Fourteen Times the Earth - Smallest Ever Extra-Solar Planet Discovered

A European team of astronomers has discovered the lightest known planet orbiting a star other than the sun (an "exoplanet"). The new exoplanet orbits the bright star mu Arae located in the southern constellation of the Altar. It is the second planet discovered around this star and completes a full revolution in 9.5 days. With a mass of only 14 times the mass of the Earth, the new planet lies at the threshold of the largest possible rocky planets, making it a possible super Earth-like object. Uranus, the smallest of the giant planets of the Solar System has a similar mass. However Uranus and the new exoplanet differ so much by their distance from the host star that their formation and structure are likely to be very different.

This discovery was made possible by the unprecedented accuracy of the HARPS spectrograph on ESO's 3.6-m telescope at La Silla, which allows radial velocities to be measured with a precision better than 1 m/s. It is another clear demonstration of the European leadership in the field of exoplanet research.

A unique planet hunting machine
The continued quest for exoplanets requires better and better instrumentation. In this context, ESO undoubtedly took the leadership with the new HARPS spectrograph (High Accuracy Radial Velocity Planet Searcher) of the 3.6-m telescope at the ESO La Silla Observatory. Offered in October 2003 to the research community in the ESO member countries, this unique instrument is optimized to detect planets in orbit around other stars ("exoplanets") by means of accurate (radial) velocity measurements with an unequalled precision of 1 metre per second.

HARPS was built by a European Consortium in collaboration with ESO. Already from the beginning of its operation, it has demonstrated its very high efficiency. By comparison with CORALIE, another well known planet-hunting optimized spectrograph installed on the Swiss-Euler 1.2-m telescope at La Silla, the typical observation times have been reduced by a factor one hundred and the accuracy of the measurements has been increased by a factor ten.

These improvements have opened new perspectives in the search for extra-solar planets and have set new standards in terms of instrumental precision.

Mu Arae planetary system

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 harbour 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.

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."

New planet
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".

This research described in this 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/

Related stories:

Researchers Say Tides Can Cut Life Short On Planets Orbiting Smaller Stars
(PhysOrg.com) -- Planet hunters searching for planets suitable for life will likely find them first around low-mass stars because it's technically easier than finding such planets around hotter, more massive stars, researchers predict.
Beta Pictoris planet finally imaged?
(PhysOrg.com) -- The hot star Beta Pictoris is one of the best-known examples of stars surrounded by a dusty 'debris' disc. Debris discs are composed of dust resulting from collisions among larger bodies like planetary embryos or asteroids. They are a bigger version of the zodiacal dust in our Solar System. Its disc was the first to be imaged — as early as 1984 — and remains the best-studied system. Earlier observations showed a warp of the disc, a secondary inclined disc and infalling comets onto the star. "These are indirect, but tell-tale signs that strongly suggest the presence of a massive planet lying between 5 and 10 times the mean Earth-Sun distance from its host star," says team leader Anne-Marie Lagrange. "However, probing the very inner region of the disc, so close to the glowing star, is a most challenging task."
Submillimeter Eagle Eyes on Mauna Kea
(PhysOrg.com) -- Three observatories on Mauna Kea have come together to form the world's most powerful facility for detailed submillimeter imaging. An exploratory project, the Extended Submillimeter Array (eSMA) will explore the Universe using light that the human eye cannot see, at wavelengths around 0.8 millimeter. There are many objects in the sky that emit radiation in this submillimeter range, especially the dusty regions in which new stars, planets and even entire galaxies are being born. These clouds of gas and tiny dust particles are completely dark in visible light, but submillimeter waves can penetrate them.
Scientists discover new planet orbiting dangerously close to giant star
A team of astronomers from Penn State and Nicolaus Copernicus University in Poland has discovered a new planet that is closely orbiting a red-giant star, HD 102272, which is much older than our own Sun. The planet has a mass that is nearly six times that of Jupiter, the largest planet in our solar system.
Hubble directly observes planet orbiting Fomalhaut
(PhysOrg.com) -- Estimated to be no more than three times Jupiter's mass, the planet, called Fomalhaut b, orbits the bright southern star Fomalhaut, located 25 light-years away in the constellation Piscis Austrinus (the Southern Fish).
First fuzzy photos of planets outside solar system
(PhysOrg.com) -- Astronomers using the Gemini North telescope and W.M. Keck Observatory on Hawaii's Mauna Kea have obtained the first-ever direct images identifying a multi-planet system around a normal star.
Solar System's Young Twin Has Two Asteroid Belts
(PhysOrg.com) -- Astronomers have discovered that the nearby star Epsilon Eridani has two rocky asteroid belts and an outer icy ring, making it a triple-ring system. The inner asteroid belt is a virtual twin of the belt in our solar system, while the outer asteroid belt holds 20 times more material. Moreover, the presence of these three rings of material implies that unseen planets confine and shape them.
Computer Simulations Reveal Exotic Weather on Distant Worlds
(PhysOrg.com) -- Computer simulations of the atmospheric circulation on Jupiter-like planets around other stars can explain temperature observations of these planets and shed light on the exotic weather experienced by these far-away worlds.

News discussion:

Space & Earth science news