Press release

The smallest known planet in the solar neighbourhood

The Spitzer space telescope detects the shadow of a ‘super-Earth' in front of a star nearby in the solar system in the zodiacal constellation of Cancer.

NASA's Spitzer telescope has detected the passage of a solid planet in front of a star situated only 42 light years away in the zodiacal constellation of Cancer. Thanks to this detection, astronomers now know that this ‘super-Earth' measures 2.1 times the size of the earth. It is the smallest known planet to date in the solar neighbourhood.

This discovery is based on data acquired by Spitzer last January. It has allowed astronomers to detect the planet's ‘transit', in other words a miniscule drop in the brightness of its host star which occurs when the planet finds itself between it and the Earth.

Up until now Spitzer's extraordinary capacities were vastly used to study known planets in transit, all of them being giant planets similar to Jupiter or Neptune. For the very first time Spitzer has been use to detect the transit of a super-Earth, a solid planet with more mass and larger than our planet,' comments Michaël Gillon, a FNRS researcher at the University of Liège and the astronomer at the head of the project which led to the discovery. ‘Thanks to Spitzer we now know the nature of this planet, and it turns out to be very different to all the planets of our solar system.'

The planet is called 55 Cnc e. With a size over twice as large as that of the Earth and a mass eight times greater, it is simply too big to be composed solely of rocks, and it must contain a significant fraction of volatile compounds. Its most probable composition is a rocky core and a thick layer made up essentially of frozen water. ‘We could say that this planet is a ‘naked Neptune,' given that it resembles Neptune beyond the fact that it does not have a hydrogen envelope surrounding its solid core,' tells us Brice-Olivier Demory of the Massachusetts Institute of Technology (MIT, USA), the main author of the publication describing the discovery. ‘But the analogy with Neptune stops there; 55 Cnc e is more than 1900 times closer to its star than Neptune is to the Sun. At such a close distance it is literally carbonised by the intense radiation of the star.'

This planet's host star has itself an interesting particularity for astronomers. It is not a star with low visibility seen only through a telescope, but on the contrary a very bright star very close to the Sun (42 light years) and visible with the naked eye. ‘The brightness of the star makes possible the first characterisation of a solid planet in orbit around a star similar to our Sun,' comments Michaël Gillon. ‘If this planet has an atmosphere we will be able to study it detail, which is not the case for the other solid planets discovered until now around stars which are further away and dimmer.'

An interesting fact: the study of the atmosphere of 55 Cnc e has maybe already begun. In effect, another team of astronomers announced last April 29 the independent detection of transits of the same planet with another space telescope, MOST. This team observed another wavelength than that of Spitzer, and measures a planetary radius 1.3 times smaller. ‘A possible explanation for this discrepancy could be the presence of a greater molecular envelope degassed by the planet, which would make the planet appear bigger for Spitzer,' tells us Brice-Olivier Demory. ‘Only new observations could corroborate this hypothesis.'

The era of the study of solid exoplanets has just begun.


Detection of a transit of the Super-Earth 55 Cnc e with Warm Spitzer, B.O. Demory, M. Gillon, D. Deming, et al., submitted for publication in Astronomy and Astrophysics,

Contact :

Michaël GILLON, chercheur FNRS,
Département d’Astrophysique, de Géophysique et d’Océanographie,
Tel +32 4 366 97 43,

Print version Page updated on 2011-06-15