First Solid Evidence for a Rocky Exoplanet

On September 16, the European Southern Observatory (ESO) announced they had the first solid evidence of a rocky exoplanet. I can hear your queries. Firstly, you ask, what is an exoplanet. And secondly, I hear, why would it be rocky?

First, an exoplanet, also called an extrasolar planet, is a planet located beyond our Solar System, orbiting a star other than our Sun. As of September 17, 2009, there were 374 exoplanets listed in the Extrasolar Planets Encyclopaedia (http://exoplanet.eu/). Yes, there are now so many of them that they have their own encyclopaedia. How do astronomers find exoplanets? They mostly use indirect methods, seeing how orbiting planets affect--wobble--the movement their parent stars. This process of detection is called radial velocity observation. The process is complex and requires lots of measurements before astronomers are satisfied with the findings and actually announce the existence of an exoplanet. Most of the exoplanets announced to date are massive gas giant planets thought to resemble our Jupiter, but the size of the planets discovered to this point is biased because of the limitations of our technology in the detection process--the planets usually have to be relatively big in order to induce a noticeable wobble in their parent stars. Projections based on the recent detections of much smaller worlds suggest that lightweight, terrestrial-type planets, or “rocky” planets as they are sometimes described, will eventually be found to outnumber extrasolar gas giants. These rocky planets would be similar in composition to our Mercury, Venus, Earth, and Mars. And this takes care of question number two.

The September 16 announcement by the ESO is in reference to CoRoT-7b, a planet orbiting the orange dwarf star CoRoT-7 (previously cataloged as TYC 4799-1733-1). The star CoRoT-7 is located in the direction of the constellation Monoceros (the Unicorn), five hundred light-years away. The star is slightly smaller and cooler than our Sun, and is also thought to be younger, with an age of about 1.5 billion years. The existence of exoplanet CoRoT-7b was first announced February 9 after about a year of observations and measurements. What makes CoRoT-7b so remarkable is that it was detected by direct observation, when it passed in front of its parent star and caused a noticeable dimming of the star's light. The difference in brightness, combined with a size estimate for the parent star, allowed astronomers to calculate the planet's size. It turns out that CoRoT-7b is the smallest and fastest-orbiting exoplanet discovered so far, with a blistering orbital period of 20.4 hours. That velocity--more than 750,000 kilometers per hour--puts the planet only 2.5 million kilometers away from its parent star, or 23 times closer than Mercury is to the Sun.

The latest observations were made using the High Accuracy Radial velocity Planet Searcher (HARPS) instrument attached to the ESO 3.6-m telescope at La Silla Observatory in Chile. The HARPS measurements were the longest made on a start to date. They reveal the planet's mass to be five times that of Earth's. Combined with CoRoT-7b's known radius, which is less than twice that of Earth, this tells astronomers that the exoplanet's density is very similar to the Earth's, suggesting a solid, rocky world. Because of the relative size and mass of planets such as this, they are described as “super-Earths.” And what’s more, the extensive observation also found another wobble that indicates the presence of another super-Earth in this alien solar system.

About a dozen super-Earths have been detected, though in the case of CoRoT-7b, this is the first time that the density has been measured for such a small exoplanet. We owe this to the fact that, from our perspective, CoRoT-7b transits--passes in front of--its parent star on every orbit.

For more details on these discoveries, and to learn more about the European Southern Observatory, check out their home page: http://www.eso.org/