Astronomers reveal new type of planet

Using Hubble space telescope, an international team of astronomers have come up with a new class of planet, according to a report published online Tuesday in The Astrophysical Journal. By analyzing the previously discovered world GJ1214b, the team led by Harvard-Smithsonian Center for Astrophysics (CfA) astronomer Zachory Berta proved that it is a water world enshrouded by a thick, steamy atmosphere. Our solar system contains three types of planets: rocky, terrestrial worlds (Mercury, Venus, Earth, and Mars), gas giants ( Jupiter and Saturn), and ice giants (Uranus and Neptune). Planets orbiting distant stars come in an even wider variety, including lava worlds and "hot Jupiters". "GJ1214b is like no planet we know of," said Berta in a statement issued Tuesday. "A huge fraction of its mass is made up of water." GJ1214b was discovered in 2009 by the ground-based MEarth Project. This super-Earth is about 2.7 times Earth's diameter and weighs almost seven times as much. It orbits a red-dwarf star every 38 hours at a distance of 1.3 million miles, giving it an estimated temperature of 230 degree Celsius. In 2010, CfA scientist Jacob Bean and colleagues reported that they had measured the atmosphere of GJ1214b, finding it likely that the atmosphere was composed mainly of water. However, their observations could also be explained by the presence of a world- wide haze in GJ1214b's atmosphere. Berta and his co-authors used Hubble's WFC3 instrument to study GJ1214b when it crossed in front of its host star. During such a transit, the star's light is filtered through the planet's atmosphere, giving clues to the mix of gases. Hazes are more transparent to infrared light than to visible light, so the Hubble observations help tell the difference between a steamy and a hazy atmosphere. They found the spectrum of GJ1214b to be featureless over a wide range of wave lengths, or colors. The atmospheric model most consistent with the Hubble data is a dense atmosphere of water vapor. "The Hubble measurements really tip the balance in favor of a steamy atmosphere," said Berta. Since the planet's mass and size are known, astronomers can calculate the density, which works out to about two grams per cubic centimeter. Water has a density of one gram per cubic centimeter, while Earth's average density is 5.5 gram per cubic centimeter. This suggests that GJ1214b has much more water than Earth, and much less rock. As a result, the internal structure of GJ1214b would be very different than our world. Theorists expect that GJ1214b formed farther out from its star, where water ice was plentiful, and migrated inward early in the system's history. In the process, it would have passed through the star's habitable zone. GJ1214b is located in the direction of the constellation Ophiuchus, and just 40 light years from Earth. It's a prime candidate for study by the next-generation James Webb Space Telescope.