The red dwarf star GJ 436 is about 30 light-years from the Sun (where one light-year is the distance that light travels in vacuum in one year at a speed of about 186,000 miles per second). The planet GJ 436b orbits the star at a distance of about 4 million kilometers (2.5 million miles), going around once every 2.6 days.

The planet, discovered in 2004, is about four times larger than the Earth (with a diameter of about 50,000 kilometers, or 31,000 miles), but is about 30 times more dense. It is about the size of the planet Neptune. The surface temperature of the planet is about 300 degrees Celsius (570 degrees Fahrenheit).

European astronomers, headed by Michael Gillon (Liege University, Belgium), made and confirmed the discovery with data provided by the OFXB Observatory in St.-Luc, Switzerland; the Wise Observatory in Israel; the W.M. Keck Observatory in Mauna Kea, Hawaii; and the Euler telescope, which is operated by Geneva University, at La Silla Observatory in Chile.

Gillon’s team measured the exoplanet (or exosolar planet, a planet orbiting a star other than our Sun) when it transited (passed) in front of the star.

The discovery indicates that the planet is made almost entirely of water. Because the subsurface water is under so much pressure and heated to many hundreds of degrees, it is in exotic forms called Ice VII and Ice X. Water is found only in these forms on the Earth through scientists’ experiments.

Ice is the common name for any of 14 known solid phases of water. What we know as ice is really hexagonal ice (Ice Ih). Ice VII (Ice-seven) is formed from liquid water when subjected to pressures greater than 3 GPa (gigapascal) (approximately 62.7 million pounds per square foot). As the pressure is raised even more it turns into Ice X (Ice-ten).

Their results appear in the journal Astronomy & Astrophysics.

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