May 22, 2007 - One of the most dramatic extrasolar planetary discoveries of the year was announced this week; unfortunately, with little fanfare. Planet hunters uncovered a Neptune-sized planet orbiting a nearby star.

This planet is close enough to its parent star that it’s extremely hot - above 250 degrees Celsius. And yet the intense pressure from gravity forces large quantities of liquid water into solid ice.

The planet was discovered orbiting the nearby M-dwarf star GJ 436 using the planetary transit technique. This is where a sensitive instrument called a photometer measures the periodic dimming and brightening of a star as a planet passes in front.

In August 2006, astronomers captured the first hint of the planet using the Observatoire Francois-Xavier Bagnoud (OFXB) observatory in St-Luc Switzerland. It was then confirmed using the Euler 1.2m telescope at La Silla Observatory in Chile.

The announcement was made in the paper Detection of transits of the nearby hot Neptune GJ 436 b, which has been accepted for publication in the journal Astronomy and Astrophysics Letters.

With more traditional planet hunting techniques, very little information can be found about the planet, other than its mass. But planetary transits offer a wealth of data.

Since the light from the star dims, and the chemical composition of the light changes, astronomers can determine the planet’s atmosphere by subtracting it from the star. They can measure both the mass, and the size of the planet, and measure the temperature of its surface.

According their calculations, GJ 436 b is approximately 50,000 km across; 4 times the radius of Earth, and approximately the size of Neptune. This makes is the smallest planet ever discovered using the planetary transit technique, and brings the possibility of uncovering Earth-sized planets tantalizingly closer.

But unlike frigid Neptune, it orbits much closer than the orbit of Mercury, completing an orbit in just a few days. Even through the dwarf star it orbits is less luminous than our Sun, the planet orbits so close that it’s heated above 250 degrees Celsius. This makes it the first “hot Neptune” ever discovered.

A planet with this amount of water ice must have formed outside the star’s “snow line”, where the protoplanetary disc is cool enough for water to condense. Some process must have brought it gradually closer to the parent star, to its current position today. Once the planet got close enough to the star its outer envelope of hydrogen and helium would have evaporated away, leaving the smaller icy core.