University of Texas Astronomers Confirm Planets Form from Disks Around Stars

9 October 2006

PASADENA, Calif. — Astronomers at The University of Texas at Austin have gone a long way toward proving that planets are born from disks of dust and gas that swirl around their home stars, confirming a theory posed by philosopher Emmanuel Kant more than two centuries ago.

G. Fritz Benedict and Barbara E. McArthur have used NASA’s Hubble Space Telescope, in collaboration with ground-based observatories, to demonstrate that Kant and scientists were correct in predicting the source of planet formation.

The results are being presented today in Pasadena, California at a meeting of the American Astronomical Society’s Division of Planetary Sciences, and will be published in the November issue of the Astronomical Journal.

Benedict and McArthur’s observations show for the first time that a known planet orbiting the nearby sun-like star Epsilon Eridani is aligned with the star’s circumstellar disk of dust and gas. The planet’s orbit is inclined 30 degrees to Earth, the same angle at which the star’s disk is tilted. Epsilon Eridani is 10.5 light-years from Earth in the constellation Eridanus.

The planets in our solar system share a common alignment, evidence that they were created at the same time in the Sun’s disk. But the Sun is a middle-aged star — 4.5 billion years old — and its debris disk dissipated long ago. Epsilon Eridani, however, still retains its disk because it is young, only 800 million years old.

The Hubble observations also helped Benedict’s team determine the planet’s true mass, which they calculate as 1.5 times Jupiter’s mass. Previous estimates measured only the lower limit, at 0.7 the mass of Jupiter. The planet, called Epsilon Eridani b, orbits its star every 6.9 years.

“Because of Hubble, we know for sure that it is a planet and not a failed star,” McArthur explained. “Some astronomers have argued that a few of the known extrasolar planets could be brown dwarfs because their precise masses are not known. If an object is less than 10 Jupiter masses, it is a planet, not a brown dwarf.

McArthur was part of a team at The University of Texas at Austin’s McDonald Observatory who discovered Epsilon Eridani b in 2000. They detected the planet using the radial velocity method, which measures a star’s subtle motion toward and away from Earth to find unseen companions.

Epsilon Eridani is a young and active star, so some astronomers claimed that what appeared as a planet-induced wobble of the star could have been the actions of the star itself. Turbulence in the atmosphere may have produced apparent velocity changes that were intrinsic to the star and not due to a planet’s influence.

The team calculated the planet’s mass and its orbit by making extremely precise measurements of the star’s location as it wobbled on the sky, a technique called astrometry. The slight wobbles are caused by the gravitational tug of the unseen planet, like a small dog pulling its master on a leash. Benedict’s team studied more than a thousand astrometric observations from Hubble collected over three years. The astronomers combined these data with other astrometric observations made at the University of Pittsburgh’s Allegheny Observatory. They then added those measurements to hundreds of ground-based radial-velocity measurements made over the past 25 years at McDonald Observatory, California’s Lick Observatory, the Canada-France-Hawaii Telescope in Hawaii and the European Southern Observatory in Chile. This combination allowed them to accurately determine the planet’s mass by deducing the tilt of its orbit.

If astronomers don’t know how a planet’s orbit is tilted with respect to Earth, they can only estimate a minimum mass for the planet. The planet’s mass could be significantly larger if the orbit were tilted to a nearly face-on orientation to Earth. The star would still move toward and away from Earth slightly, even though it had a massive companion.

“You can’t see the wobble induced by the planet with the naked eye,” Benedict said. “But Hubble’s fine guidance sensors are so precise that they can measure the wobble. We basically watched three years of a nearly seven-year-long dance of the star and its invisible partner, the planet, around their orbits. The fine guidance sensors measured a tiny change in the star’s position, equivalent to the width of a quarter 750 miles away.”

Epsilon Eridani has long captivated the attention of science fiction writers, as well as astronomers. In 1960, years before the first extrasolar planet was detected, astronomer Frank Drake listened for radio transmissions from inhabitants of any possible planets around Epsilon Eridani, as part of Project Ozma’s search for intelligent extraterrestrial life. In the fictional “Star Trek” universe, Epsilon Eridani is considered by some fans to be the parent star for the planet Vulcan, Mr. Spock’s home.

No Vulcan or any other alien could live on this gas giant planet. If moons circled the planet, they would spend part of their orbit close enough to Epsilon Eridani to have surface temperatures like that of the Earth, and possibly water. However, the planet’s looping, “roller-coaster” orbit also would carry the moons far away from the star, a distance equal to Jupiter’s 500-million-mile separation from the Sun, where oceans would freeze. If a moon were massive enough, like Saturn’s giant moon Titan, it could have a sufficiently dense atmosphere that would retain heat. Such an atmosphere would suppress wide swings in surface temperatures, like covering up with a heavy blanket on a cold night. This could make such a moon potentially habitable for life as we know it, Benedict said.

Although Hubble and other telescopes cannot image the gas giant planet now, they may be able to snap pictures of it in 2007, when its orbit is closest to Epsilon Eridani. The planet may be bright enough in reflected sunlight to be imaged by Hubble, other space-based cameras and large ground-based telescopes.

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Additional Contact Information: Donna Weaver, Space Telescope Science Institute, phone 410-338-4493, e-mail dweaver@stsci.edu.

Artist's concept of Epsilon Eridani, the nearest extrasolar planet to our solar system. (Credit: NASA, ESA & G. Bacon/STScI)

Click to download the high-resolution image of Epsilon Eridani's Tilt


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