have another payoff: boosting the odds of finding planets that lie outside the solar system but within our own galaxy.

In the May 25 Nature, Adam S. Burrows and Jonathan I. Lunine of the University of Arizona in Tucson and their colleagues consider the feasibility of looking for young, giant planets--brighter and therefore more visible than older, smaller planets--at various distances from their parent stars.

They conclude that planets more massive than Jupiter and younger than 1 billion years have the best chance of showing up in telescope searches--especially if they lie farther from their stars than Jupiter does from the sun.

"That a young 'Jupiter' or 'Saturn' may be bright has been known for some time, but ours are the first detailed calculations for objects [with masses greater than Jupiter's] and ages greater than 10 million years," the team writes.

But brightness isn't the only consideration. To hunt planets, a telescope must detect faint objects and have high spatial resolution. Resolution allows the instrument to distinguish a planet from the star it orbits. The planned Large Binocular Telescope, the Near Infrared Camera and Multi-Object Spectrometer (NICMOS) due for installation on the Hubble Space Telescope in 1997, and the proposed Space Infrared Telescope Facility have the capacity to discover a variety of massive planets, the scientists note.

In particular, NICMOS can detect a planet 400 light-years away-- as distant as the Pleiades star cluster--if the body has at least six times Jupiter's mass and travels in a sufficiently wide orbit.

More generally, the team says that none of the new or upgraded telescopes will have the resolution to find a massive planet in the Pleiades unless the object lies at least as far from its parent as Neptune's separation from the sun. Infrared telescopes can more easily detect very massive planets because these objects generate enough heat to emit a copious amount of infrared light.

Scientists have noted that giant planets like Jupiter may be rare (SN: 4/22/95, p.251). But it still pays to look, Lunine says, because knowing the population of big planets "cuts to the heart of whether we will find inhabitable planets." At least in our solar system, large outer planets protect smaller ones, such as Earth, from bombardment by comets.