Detection of transits of the nearby hot Neptune GJ 436 b

TL;DR

This paper reports the photometric detection of transits of GJ 436 b, a nearby Neptune-mass exoplanet, marking the closest and smallest transiting planet discovered so far, with implications for its composition.

Contribution

It presents the first detection of transits of GJ 436 b, a Neptune-mass planet around an M-dwarf, providing detailed measurements of its size, orbit, and possible composition.

Findings

GJ 436 b is the closest, smallest, and least massive transiting planet detected.

The planet's radius is approximately 3.95 Earth radii, similar to Uranus and Neptune.

The planet likely has a water ice composition with a possible H/He envelope.

Abstract

This Letter reports on the photometric detection of transits of the Neptune-mass planet orbiting the nearby M-dwarf star GJ 436. It is by far the closest, smallest and least massive transiting planet detected so far. Its mass is slightly larger than Neptune's at M = 22.6 +- 1.9 M_earth. The shape and depth of the transit lightcurves show that it is crossing the host star disc near its limb (impact parameter 0.84 +- 0.03) and that the planet size is comparable to that of Uranus and Neptune, R = 25200 +- 2200 km = 3.95 +- 0.35 R_earth. Its main constituant is therefore very likely to be water ice. If the current planet structure models are correct, an outer layer of H/He constituting up to ten percent in mass is probably needed on top of the ice to account for the observed radius.