Exoplanet Research


EXOPLANET DETECTION — The transiting, steaming super-Earth 55 Cancri e

EXOPLANET CHARACTERIZATION — The large albedo of the hot Jupiter Kepler-7b

Since the startling discovery by M. Mayor and D. Queloz in 1995 of the first extrasolar planet orbiting a solar-type star, 51 Peg b, the field has been developing at a tremendous pace. As of March 14th, 2012, 760 exoplanets are known. These objects exhibit an extraordinary diversity: some are significantly larger than our Solar System’s Jupiter, while other are smaller than the Earth. A few are telluric, a handful may have the right temperature to allow liquid water to exist, while some are roasting at more than 2’000°C because of the proximity of their host star. And we are only seeing the tip of the iceberg, since recent studies, employing different observing techniques, show that planets are ubiquitous in our galaxy.

By employing ground-based (VLT, Magellan) and space-based (Spitzer, Kepler) observatories, we aim at detecting and characterizing these new worlds. Here are two results featuring our work.

Transit lightcurve of 55 Cancri e obtained by the Spitzer Space Telescope on January 6, 2011. The x-axis represents time in barycentric Julian days (BJD) and the y-axis is the relative flux from the star.

55 Cancri is one of the richest exoplanetary systems known to date. Five planets have been discovered orbiting this naked-eye star, located at 40 light years.

With an orbital period less than 18 hours, the fifth planet (55 Cancri e) is the smallest, innermost and latest discovered in this system. In 2011, 55 Cancri e was found to transit its host star (see here and here), providing a direct measurement of its size: about twice larger than the Earth.

Top: Kepler-7 b orbital phase curve. Transits occur at phase = 0.0 but are omitted on this figure. Bottom: Kepler-7 b phase-folded occultation light curve.

Since 55 Cancri e’s mass was previously known from spectroscopic Doppler surveys, the planetary size measured from these new transit observations provide an estimate of the planetary mean density. The figure on the left shows a transit of 55 Cancri e obtained with the Spitzer Space Telescope. Spitzer measured with a remarkable precision the dip in stellar flux occuring during a transit, when the planet passes in front of the star.

One possible interpretation of these results is that 55 Cancri e is an Earth-like nucleus, surrounded by an envelope of volatiles, possibly water. Because of its proximity to its sun, the temperature at 55 Cancri e’s orbital distance exceeds 2’000 °C.

Kepler-7b belongs to the first set of transiting planets discovered by the Kepler mission. Kepler-7b is called a «hot Jupiter» because of its large size and its high equilibrium temperature (1’600-1’800 °C). Being larger than our Jupiter by more than 50% and half massive, Kepler-7b is one of the least dense exoplanets discovered so far.

The figure on the left (top) reveals the planetary flux (y axis) isolated from the star, along its orbit. The occultation (bottom panel) occurs when the planet disappears behind its host star, providing a measure of the planetary light. About 100 orbits were observed to obtain this result.

The work presented here has been published in Astronomy and Astrophysics.

The work presented here has been published in the Astrophysical Journal Letters.

The direct mesurement of Kepler-7b’s planetary flux enabled a detailed study of its atmospheric properties, and more specifically regarding the origin of the planetary light. Our findings suggest that most of the observed flux is likely due to scattered light, resulting in a geometric albedo of ~0.3, similar to the Earth. Interestingly, most hot Jupiters are expected to be darker than coal in visible bandpasses, because of prominent atomic and molecular absorption occuring in their atmospheres. Kepler-7b seems to show that some hot Jupiters have much higher reflectivity, we still have to understand why.

Planet projected path

Star’s apparent brightness