Artist's impression of the 'hot snowball' planet, GJ 436b, orbiting a small red dwarf star 33 light years away
Credit: NASA
SYDNEY: A planet resembling a hot snowball discovered orbiting a distant star is the latest addition to a growing list of extrasolar planets with bizarre properties.
The Neptune-sized planet, named Gliese 436 b (or GJ 436 b), is covered in hot, solid ice and has a steamy, gaseous atmosphere. According to a paper published this week in the journal Astronomy & Astrophysics, these conditions result from intense pressure forcing water to solidify into ice, despite a surface temperature of close to 250°C.
The 'hot snowball' planet orbits a small red dwarf star around half the mass of our Sun and 100 times dimmer, about 33 light years away. Each orbit takes three days and occurs at a distance of four million kilometres – around 14 times closer than Mercury is to our Sun.
GJ 436 b was first discovered in 2004, but more recent observations of the planet in transit across its sun revealed its unusual properties. After measuring the planet's mass and radius, astronomers concluded that it was too compact to be made of hydrogen, but not compact enough to be a rocky planet like our own.
The most likely explanation, according to Michael Gillon of Liege University, Belgium, who led the study, is a planet composed primarily of ice, with an atmosphere that may contain hydrogen, helium and water vapour.
A host of exotic worlds
"No one has, until now, thought of a world being surrounded by hot ice – it is really a new and exotic idea," said astronomer Brad Carter of the University of South Queensland, Australia. "It is certainly unlike anything in our solar system and tells us of the incredible diversity of the planets that seem to exist in the universe".
Indeed, GJ 436 b is only the latest in a string of quirky worlds discovered in the far reaches of our galaxy in recent years. Improved planet-hunting techniques and advances in telescopic imagery have led to the discovery of more than 230 extrasolar planets – some as far as 26,000 light years from Earth – since the first, Pegasi 51 b, was found 12 years ago. Let's take a look at a few of them.
Black hot
Travel 30 light years beyond Gliese 436 b and you hit another exotic world. HD149026b orbits its sun at a distance 25 times closer than Earth is to our Sun, but even this proximity doesn't explain the surface temperature – a scorching 2,040 degrees Celsius. Scientists believe that the planet must absorb almost all of the light that strikes it, so it must be completely black.
Water world
20 light years away, orbiting a dim red dwarf, is a planet that might almost be like home to us. About five times the mass of Earth, the planet orbits close enough to its parent star to have just the right surface temperature to support liquid water.
The star, Gliese 581, has already been identified as hosting a planet similar in size to Neptune, the frigid gas giant on the edge of our own Solar System.
The new planet is 14 times closer to Gliese 581 than the Earth is to the Sun. But because Gliese 581 is so cool, the planet is not scorched by solar radiation. It zips around the star at express speed, making just 13 days to complete an orbit.
Eccentric
Standing on the surface of HAT-P-2b, 440 light years away in the constellation of Hercules, would not be good for your self-esteem: a person weighing in at 70 kg on Earth would find themselves tipping the scales at around 950 kg. With a mass eight times that of Jupiter, gravity squashes HAT-P-2b so that – despite being made primarily of hydrogen – the planet is as dense as the Earth.
Another quirk of HAT-P-2b is that it has one of the most eccentric orbits known, ranging from 500,000 km from its sun, to three times that at its furthest distance. That would be like Earth traversing the skies to reach Mercury, then swinging back to pass near the surface of Mars.
Giant cork
A little bit further on, 450 light years from Earth, is HAT-P-1b – one of the largest planets known. With a radius almost 1.4 times greater than Jupiter, but only half the mass, the planet has a quarter of the density of water. This means it's lighter than a giant ball of cork, and suggests it's formed entirely of light gases without a rocky or metallic core. HAT-P-1b, which does not appear to have a solid core, is the first in an entirely new class of planets.
