CoRoT-Exo-7b - one small planet can hide another

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CoRoT-Exo-7b - one small planet can hide another

CoRoT-Exo-7b is just 1.7 times the diameter of Earth. This makes it a distant cousin of our planet, probably a rocky one. As it orbits very close to its star, most of this new world must be baking at a hellish temperature of more than 1,500°C.

CoRoT spacecraft. Credits: CNES/Ill. D. Ducros.

However, despite the infernal heat on its surface, CoRoT-Exo-7-b is paradoxically a major discovery: it is the smallest planet yet found outside our solar system, precisely the sort of object that CoRoT was launched to see.

“The signal in the CoRoT data was really very faint, almost at the limit of what the instrument can detect. To be honest, nobody thought it could be a planet,” admits Pierre Barge.

Nobody except Roi (pronounced Roy) Alonso, a young Spanish post-doctorate researcher hired by the Marseille-Provence astronomy observatory (OAMP) to assist in analysing CoRoT data.

CoRoT-Exo-7b light curve. Credits: CoRoT exo team.

“As he delved deeper into the data, Roi found that it really looked a lot like a transit. So, we put the new candidate on our list of objects to be tracked from the ground, but our tracking colleagues weren’t convinced! They were sure it was a false positive, and nobody was willing to sacrifice observing time to follow it up.”

But the more he looked at the data, the more the young researcher became convinced there was something lurking behind the tiny variations in the light curve.

“Roi kept working on this tiny signal until he succeeded in convincing the astronomers to track it.”

CoRoT-Exo-7b animation. Credits: CNES.

When follow-up ground observations finally got underway, the scientists were in for a huge surprise: using a different method, called the radial velocity method, the tracking teams found the probable signature of a planet orbiting the distant star, only it was a very different planet to the one found by CoRoT.

“It looks like there are 2 planets orbiting this star,” confirms Pierre Barge. “The smaller one, which CoRoT observed transiting the star, and a larger one, about the size of Neptune, orbiting 3½ days from its star.”

“But because it doesn’t pass in front of its star, CoRoT couldn’t have seen it.”

In fact, the small dip in the CoRoT light curve was hiding another planet… and maybe that’s not all.

Without a young researcher’s perseverance, nobody would ever have known.