University of Pittsburgh
October 8, 2006

Pitt Measurements of Jupiter-size Planet Confirmed by Hubble

In 2000, Allegheny Observatory estimated mass of new planet, nearest to our solar system
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PITTSBURGH-NASA's Hubble Space Telescope, in collaboration with the University of Pittsburgh's Allegheny Observatory and other ground-based observatories, has provided definitive evidence for the existence of the nearest planet outside our solar system. The results are being presented today in Pasadena, Calif., at a meeting of the American Astronomical Society's Division of Planetary Sciences and will appear in the November issue of the Astronomical Journal.

The planetary system around the star Epsilon Eridani was first detected in 1988 by a Canadian team led by Bruce Campbell of the University of British Columbia. In 2000, it was confirmed by Barbara McArthur of the University of Texas at Austin using radial velocity measurements that were interpreted as a wobble in the motion of Epsilon Eridani caused by the gravitational tug of an unseen planet.

University of Pittsburgh Professor of Physics and Astronomy George Gatewood strengthened this result the same year. Using astrometric data from the Allegheny Observatory, which he directs, he estimated the mass of the planet to be 1.2 times that of Jupiter.

Some astronomers wondered whether the turbulent motion of the young star's atmosphere was merely mimicking the effects of the star being nudged by a planet's gravitational pull. The new Hubble observations, made by a team led by McArthur and G. Fritz Benedict of the University of Texas at Austin, settle any uncertainty, calculating the planet's mass as 1.5 times Jupiter's, within the range calculated by Gatewood.

The planet orbits the Sun-like star Epsilon Eridani, which is 10.5 light-years away (approximately 63 trillion miles)-so close it may be observable by Hubble and large ground-based telescopes in late 2007.

Hubble also found that the planet's orbit is tilted 30 degrees to our line of sight, which is the same inclination as a disk of dust and gas that also encircles Epsilon Eridani. The researchers feel this is a particularly remarkable result because, although it has long been inferred that planets form from such disks, this is the first time that the two objects have been observed around the same star.

The research team emphasized that the alignment of the planet's orbit with the dust disk provides compelling direct evidence that planets are created from disks of gas and dust debris around stars. 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, is a young 800 million years old and still retains its disk.

The Benedict-McArthur team calculated the planet's mass and its orbit by making extremely precise measurements of subtle changes in the star's location in the sky, a technique called astrometry. The slight variations are unmistakably caused by the gravitational tug of the unseen companion object. Benedict's team studied more than a thousand astrometric observations from Hubble collected over three years.

The astronomers combined these data with Gatewood's observations made at the Allegheny Observatory. They then added those measurements to hundreds of ground-based radial-velocity measurements made over the past 25 years at McDonald Observatory at the University of Texas, Lick Observatory at the University of California Observatories, 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.

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 starlight to be imaged by Hubble, other space-based cameras, and large ground-based telescopes.

The Hubble Space Telescope is an international cooperative project between NASA and the European Space Agency. The Space Telescope Science Institute is operated for NASA by the Association of Universities for Research in Astronomy, Inc., in Washington, D.C.

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