Our second transiting Neptune-mass planet has been discovered via the HAT Network of small, automated telescopes maintained by the Harvard-Smithsonian Center for Astrophysics. HAT-P-11b is described by Greg Laughlin at systemic (thanks to many who sent this link):
HAT-P-11b is quite similar in mass and radius to Gliese 436b, and it’s actually somewhat larger than Neptune on both counts. When the mass and radius are compared to theoretical models, it’s clear that, like Gliese 436, it’s mostly made of heavy elements (that is, some combination of metal, rock and “ice”) with an envelope of roughly 3 Earth masses of hydrogen and helium). It’s completely dwarfed when placed next to an inflated hot Jupiter, HAT-P-9b, for instance…
The advantages of a detected transit are great. Couple the transit light curve with radial velocity measurements and you can work out the mass and radius of the transiting planet. Moreover, the opportunity to investigate planetary atmospheres comes into play through follow-up observations that can also tell us something about planetary temperatures. And because sixteen out of the twenty ‘hot Neptune’ planets we’ve found through various methods have been part of multiple planet systems, it’s no surprise that there is at least some evidence of a second planet to accompany HAT-P-11b.
It’s helpful indeed that HAT-P-11 will be in the Kepler mission’s observational field. With Kepler designed for a minimum mission of 3.5 years, the space-borne observatory should have this Neptune-class world available for more than 250 transits. That will firm up all our parameters for the planet based on the transit light curve, and will allow new evidence to be gathered for a second planet in the system. Knowing in advance that a particular Kepler star has a transiting world around it allows detailed observations to begin from the mission’s outset.
More in Bakos et al., “HAT-P-11b: A Super-Neptune Planet Transiting a Bright K Star in the Kepler Field,” submitted to the Astrophysical Journal and available online. Related: Stringellow et al., “Transit Timing Observations of the Extrasolar Hot-Neptune Planet GL 436b,” available here.