The planetary system around mu Arae The star mu Arae is about 50 light years away. This solar-like star is located in the southern constellation Ara (the Altar) and is bright enough (5th magnitude) to be observed with the unaided eye. Mu Arae was already known to harbor a Jupiter-sized planet with a 650 days orbital period. Previous observations also hinted at the presence of another companion (a planet or a star) much further away.
HARPS radial velocity measurements phase-folded with the orbital period of the newly found exoplanet (9.5 days). The measurements have been corrected from the effect of the two longer period companions. The semi-amplitude of the curve is less than 5 m/s! Coupled with the 9.5 days orbital period, this implies a minimum mass for the newly discovered planet of 14 times the mass of the Earth. The new measurements obtained by the astronomers on this object, combined with data from other teams confirm this picture. But as François Bouchy, member of the team, states, "Not only did the new HARPS measurements confirm what we previously believed to know about this star but they also showed that an additional planet on short orbit was present. And this new planet appears to be the smallest yet discovered around a star other than the sun. This makes mu Arae a very exciting planetary system."
"Listening" to the star During 8 nights in June 2004, mu Arae was repeatedly observed and its radial velocity measured by HARPS to obtain information on the interior of the star. This so-called astero-seismology technique (see ESO PR 15/01) studies the small acoustic waves which make the surface of the star periodically pulsate in and out. By knowing the internal structure of the star, the astronomers aimed at understanding the origin of the unusual amount of heavy elements observed in its stellar atmosphere. This unusual chemical composition could provide unique information to the planet formation history.
Says Nuno Santos, another member of the team: "To our surprise, the analysis of the new measurements revealed a radial velocity variation with a period of 9.5 days on top of the acoustic oscillation signal!"
This discovery has been made possible thanks to the large number of measurements obtained during the astero-seimology campaign. From this date, the star, that was also part of the HARPS consortium survey program, was regularly monitored with a careful observation strategy to reduce the "seismic noise" of the star.
These new data confirmed both the amplitude and the periodicity of the radial velocity variations found during the 8 nights in June. The astronomers were left with only one convincing explanation to this periodic signal: a second planet orbits mu Arae and accomplishes a full revolution in 9.5 days.
But this was not the only surprise: from the radial velocity amplitude, that is the size of the wobble induced by the gravitational pull of the planet on the star, the astronomers derived a mass for the planet of only 14 times the mass of the Earth! This is about the mass of Uranus, the smallest of the giant planets in the solar system. The newly found exoplanet therefore sets a new record in the smallest planet discovered around a solar type star.
At the boundary The mass of this planet places it at the boundary between the very large earth-like (rocky) planets and giant planets. As current planetary formation models are still far from being able to account for all the amazing diversity observed amongst the extrasolar planets discovered, astronomers can only speculate on the true nature of the present object. In the current paradigm of giant planet formation, a core is formed first through the accretion of solid "planetesimals". Once this core reaches a critical mass, gas accumulates in a "runaway" fashion and the mass of the planet increases rapidly. In the present case, this later phase is unlikely to have happened for otherwise the planet would have become much more massive.
Furthermore, recent models having shown that migration shortens the formation time, it is unlikely that the present object has migrated over large distances and remained of such small mass. This object is therefore likely to be a planet with a rocky (not an icy) core surrounded by a small (of the order of a tenth of the total mass) gaseous envelope and would therefore qualify as a "super-Earth".
Further prospects The HARPS consortium, led by Michel Mayor (Geneva Observatory, Switzerland), has been granted 100 observing nights per year during a 5-year period at the ESO 3.6-m telescope to perform one of the most ambitious systematic searches for exoplanets so far implemented worldwide. To this aim, the consortium repeatedly measures velocities of hundreds of stars that may harbor planetary systems.
The detection of this new light planet after less than 1 year of operation demonstrates the outstanding potential of HARPS for detecting rocky planets on short orbits. Further analysis shows that performances achieved with HARPS make possible the detection of big "telluric" planets with only a few times the mass of the Earth. Such a capability is a major improvement compared to past planet surveys. Detection of such rocky objects strengthens the interest of future transit detections from space with missions like COROT, Eddington and KEPLER that shall be able to measure their radius.
More information The research described in this Press release has been submitted for publication to the leading astrophysical journal, Astronomy and Astrophysics. A preprint is available as a postscript file at http://www.oal.ul.pt/~nuno/.
Notes  The team is composed of Nuno Santos (Centro de Astronomia e Astrofisica da Universidade de Lisboa, Portugal), François Bouchy and Jean-Pierre Sivan (Laboratoire d'astrophysique de Marseille, France), Michel Mayor, Francesco Pepe, Didier Queloz, Stéphane Udry, and Christophe Lovis (Observatoire de l'Université de Genève, Switzerland), Sylvie Vauclair, Michael Bazot (Toulouse, France), Gaspare Lo Curto and Dominique Naef (ESO), Xavier Delfosse (LAOG, Grenoble, France), Willy Benz and Christoph Mordasini (Physikalisches Institut der Universität Bern, Switzerland), and Jean-Louis Bertaux (Service d'Aéronomie de Verrière-le-Buisson, Paris, France).
 A fundamental limitation of the radial-velocity method is the unknown of the inclination of the planetary orbit that only allows the determination of a lower mass limit for the planet. However, statistical considerations indicate that in most cases, the true mass will not be much higher than this value. The mass units for the exoplanets used in this text are 1 Jupiter mass = 22 Uranus masses = 318 Earth masses; 1 Uranus mass = 14.5 Earth masses.
 HARPS has been designed and built by an international consortium of research institutes, led by the Observatoire de Genève (Switzerland) and including Observatoire de Haute-Provence (France), Physikalisches Institut der Universität Bern (Switzerland), the Service d'Aeronomie (CNRS, France), as well as ESO La Silla and ESO Garching.
Read the original news release at http://www.eso.org/outreach/press-rel/pr-2004/pr-22-04.html.
Additional articles on this subject are available at:
25 August 2004
David Morrison is the senior scientist for the NASA Astrobiology Institute (NAI), an international research consortium of more than a dozen universities and space research centers. Astrobiology Magazine had the opportunity to talk with David Morrison about how astrobiology has changed since its conception as a scientific discipline a decade ago. Morrison was recently honored by the world's largest organization of planetary scientists—the Division for Planetary Sciences (DPS). The DPS awarded its 2004 Carl Sagan Medal to Morrison, a former student of Sagan's. The Sagan Medal is awarded annually to an active DPS member and researcher for their long-term excellence in communicating planetary science to the public. Morrison will receive the award at the organization's annual meeting to be held November 8-12, 2004, in Louisville, KY.
"We are honored by David's award," said G. Scott Hubbard, director of NASA Ames Research Center, Moffett Field, CA. "A doctoral student of Carl Sagan, David is that rare breed of scientist who combines research depth with the ability to popularize technical topics to non-scientists."
Morrison has been instrumental in illuminating the scientific basis for potential hazards due to asteroid and comet impacts, through refereed papers and popular articles and books. He created and implemented the impact hazard web site. Throughout his distinguished science career—as an expert on solar system small bodies and as an investigator for numerous spacecraft missions, including Voyager and Galileo—Morrison has dedicated himself to sharing the excitement of planetary exploration. In his testimony to the President's blue ribbon commission "Moon to Mars and Beyond", Morrison spoke to the limits of and opportunities for scientists as "we are in that transition from being citizens of planet Earth to being citizens of the solar system."
Astrobiology Magazine (AM): In early July, the Europeans proposed what they called the "Don Quixote" mission profile for asteroid mitigation. Their scenario entails detecting a dangerous asteroid on a terrestrial collision course, then intercepting the incoming rock with a scout probe followed by a destroyer probe. For the mission, the Europeans are soliciting international partners. Do you think this seek-and-destroy profile has a consensus among international partners as the best approach—or are there alternatives that merit consideration?
David Morrison (DM): The proposed Don Quixote mission, like the NASA Deep Impact comet mission, will certainly add to our knowledge about asteroids and comets—knowledge that will be needed if we ever have to defend against an impact. Today we know far too little to try for a consensus as to how we would deflect an asteroid. One other approach under study is gradual deflection using a low-thrust ion engine, as proposed by the B612 Society for a follow-on Prometheus mission. Furthermore, different targets may well require different technologies to deal with them. A number of interesting ideas that have been suggested, but not much of the hard work to actually develop these approaches has been done.
Left: fragments of Comet P/Shoemaker-Levy 9 colliding with Jupiter (July 16-24, 1994). Image Credit: NASA. Right: HD 28185b is the first exoplanet discovered with a circular orbit within its star's habitable zone. Image credit: STScI Digitized Sky Survey.