CfA: Hubble Reveals a New Type of Planet (GJ 1214b)

[bystander]

Hubble Reveals a New Type of Planet (GJ 1214b)
CfA | Smithsonian | NASA/STScI | ESA/HEIC | 2012 Feb 21

[i]Artist's Impression of Exoplanet GJ 1214b (Credit: NASA, ESA, and D. Aguilar (Harvard-Smithsonian Center for Astrophysics)[/i]

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Waterworld enshrouded by a thick, steamy atmosphere is new class of planet

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."

Observations by NASA's Hubble Space Telescope have added a new type of planet to the mix. By analyzing the previously discovered world GJ1214b, astronomer Zachory Berta (Harvard-Smithsonian Center for Astrophysics) and colleagues proved that it is a waterworld enshrouded by a thick, steamy atmosphere.

"GJ1214b is like no planet we know of," said Berta. "A huge fraction of its mass is made up of water."

GJ1214b was discovered in 2009 by the ground-based MEarth (pronounced "mirth") Project, which is led by CfA's David Charbonneau. This super-Earth is about 2.7 times Earth's diameter and weighs almost 7 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 450 ° Fahrenheit.

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.

"We're using Hubble to measure the infrared color of sunset on this world," explained Berta.

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 wavelengths, 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 2 grams per cubic centimeter. Water has a density of 1 g/cm3, while Earth's average density is 5.5 g/cm3. 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.

"The high temperatures and high pressures would form exotic materials like 'hot ice' or 'superfluid water' - substances that are completely alien to our everyday experience," said Berta.

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. How long it lingered there is unknown.

GJ1214b is located in the direction of the constellation Ophiuchus, and just 40 light-years from Earth. Therefore, it's a prime candidate for study by the next-generation James Webb Space Telescope.

The Flat Transmission Spectrum of the Super-Earth GJ1214b from Wide Field Camera 3 on the Hubble Space Telescope - Zachory K. Berta et al

• Astrophysical Journal 747(1) 35 (2012 Mar 01) DOI: 10.1088/0004-637X/747/1/35
  arXiv.org > astro-ph > arXiv:1111.5621 > 23 Nov 2011

ScienceShot: Water World Is New Type of Planet
Science NOW | Sid Perkins | 2012 Feb 21

More Details from Hubble Reveal Strange Exoplanet is a Steamy Waterworld
Universe Today | Nancy Atkinson | 2012 Feb 21

Super-Earth exoplanet likely to be a waterworld
Discover Blogs | Bad Astronomy | 2012 Feb 21

A Waterworld Around GJ1214
Centauri Dreams | Paul Gilster | 2012 Feb 22

[bystander]

Possible Water in the Atmosphere of a Super-Earth
Smithsonian Astrophysical Observatory
Weekly Science Update | 2012 Mar 02

[i]The infrared spectrum of the super-Earth GJ1214b. The circles show the measured intensity at each wavelength; the bars show the uncertainties. Also plotted are some proposed model atmospheres - the best fitting ones contain water vapor. (Credit: Zachory Berta, 2012)[/i]

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A "super-Earth" is an exoplanet (a planet around another star) whose mass is between about two and ten Earth-masses. Planets larger than this are closer to Uranus and Neptune in size (and perhaps in other physical properties as well). The category of "super-Earth" currently refers only to the mass of the object, and not to its radius, its orbital distance from the star, its surface temperature, or its atmospheric properties, although naturally astronomers are working hard to identify super-Earths that might offer clues about the Earth in these features. Of the 576 exoplanets whose basic parameters are currently approximately known, there are 36 in the super-Earth category.

Four CfA astronomers, Zachory Berta, David Charbonneau, Jean-Michel Desert, and Jonathan Irwin, together with six colleagues, used the Hubble Space Telescope to probe the atmosphere around the transiting super-Earth known as GJ1214b. This exoplanet has a mass of 6.5 Earth-masses and a radius of 2.7 Earth-radii, and it orbits a small M-dwarf star (its diameter is only 21% of the Sun's). They used the Hubble infrared spectrometer to observe the planet as it transited across the face of the star; as it did so, the planet's atmosphere absorbed light from the star, thus subtly altering the star's intrinsic spectrum as we observe it. The intrinsic spectrum itself was carefully measured and subtracted after the transit.

Earlier models tried to predict what the atmosphere of this super-Earth might contain, based on ground-based transit spectra of it and on our current knowledge of the atmospheres of the planets in the solar system. For example, an atmosphere dominated by molecular hydrogen is one possibility; this is the situation in Jupiter. But measurements made in three separate transits found no evidence for this scenario - nor were they consistent with an equilibrium atmosphere composed of elements whose relative abundances are those of the average solar system. Instead, the astronomers concluded that, unless the atmosphere has a thick top layer of clouds obscuring our view, its probable composition includes abundant water vapor. Probing the atmosphere around a planet orbiting a star 42 light-years away is a remarkable achievement and an early step. Future observations of this and other transiting exoplanets will tell us more about the atmospheres of exoplanets, and lead to a better understanding of how atmospheres form, evolve, and behave under a wide range of physical conditions.

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