Mystery find may turn out to be planet

Space: With Hubble's help, a Penn State astronomer may have seen for the first time a planet outside our solar system.

Medicine & Science

May 24, 2004|By Frank D. Roylance | Frank D. Roylance,SUN STAFF

Six years ago this week, California astronomer Susan Terebey presented a Hubble Space Telescope photo that appeared to be the first ever of a planet outside our solar system.

The National Aeronautics and Space Administration was skeptical. Such "exoplanets" had always been too dim, small and distant to be seen directly amid the glare of their parent stars.

But after a 90-minute "grilling" of Terebey and her data by independent scientists, NASA called a news conference to describe her unpublished discovery. Scientists unconnected to the research called Hubble's find "exciting," "a landmark" and "a watershed event."

A year later, the "planet" was shown to be no planet at all, but a distant background star that just happened to align with the double star system visible in the foreground.

Caution prevails

With that sort of history, Pennsylvania State University astronomer Steinn Sigurdsson was understandably cautious last week in describing the object he's found with Hubble, glowing beside a dim "white dwarf" star.

"What we see is in the right place, and faint enough to be a planet," he said. "There's definitely something there. ... But we're being careful about not claiming anything right now."

Sigurdsson's find has not been peer-reviewed or published. It was reported at a scientific meeting and picked up as a news report May 14 in the journal Nature.

He is seeking telescope time for more observations this year. Movement by the fast-traveling star in the coming months will reveal whether the mystery object is really a planet, moving in tandem with its star, or an unrelated background object that gets left behind.

In the meantime, he has declined to identify the star lest other astronomers beat him to the prize.

`Holding our breath'

Alan P. Boss, an astrophysicist at the Carnegie Institution in Washington unconnected to Sigurdsson's research, said previous surveys of scores of white dwarf stars like Sigurdsson's have turned up no planets.

"It would be a little bit astounding if these guys managed to find one," Boss said. But not impossible. "We're all holding our breath to see if it's real." If it's not, "hopefully everyone will remember that this wasn't claimed to be a planet."

Since 1995, astronomers have identified more than 120 giant exoplanets, but they have never seen one directly because a star's glare typically swamps its planet's feeble light.

They've only been revealed indirectly by the "wobble" of their parent stars as the planets circle around them, or by the periodic dimming of the star's light as planets orbit in front of them.

"If the object we found is a planet, we are seeing it directly ... sitting there glowing from its own light," Sigurdsson said.

Reducing glare

The Penn State team tackled the glare problem by searching out relatively dim white dwarf stars in nearby regions of our own Milky Way galaxy. White dwarfs are the glowing remnants of suns that have aged and exploded, extinguishing their thermonuclear fires.

Instead of being a billion times brighter than their planets, white dwarfs are only 10,000 times brighter.

To further reduce the blinding contrast between stars and planets, the astronomers observed them with Hubble's Near Infrared Camera, imaging their heat rather than their light.

They also masked the star's glare with a coronagraph, a black dot in Hubble's tool kit that blocks a star's glare like an automobile sun visor.

The strategy allowed three objects to emerge from the glare of three of the seven white dwarfs they targeted. Two appeared too large to be planets - more than 10 times the size of Jupiter. They're likely "brown dwarfs," failed companion stars too small to trigger nuclear fusion and light up.

It's `extremely red'

But the third one seemed promising. First, it's a point source, not a spread-out object like a background galaxy.

Second, Sigurdsson said, "this thing is extremely red." That suggests it's something very cold and close to the white dwarf star - likely a planet. Or it could be something very luminous and hot, but very far away, perhaps even a quasar far outside our galaxy. But sky surveys suggest the odds are 25-to-1 against a quasar turning up in that spot.

If it is a planet, he said, it appears to be a gas giant perhaps five times the size of Jupiter, orbiting as far from its star as Neptune is from our sun.

"It would be between 1 billion and 3 billion years old," Sigurdsson said, and a survivor of the blast that dimmed its star and incinerated any close-in terrestrial planets.

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