Washington, D.C. "Our ability to find planets in the Neptune-mass range tips the scales for finding other Earths sooner rather than later," remarked R. Paul Butler, lead author of one* of two papers announcing the discoveries of Neptune-mass planets--the smallest planets confirmed to date. Butler, with the Department of Terrestrial Magnetism at the Carnegie Institution of Washington, is also a contributing author to a second paper** announcing the detection of another Neptune-massed planet. Both papers have been accepted to the Astrophysical Journal and the announcement of the discoveries was formally made at a NASA science update briefing today (http://planetquest.jpl.nasa.gov/webcasts/ssu_0804.html).

Butler and Geoffrey Marcy of U.C. Berkeley lead the team that has found most of the extrasolar planets to date. The newest planet found by the team orbits a nearby, low-mass, 3-billion-year-old M-dwarf" star named Gliese 436 in the Leo constellation, 33 light-years away (1 light-year is 5.8 trillion miles) . "Up until now, the technology has limited planet detection to those in the Jupiter and Saturn-mass range," said Butler. "We've entered a new era for planet hunting." The second planet orbits 55 Cancri, a Sun-like star about 41 light-years from Earth; three other known planets orbit that star.

31 August 2004: Detection of a Neptune-mass planet in the rho Cancri system using the Hobby-Eberly Telescope

31 August 2004: A Neptune-Mass Planet Orbiting the Nearby M Dwarf GJ 436

These two newest planets join a growing roster of some 135 exoplanets, so-called because they are outside of our solar system. Both new Neptunes are very close to their parent stars, orbiting once every 2 to 3 days, and have similar likely masses, estimated to be between about 17 and 21 Earths--close to Neptune's mass.

The Butler/Marcy team are engaged in a multi-year project monitoring the telltale wobbles of about 2,000 Sun-like stars within 150 light-years of our solar system. Stellar wobbles can indicate orbiting planets. The team uses the precision Doppler velocity technique, which they developed, to detect these movements. By refining their methods, they are the first to detect planets with less mass than Saturn.

Only one other planetary system has been found around an M Dwarf star. Of the stars the team is measuring, 150 are M dwarfs. They monitored Gliese 436 with the Keck telescope in Hawaii from January 2000, to July 2004, and determined that the planet orbited every 2.64 days, has a mass of 1.2 Neptune masses, and is about 2.6 million miles from the star ( .028 astronomical units) --close enough to sear the surface at 650 deg F.

So what kind of planet is it and how did it evolve? The fact that it's so close to its star suggests it may be gravitationally locked so as to show only one face to its sun. If it were a gaseous planet like Jupiter or had a thick atmosphere like Venus, the heat would build up, keeping the entire planet very hot. If it's a rocky world, then the back side would remain dark and frozen. For now, its composition, origin, and whether it formed close to its star or migrated there will remain a mystery.

However, a pattern is beginning to emerge that shows a correlation between star types and planet mass. Giant Jupiter-like planets, thus far are rarely found around low-mass M dwarf stars--the authors noted in their paper. But 41 high-massed planets have been found orbiting within 1 astronomical unit, or the distance between Earth and our Sun, around higher-mass F, G, and K stars. "I expect that in the next few years we'll be able to tell with some certainty what kind of stars are more likely to harbor different types of planets,' commented Alan Boss, a planet-formation theorist also with the Carnegie Institution, but not involved in the research. "We've just made a huge leap toward finding planets that look like our own."

*Authors of "A Neptune-Mass Planet Orbiting the Nearby M Dwarf GL 436,"

to appear in the Astrophysical Journal, are R. Paul Butler, Carnegie Institution of Washington; Steven Vogt, UC Santa Cruz; Geoffrey Marcy and Debra Fischer, UC Berkeley/ San Francisco State University; Jason Wright, UC Berkeley; Gregory Henry, Tennessee State University; Greg Laughlin, UC Santa Cruz; and Jack Lissauer, Ames Research Center. The research was funded by NASA and the National Science Foundation.

** Authors of "Detection of a Neptune-mass planet in the 55 Cancri system using the Hobby-Eberly Telescope," to appear in the Astrophysical Journal, are Barbara McArthur, Michael Endl, William Cochran and G. Fritz Benedict of the McDonald Observatory, University of Texas, Austin; Debra Fischer and Geoffrey Marcy, UC Berkeley/ San Francisco State University; R. Paul Butler, Carnegie Institution of Washington; D. Naef, M. Mayor, D. Queloz, and S. Udry, Observatoire de Geneve; and T. E. Harrison, New Mexico State University. The research was funded by NASA and the National Science Foundation.

The Carnegie Institution of Washington (www.CarnegieInstitution.org) has been a pioneering force in basic scientific research since 1902. It is a private, nonprofit organization with six research departments in the U.S.: Embryology, in Baltimore, MD; The Observatories in Pasadena, CA, and Chile; and Plant Biology and Global Ecology in Stanford, CA; the Geophysical Laboratory and the Department of Terrestrial Magnetism in Washington, DC.

Contact

Dr. Paul Butler at Carnegie's Department of Terrestrial Magnetism (DTM), 202- 478- 8866, email Paul@dtm.ciw.edu;

or

Dr. Geoff Marcy at UC Berkeley, 510-642-1952, email gmarcy@stars.sfsu.edu.

For a copy of the papers contact the Astrophysical Journal at 520 621-5145, email apj@as.arizona.edu