Extrasolar Planets





Mercury News Staff Writer


Observation expected to help scientists learn


For the first time, scientists have watched a planet outside our solar system cross in front of its home star -- a  milestone that will allow them to learn more about the nature  of these distant planets than ever before possible, including  their exact size and weight.


``It's a moment I don't think I'll ever forget,'' said  astrophysicist Geoffrey Marcy of the University of California- Berkeley, leader of the team that announced the discovery on  Friday.


``Even now, describing it to you, I can feel the hairs popping up on the back of my neck,'' he said. ``With all the planet discoveries, this is the first time we're almost 100 percent sure that we have a planet and we know everything about it.''


Jack Lissauer, a planetary scientist at NASA's Ames Research Center in Mountain View, said Friday's announcement, which went out on an e-mail bulletin to astronomers all over the world, ranks with the discovery last April of the first solar system outside our own.




The planet, which has no formal name, goes around the star HD 209458 every 3 1/2 days, orbiting so closely that it must be extremely hot -- in the neighborhood of 3,600 degrees.


It's the 29th planet discovered so far outside our solar system, 19 of which have been found by the Marcy team.  Until now these planets were detected only indirectly, by observing how their gravity tugged their home stars rhythmically back and forth. Using the Keck Telescope in Hawaii, Marcy and his colleagues had been measuring such a wobble in HD 209458 -- a star that's 153 light years (859,000 billion miles) away in the constellation Pegasus.


But this method yields only limited information. Astronomers can determine the planet's minimum weight, or mass. They can tell how closely and how fast it orbits. But they can't determine how big it is -- and without an exact size and mass, they can't calculate its density or tell much about its  character.


About one out of 20 planets, though, should be in an orbit  that happens to be perfectly aligned from the perspective of a scientist on Earth. It's like a spinning phonograph record viewed edge-on, Marcy said, so that the planet passes directly in front of its home star on each orbit. Each time  this happens, the starlight dims ever so slightly.



 Eight days ago, after confirming the existence of a planet around HD 209458, Marcy passed the information along to Greg Henry of Tennessee State University, a leading expert in the measurement of star brightness.


On Tuesday, with a robotic telescope in Arizona operated remotely from his office, Henry found that the star's light  did, indeed, fade by just 1.7 percent.


It happened at exactly the time Marcy had predicted from his observations, Henry said, ``confirming absolutely the presence of a companion. We've essentially seen the shadow of the planet.''


With the new data, the team calculated the size and mass of the planet and came up with another surprise: Although the planet weighs about two-thirds as much as Jupiter, it's 60 percent wider.


Apparently it's been puffed out and bloated by the extreme heat of the star so close by, Marcy said -- a phenomenon that had been predicted by scientists at the University of Arizona.


``We've been looking for such a thhing for a fairly long  time,' said Adam Burrows, one of the group that predicted the bloating effect.


``We want to be able to, as theorists and scientists, understand the character of these objects,'' he said.  ``Getting their radius and mass is a grreat milestone in the  study of these beasts.''


NASA Ames' Lissauer has been working with William Borucki, a planetary astronomer at Ames, to demonstrate a way to look for Earth-sized planets from a space telescope by watching them cross in front of stars.


They've set up a small, earthbound version of the system at Lick Observatory on Mount Hamilton east of San Jose, where they have been looking for larger planets -- so far, without success.


But on Sunday night, Lissauer said, they will train the telescope on HD 209458 and try to confirm the dimming of the star -- a confirmation that would also show their own method works.




Lissauer said the chances are better than 90 percent that the crossing of the planet in front of HD 209458 will be independently verified -- removing any speculation that it might be a failed star or some other object, as skeptics have suggested for some of the planet discoveries.


``To the naysayers that claim these things are not planets,''  he said, ``it just knocks that completely away.''


Contact Glennda Chui at gchui@s... or

(408) 920-5453.



Published Saturday, November 13, 1999,

in the San Jose Mercury News