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Ground Detection of a Super-Earth Transit

Forty light years from Earth, the planet 55 Cancri e was detected about a decade ago using radial velocity methods, in which the motion of the host star to and from the Earth can be precisely measured to reveal the signature of the orbiting body. Now comes news that 55 Cancri e has been bagged in a transit from the ground, using the 2.5-meter Nordic Optical Telescope on the island of La Palma, Spain. That makes the distant world’s transit the shallowest we’ve yet detected from the Earth’s surface, which bodes well for future small planet detections.

Maybe ‘small’ isn’t quite the right word — 55 Cancri e is actually almost 26000 kilometers in diameter, a bit more than twice the diameter of the Earth — which turned out to be enough to dim the light of the parent star by 1/2000th for almost two hours. The planet’s period is 18 hours, bringing it close enough to reach temperatures on the dayside of 1700° Celsius. As the innermost of the five known worlds around 55 Cancri, 55 Cancri e is not a candidate for life.


Image: A comparison between the Earth and 55 Cancri e. Credit: Harvard-Smithsonian Center for Astrophysics.

Previous transits of this world were reported by two spacecraft, MOST (Microvariability & Oscillations of Stars) and the Spitzer Space Telescope, which works in the infrared. What’s exciting about the new transit observation is that as new observatories like TESS (Transiting Exoplanet Survey Satellite) come online, the number of small candidate worlds will zoom. Each will need follow-up observation, but the sheer numbers will overload both space-based and future ground observatories. Learning how to do this work using existing instruments gives us a way to put smaller telescopes into play, a cost-effective option for continuing the hunt.

The work on 55 Cancri e grows out of an effort to demonstrate the capacity of smaller telescopes and associated instruments for such detections. Having succeeded with one planet, the researchers look at some of the problems that arise when using moderate-sized telescopes:

In the case of bright stars, scintillation noise is the dominant limitation for small telescopes, but scintillation can be significantly reduced with some intelligent planning. With the current instrumentation, the main way to do this is to use longer exposure times and thereby reduce the overheads. However, longer exposure times will likely result in saturation of the detector, which can be mitigated by either defocusing the telescope (resulting in a lower spectral resolution, and possible slitlosses), using a higher resolution grating (which will decrease the wavelength coverage), or using a neutral density filter to reduce the stellar flux. The latter option will be offered soon at the William Herschel Telescope. Another possible solution is to increase default gain levels of standard CCDs.

So if we do find numerous super-Earths in upcoming survey missions, we can spread the task of planet confirmation to a broader pool of participants as our expertise develops. As for 55 Cancri itself, it continues to intrigue us, the first system known to have five planets. It’s also a binary system, consisting of a G-class star and a smaller red dwarf separated by about 1000 AU. While 55 Cancri e does transit, the other planets evidently do not, though there have been hints of an extended atmosphere on 55 Cancri b that may at least partially transit the star.

The paper is de Mooij et al., “Ground-Based Transit Observations of the Super-Earth 55 Cnc e,” accepted for publication in the Astrophysical Journal Letters (preprint).


Comments on this entry are closed.

  • Andrew LePage December 2, 2014, 14:10

    Interestingly, 55 Cancri was among the first Sun-like stars to have planets discovered orbiting it back in 1996. While none of the five planets currently known to orbit 55 Cancri could be habitable (they are either gas giants and/or too hot like 55 Cancri e), 55 Cancri f with a mass half that of Saturn is located at the inner edge of this stars habitable zone and could support a habitable moon if it were large enough.


  • andy December 2, 2014, 19:48

    Unfortunately going to be a while before the carbon planet hypothesis can be definitively refuted. Not especially convinced by the idea that 55 Cancri e is a carbon planet though, particularly given that the stellar C/O ratio has been revised downwards since the idea was proposed.

    The orbital eccentricity of 55 Cancri f is rather poorly constrained, last I heard the constraint is that it is below ~0.5 (which would give a ~15% increase in the orbit-averaged insolation) though circular orbits do remain a possibility – this might allow for a planet at the outer boundary of the HZ.

    As for habitable moons, I guess the best possibility is for an irregular (captured) satellite rather than a regular satellite produced in a circumplanetary disc, given the relatively low mass of the planet. Exomoons still are proving elusive, e.g. recently the candidate moon of Kepler-90g has been shown to be very likely a false positive. Co-orbital planets also don’t seem to have shown up yet.

  • Ronald December 3, 2014, 7:08

    Great achievement. Can this ground-based methodology also be used for transits of roughly earthsized planets in earthlike orbits around solar type stars?

  • Harry R Ray December 3, 2014, 10:50

    TO RONALD: The answer is no in the very long forseeable future. But, they might be able to detect the transits of earth sized planets orbiting Proxima Centauri analogs! One ground based attempt has tried and failed to find anything. MOST (which could detect planets SLIGHTLY smaller than Earth) has made a RECENT attempt, but the data has not been fully reduced. Speaking of Proxima Centauri analogs, KOI 3138 has been RECENTLY ADDED to the HEC habitable sone CANDIDATE list! For the FIRST TIME< we have a POSSIBLE true Mars analog (both in size AND temperature) , KOI3138b, in a 8.7 day orbit! Check it out( I would also appreciate it if someone can find the "2MASS" designation for the star)! My gut feeling on the MOST search is that they will find nothing, because a Mars-sized planet may be the biggest one that CAN form around this kind of star( or even Barnard's Star analogs like Kepler 42), but I hope I am wrong!

  • Harry R Ray December 3, 2014, 13:44

    Sorry, I meant KOI 3138.01, not KOI 3138b.