Evidence for a solid, rocky world 500 light-years away

This is the first time that the density has been measured for such a small exoplanet, and the measurements suggests a solid, rocky world.

In February of 2009, astronomers announced that an Earth-orbiting satellite known as CoRoT had discovered an exoplanet orbiting around a distant star. The star was unremarkable, but its planet has become known as the smallest and fastest-orbiting of all the exoplanets known. Plus the European Southern Observatory (ESO) has announced that it has ‘firmly established’ the nature of this world as being solid and rocky, like Earth.

Astronomers – who don’t give stars the romantic names they deserve – originally called this star TYC 4799-1733-1. But they renamed it for CoRoT-7, and they called its little planet by the name CoRoT-7b, for the satellite that provided us our first earthly awareness of it.

The image at the top of this page is an artist’s impression of the star CoRoT-7 as seen from the surface of its planet. What can we glean from this image? First, notice that the planet seems very close to its star, closer than Earth is to our sun. See how big this sun looks in this planet’s sky? In fact, CoRoT-7b orbits only 2.5 million kilometers away from CoRoT-7. That’s close indeed, 23 times closer to its star than the innermost planet in our sun’s family, Mercury, is to our sun. And remember that Mercury is so close that its sun-facing surface reaches temperatures of at least 750 degrees F. or hot enough to melt some metals. So CoRoT-7b must be hot indeed on its day side. Theoretical models suggest its surface may feature lava or boiling oceans.

Mercury’s radius is 38% that of Earth, however. CoRoT-7b has a radius that is about 80% greater than Earth’s. So although I’ve been calling it ‘little,’ in fact this distant world is bigger than our home planet, almost double in size in fact.

The information about CoRoT-7b’s being solid and possibly rocky came from a long examination with an instrument called HARPS, the High Accuracy Radial velocity Planet Searcher at the ESO La Silla 3.6m telescope. This instrument is dedicated to the discovery of extrasolar planets, which were unconfirmed until 1995. At this writing, there are 374 listed in the Extrasolar Planets Encyclopaedia. The HARPS instrument at the La Silla telescope has revealed CoRoT-7b’s mass as five times that of Earth’s. It’s larger in size than Earth, and more massive, so that makes sense. Combined with CoRoT-7b’s known radius, which is somewhat less than twice that of Earth, astronomers have deduced that the exoplanet’s density is quite similar to Earth’s. This is the first time that the density has been measured for such a small exoplanet, by the way, and the measurements suggests a solid, rocky world.

corot-7_starfield_250Didier Queloz, leader of the team that made the observations, called this ‘science at its thrilling and amazing best’ and who could disagree?

The star CoRoT-7 and its planet CoRoT-7b are located in the direction of our constellation of Monoceros the Unicorn, a small star pattern south of the very noticeable constellation Orion the Hunter. The photo at right shows the star field where it resides. The star is located at a distance of about 500 light-years, and it is known to be slightly smaller and cooler than our sun. CoRoT-7 is also thought to be younger than our sun at about 1.5 billion years, in contrast to 4.5 billion years for Sol.

Astronomers love systems like this one. Every 20.4 hours, the planet – CoRoT-7b – eclipses a small fraction of the light of its star for a little over one hour. The eclipse is subtle, with the light of the star diminished by only one part in 3000, but the instruments of astronomers are even more subtle, and thus they were able to detect a regular dip and rise in the star’s light and gain much information from it.

By the way, the astronomers also found in their dataset that CoRoT-7 hosts another exoplanet slightly more distant from its parent star than CoRoT-7b. They named it – of course – CoRoT-7c.

This newly discovered planet circles its host star in 3 days and 17 hours – in contrast to 365 days for Earth’s orbit around its star – and the planet has a mass about eight times that of Earth. Both CoRoT-7b and CoRoT-7c are classified as super-Earths. Unlike CoRoT-7b, this sister world does not pass in front of its star as seen from Earth, so astronomers cannot measure its radius or density.

Given these findings, CoRoT-7 stands as the first star known to have a planetary system made of two short period super-Earths with one that transits its host.

You can find more details about this star system, along with images and video, here.

Deborah Byrd