Hot Ice: A Planet, But Not As We Know It

A bizarre world of scorching hot ice shrouded in a steamy atmosphere may have been found, according to new observations. Characterising the Neptune-size planet is an important milestone on the way to detecting and characterising Earth-like planets that could harbour life.

Astronomers have discovered more than 200 planets orbiting other stars, called extrasolar planets or exoplanets. Almost all of these were detected by the way their gravity makes their parent stars wobble. But this technique, called the radial velocity method, reveals very little about the planet except for the size of its orbit and an estimate of its mass.

Astronomers can learn a lot more by watching "transits" of planets that pass in front of their parent stars as seen from Earth. Careful analysis of the dimming this causes can provide clues to the planet's composition and structure. But the brightness dips are small and difficult to detect for all but the largest planets.

Now, astronomers have observed the smallest ever transiting planet. It has turned out to be a strange world, unlike anything seen before.

...[Scientists] have been able to measure the planet's width, which provides clues
to its composition and structure. It turns out to be about 50,000
kilometres wide, roughly four times the width of Earth and about the
size of Neptune.

The
planet is therefore too compact to be made mostly of hydrogen gas, like
Jupiter, the researchers say, but not compact enough to be a rocky
'super Earth', as some had speculated. Instead, they believe it must be
made mostly of an exotic form of water.