McDonald Observatory Astronomers Discover Neptune-Sized Planet with Hobby-Eberly Telescope

31 August 2004

Austin, Texas —A team of astronomers led by Barbara McArthur, and including Michael Endl, William Cochran and Fritz Benedict, of The University of Texas at Austin's McDonald Observatory has used the Hobby-Eberly Telescope (HET) and its High Resolution Spectrograph to discover a very small planet orbiting a nearby star known as rho1 Cancri (also called 55 Cancri).

Astronomers already knew that three planets, with periods of 14.6, 44 and 4520 days, orbit rho1 Cancri, a star about the same size as the Sun. This new planet is closer to the star than the other three. Named rho1 Cancri e, it orbits the star every 2.8 days at a distance of only 0.038 Astronomical Units (3,534,000 miles). This makes the system the first known extrasolar four-planet system. The research has been accepted for publication in Astrophysical Journal Letters.

The highlight of the work is the fact that the newly discovered planet, rho1 Cancri e, has a minimum mass of only 14 Earth masses, and a most likely mass of just 18 Earth masses — about the mass of Neptune. It is the lowest mass planet that has been discovered. Most of the 120 or so known extrasolar planets are Jupiter-sized. (At about 300 times the mass of Earth, Jupiter is the most massive planet in our solar system.) This discovery with the HET indicates that with improved instrumentation astronomers are coming closer to finding extrasolar Earths.

McArthur's quest began when she decided to reanalyze archival Hubble Space Telescope (HST) data to look for the motion of the star on the sky as it orbits the center of gravity of the star and its planets — a technique known as "astrometry." Thanks to the exquisite sharpness of HST's Fine Guidance Sensors, the anticipated motion of the star was detected. Thomas Harrison, an astronomer from New Mexico State University, provided supporting observations of stars in the HST field of view.

As she and colleague Benedict have done with other stars, McArthur decided to collect "radial velocities" of rho1 Cancri to complement the Hubble data. Radial velocity observations involve measurements of changes in a star’s velocity toward and away from Earth — its wobble. Astronomers from the California and Carnegie Planet Search  team and Geneva Observatory contributed radial velocity data. McArthur, seeking highly precise, intensive data, then collaborated with McDonald Observatory astronomers Endl and Cochran to make radial velocity observations of rho1 Cancri with the Hobby-Eberly Telescope in West Texas.

"These data were taken very quickly," McArthur said. "In 180 days, we got over 100 observations of the star. This is amazing access to a high precision instrument," she said, referring to the HET's queue-scheduling operation.

Astronomers do not travel to the observatory to operate the telescope themselves. Rather, a resident astronomer at McDonald Observatory has a list of all HET research projects and selects the ones best suited to any given night's weather conditions and Moon phase. This way, many targets for different research projects can be observed each night, and any particular target can be observed dozens of night in a row. McArthur said getting the data over a short period of time helps reduce introduction of errors that can build up over months or years due to changes in a telescope, its instrumentation or even the star system under study.

Combining the new HET data with earlier data from the other planet search teams and with archival data from NASA's Hubble Space Telescope, McArthur was able to determine the size, shape and orientation of the orbit of the outer planet (rho1 Cancri d). Next, McArthur was able to model the inner planets, revealing a fourth planet in this system. On the assumption that rho1 Cancri's planets all lie in the same plane, as planets do in our solar system, the mass of the newly discovered planet was found to be a mere 18 times that of the Earth, equal to the mass of Neptune. Because of the HST astrometry measurements, the mass of this planet is a true mass, not the lower limit usually reported by radial velocity-alone techniques.

"This discovery is a leap forward into a new domain of extrasolar planets," Endl said. "Finally, we find planets with masses that probably mean that they resemble more our Neptune or Uranus, which consist mostly of a rocky/ice core and a small gaseous envelope.";

"This planet detection demonstrates that the HET is an absolutely wonderful planet-hunting machine," Cochran added. "The combination of the queue-scheduled operation, which allows us to get data when we need it, and the exquisite spectrograph, which gathers outstanding data, makes the HET uniquely suited for this task."

"It's a remarkable discovery," said McDonald Observatory Director David L. Lambert. "It's taking extrasolar planet discoveries to a new dimension. This challenges theoretical understanding of how planets form and evolve around stars like our Sun. I am proud of the team."

The complete list of authors for this paper include: McArthur, Endl, Cochran and Benedict, McDonald Observatory, The University of Texas at Austin; Debra A. Fischer and Geoffrey W.  Marcy, Department of Astronomy, University of California, Berkeley; R. Paul Butler, Department of Terrestrial Magnetism, Carnegie Institution of Washington; D. Naef, M. Mayor, D. Queloz, and S. Udry, Observatoire de Geneve, Sauverny, Switzerland; and Harrison, Department of Astronomy, New Mexico State University.

The Hobby-Eberly Telescope is a joint project of The University of Texas at Austin, The Pennsylvania State University, Stanford University, Ludwig-Maximilians-Universität München and Georg-August-Unversität Göttingen. The researchers would like to thank the following people with McDonald Observatory who made this discovery possible: Phillip J. MacQueen, Robert G. Tull, John Good, John Booth, Matthew Shetrone, Brian Roman, Stephen Odewahn, Frank Deglman, Michelle Graver, Michael Soukup, Martin L. Villarreal Jr.


Notes to editors: Additional graphics are available from NASA's Jet Propulsion Laboratory. More information on this discovery is available online at:

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