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A team of ICE/IEEC astronomers announces the discovery of a possible terrestrial-type exoplanet orbiting a star in the constellation of Leo

2008-04-10
A dynamical analysis indicates that the new exoplanet orbits the star GJ 436 every 5.2 days and lies at a distance of 30 light years from the Earth

According to current models, the planet would be of terrestrial type, with a mass 5 times that of the Earth, making it the smallest found to date

A team of three scientists Ignasi Ribas, astronomer at the Spanish Research Council (CSIC) and the Institute for Space Studies of Catalonia (IEEC), Andreu Font-Ribera, PhD student at CSIC, and Jean-Philippe Beaulieu astrophysicist at the Institut d’Astrophysique de Paris and visiting Scientist at University College of London, have predicted the existence of a new exoplanet orbiting a star in the constellation of Leo, at 30 light years from the Earth. The simulations and models indicate that the exoplanet, dubbed GJ 436c [because it obits the star GJ 436, which is number 436 of the catalogue of Gliese and Jahreiss], is of terrestrial type and has a mass of five times that of the Earth. The study appears on the latest issue of the Astrophysical Journal Letters.

Ignasi Ribas, the lead author of the study points out the importance of the new discovery: “After final confirmation, the new exoplanet will be the smallest found to date with a frozen super Earth detected by microlensing. It is the first one to be identified from the perturbations exerted on another planet of the system. Because of this, the study opens a new path that should lead to the discovery of even smaller planets in the near future, with the goal of eventually finding worlds more and more similar to the Earth”.

The researchers have succeeded in explaining the existence of the exoplanet from dynamical simulations. The study predicts the presence of a small exoplanet that perturbs an inner planet (already known) producing changes on its orbit. A re-analysis of archival radial velocities has also permitted the identification of a signal that perfectly matches the simulations and corresponds to a planet in resonance with the inner one, meaning that for every two orbits of the known planet the new planet completes one.

“While the Earth employs 365 days to cover a full orbit around its star, the Sun, the new exoplanet orbits its host star, GJ 436, in only 5.2 Earth days. Also, and according to current theory, while the Earth completes a revolution in 24 hours, this planet may complete it in 4.2 Earth days”, states Ribas.

In the Earth, a full day, the time lapse from two consecutive sunsets, coincides quite closely with the rotation period. In the new planet these two periods do not coincide, since the orbital translation period and the rotation period are very similar. For this reason, a full day in the planet, this is, the time between two consecutive sunsets, occurs in 4 planetary years, which corresponds to 22 Earth days.

“The calculations indicate that the temperature of the planet is within 400-700 K, but it can locally be as low as 350 K at the poles” remarks Giovanna Tinetti (University College of London) who recently calculated the putative properties of this planet.

Most of the approximately 280 planets discovered to date are gas giants similar to our Jupiter, although some with masses below 10 times that of the Earth have already been found. This kind of planets, with masses between 1 and 10 Earth masses, are often dubbed Super-Earths. In this case, current models predict that they are of rocky type and that GJ 436c has a radius some 50% larger than the Earth.

“This is the fourth super-Earth planet. The previously discovered planets of this class are the frozen super Earth OGLE-2005-BLG-390lb detected by microlensing, and the two hot super-Earths Gl 581b and Gl 876d detected by their Doppler wobble,“ adds Beaulieu.

A peculiar planetary system


In 2004, a team of American astronomers studied the star GJ 436 using the radial velocity technique [velocity of an object along the line of sight] and found a planet around it. The planet, of gaseous type and with a mass 22 times that of the Earth, is close to the star and completes an orbit in 2.6 days, which happens to be quite elongated and constitutes somewhat of a surprise.

In mid 2007 a Swiss group detected the existence of transits caused by this planet [seen from the Earth as a dimming of the light of the star as the planet occults part of its disc]. This fact had not been observed in the previous analysis. The team led by Ribas studied these two surprising characteristics, namely its elliptical orbit and the absence of transits in 2004 and subsequent discovery in 2007.

“The computer simulations predicted the existence of a small planet orbiting GJ 436 that is responsible for perturbing the inner planet and altering its orbit. In addition, we studied available data from radial velocities and found the possible weak signal of a planet that closely matches the predictions of the simulations”, points out the Spanish astronomer. “Observations with the European Southern Observatory VLT telescope will help us to confirm the presence of this planet soon”.

A similar system was used in the 19th century to detect the Solar System planet Neptune. Between 1843 and 1845, astronomers Urbain Le Verrier and John Adams studied the motion of Uranus and detected deviations that they attributed to the perturbation exerted by another planet. A year later, thanks to the predictions, Johann Gottfried Galle discovered Neptune.

Together with Ignasi Ribas, the study is co-authored by Andreu Font-Ribera, from the Spanish CSIC, and by Jean-Philippe Beaulieu, from the Institute d’Astrophysique de Paris (France) and University College London (UK). This research was partly supported by the HOLMES project (Hunting for Low Mass Extrasolar planets);

Contacts :
I. Ribas , iribas@ieec.uab.es
JP Beaulieu, beaulieu@ iap.fr Mobile +33 6 03 98 73 11