55 Cancri e: Small or Big?

Detecting and characterizing exoplanets remains an extremely challenging occupation. A very good example of this has been the recent detection of the transit of a planet around a star in the nearby stellar system 55 Cancri. This is an interesting place. It contains a binary star consisting of a G-dwarf and an M-dwarf separated by at least 1000 astronomical units. At only 41 light years away the G-dwarf is visible to the naked human eye and, conveniently, harbors at least 5 planets.

These worlds had been detected using radial velocity measurements, with the first detections back in 1997. A recent reanalysis of the notoriously tricky radial velocity data (made even more so with possible mean-motion orbital resonances in the system) by Dawson & Fabrycky suggested that the innermost planet "e" was actually on a much tighter orbit than had previously been thought - with the incredibly short orbital period of 0.74 days. This also suggested it was rather lighter than thought, a super-Earth of at least 8 Earth masses.

This was intriguing because the probability of a transit increases with decreasing orbital radius. In this case it went up by a factor of three to about 33%. This has prompted a number of groups to go look for the transit again, after earlier attempts came up empty. Lo and behold the first reported transit of 55 Cancri e appeared from Winn et al. on April 27th 2011. Using the visible light telescope of the Canadian MOST mission they spotted a transit right on queue. Hopefully Dawson and Fabrycky had some champagne at the ready.

Analyzing the depth of the transit curve (a tiddly drop in starlight of about 0.02%) Winn and colleagues came up with a planetary radius of approximately 1.6 times the size of the Earth. Combined with a mass of almost 9 times that of the Earth this implies a remarkable density of about 11 grams per cubic centimeter. That's almost as dense as lead, and a clear indicator that this is an almost pure rock-iron composition.

At least that would be the case, except on May 2nd 2011 another paper appeared by Demory and colleagues that uses the Spitzer infrared telescope to detect the very same transiting planet. With this different telescope and different waveband, a transit depth is found that indicates 55 Cancri e is about 2.1 times the radius of the Earth. The statistics are good enough that this is a "3-sigma" difference from the MOST detection, indicating that this difference in measured radius would only occur by random error about 0.3% of the time.

So, with a radius of 2.1 times that of the Earth this devious planet would have to contain a significant amount of volatiles and be much less dense. Possible compositions would involve a rocky-iron Earth-like core surrounded by either 20% by mass of water or 0.1% by mass hydrogen and helium. The problem is that 55 Cancri e is so close to its parent G-dwarf star that it should have an effective temperature of about 2000 Kelvin - all over. Hardly an environment for anything that could be called a "volatile".

Its quite a conundrum. Demory et al. perform their analyses very carefully. The most likely explanation - and one that Demory and colleagues offer up - is that 55 Cancri e harbors an extended atmosphere. Perhaps carbon monoxide or carbon dioxide as a super heated "exosphere" could produce an anomalous infrared surface that makes the planet look fatter than it is. That would likely indicate some interesting compositional properties if the planet is indeed as dense as the optical data suggest. Winn et al. investigate the constraints on an atmosphere from their data. It's very interesting. The odds seem slim. The only option appears that if such an atmosphere exists it is being constantly replenished by perhaps geophysical activity on the planet itself. The current status is therefore open to question.

There is hope however. Because the star 55 Cancri shines so brightly in our skies it provides a marvelous flood of photons and the potential for extremely high quality astronomical observations. Unlike many of the mysterious new worlds we merely catch glimpses of, this one may well reveal its secrets to us in due course. Exoplanetary science continues to grow in richness.

As I write this 55 Cancri is up in the western sky of the Eastern seaboard of the United States, amongst the constellation of Cancer, the Crab. Perhaps you should run out in case you can spot it - you never know, you might just catch a transit.