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).