A ‘Super-Earth’ with an Atmosphere

by Paul Gilster on December 16, 2009

Picking up on yesterday’s theme of planetary detections from ground-based observatories, we now get word of the detection of a transiting ‘super-Earth’ — one that may well have an atmosphere we can study — with the kind of equipment many amateurs already use to observe the sky. The new world is GJ 1214b, about 6.5 times as massive as the Earth, orbiting a small M-dwarf about a fifth the size of the Sun some forty light years from Earth.

But there’s more, a good deal more. At a distance of 1.3 million miles, the planet orbits its star every 38 hours, with an estimated temperature a little over 200 degrees Celsius. Because GJ 1214b transits the star, astronomers are able to measure its radius, which turns out to be 2.7 times that of Earth. The density derived from this suggests a composition of about three-fourths water and other ices and one-fourth rock. Some of the planet’s water should be in the form of exotic materials like Ice VII, a crystalline form of water that is found at pressures greater than 20,000 Earth atmospheres:

“Despite its hot temperature, this appears to be a waterworld,” said Zachory Berta, a graduate student at the Harvard-Smithsonian Center for Astrophysics (CfA) who first spotted the hint of the planet among the data. “It is much smaller, cooler, and more Earthlike than any other known exoplanet.”

That’s quite a find for the MEarth Project, which uses an array of eight 16-inch telescopes that monitor a list of 2000 red dwarf stars. The MEarth array is located at the top of Mount Hopkins, Arizona. MEarth looks for the tiny drop in brightness that indicates a transit, using data processing technologies to extract the planetary signature. If ever there was a find that should galvanize the amateur astronomy community, it’s this one, as David Charbonneau (CfA), who heads MEearth, is quick to note:

“Since we found the super-earth using a small ground-based telescope, this means that anyone else with a similar telescope and a good CCD camera can detect it too. Students around the world can now study this super-earth!”

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Image: This artist’s impression shows how the newly discovered super-Earth surrounding the nearby star GJ1214 may look. Discovered by the MEarth project and investigated further by the HARPS spectrograph on ESO’s 3.6-metre telescope at La Silla, GJ1214b is the second super-Earth exoplanet for which astronomers have determined the mass and radius, giving vital clues about its structure. It is also the first super-Earth around which an atmosphere has been found. Credit: ESO/L. Calçada.

But detecting the transit was only the first step in the process of collaboration between small telescopes and larger instrumentation. Confirming the MEarth find involved studying it with the HARPS spectrograph attached to the European Southern Observatory’s 3.6-meter telescope at La Silla. HARPS data were also crucial in determining the mass and radius of CoRoT-7b.

Sifting through this material, what stands out is that the radius measured for GJ 1214b is larger than expected by current models. Remember, this is the second time we’ve found a transiting super-Earth, the first being CoRoT-7b. The latter has a similar mass but the radius of GJ 1214b is much larger. Indications are that a surrounding atmosphere some 200 kilometers thick is adding to the drop in stellar light measured in these transits. Charbonneau again:

“This atmosphere is much thicker than that of the Earth, so the high pressure and absence of light would rule out life as we know it, but these conditions are still very interesting, as they could allow for some complex chemistry to take place.”

We should be able to learn more about this atmosphere, for GJ 1214b is close enough to Earth that the Hubble telescope should be able to characterize its atmosphere. The paper is Charbonneau et al., “A Super-Earth Transiting a Nearby Low-Mass Star,” Nature 462 (17 December 2009), pp. 891-894 (abstract).

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Ron S December 16, 2009 at 15:25

The abstract doesn’t mention the 200 degree temperature, but presumably that would refer to the liquid and/or solid surface and be in degrees C (mostly European authors). However 200 degrees is kind of warm for a water surface, unless the atmospheric pressure of that 200 km thick (steam) atmosphere is high. I also don’t see how the surface temp could be known if the atmosphere is so opaque. So maybe it’s the temp of the atmosphere.

I seem to be missing something(s) here.

andy December 16, 2009 at 15:44

Ron S: you can access the figures and tables in Nature articles without having to breach the paywall – planetary parameters are given here. The equilibrium temperature given for zero Bond albedo is given as 555 K. This temperature does not include the warming effects of an atmosphere.

Interestingly enough the planet has a radius implying it has a hydrogen atmosphere. I’m curious as to whether this would cause the planet’s interior to be fluid instead of in the form of high pressure ice because of mixing in the hydrogen with the ice, as occurs in Uranus and Neptune. This appears to be a planet at the low-mass end of the scale of ice giant planets, a “mini-Neptune” rather than a “super-Earth”.

This discovery has implications for the habitability of “super-Earths” located in or near habitable zones (for example those in the Gliese 581 system) – they may be mini-Neptunes with massive hydrogen atmospheres, resulting in surface pressures too high to be compatible with liquid water.

kurt9 December 16, 2009 at 15:49

At 200degC, the planetary environment might be mostly supercritical steam. I think liquid water could exist at this temperature, but the atmospheric pressure would have to be at least 40 atmospheres or so. So, a liquid water surface with an incredibly steamy (supercritical) atmosphere. Essentially a wet Venus. Not the most hospitable of places. I’m sure that wet Venuses exist.
Nevertheless, a nice try for Earth-based detection of an near Earth sized planet. Kudos for those who found it.

Adam December 16, 2009 at 16:33

If it was a bit cooler it’d be like Hal Clement’s Kainui from his last book “Noise”. No doubt we’ll be finding a lot more of these – latest estimates put Super-Earths around 38-56% of stars – and in all sorts of combinations. In spite of all the water they really aren’t “Earth-like” even if they’re the right temperature. Too much water.

If Earth’s atmosphere really did come from exogenic sources then it’s likely the ocean did too. We owe Jupiter for delivering our air and water via perturbing Main Belt comets our way in the early days, and secondly for keeping infalling Oort Cloud comets away from the Inner System. But that also means the amount of volatiles on terrestrial planets is a stochastic thing – Venus, for example, might’ve formed drier than Earth did, thus the apparently stifled plate tectonics due to a lack of lubricating water.

Tobias Holbrook December 17, 2009 at 6:26

Hmmm… nice terraforming challenge…

Get rid of the thick atmosphere, possibly by sequestering it in the coean, and shade the planet to lower the temperature. Then build a floating, semi ‘Supra-mundane’ planet on the surface of the ocean. Just some floating Islands to start with, getting larger and larger until most of the surface is covered. Hopefully there will be resources dissolved in the ocean that we can extract.

One more thing – if the atmosphere is Hydrogen, surely it will have a higher scale hieight anyway to get the same surface pressure?

Eniac December 17, 2009 at 10:42

Carbon based life on a planet with excess hydrogen would work differently than on Earth. To form carbohydrates, organisms would split hydrogen molecules instead of water, and no oxygen would be generated. Not to mention that any such oxygen would immediately burn up with the hydrogen, anyway. Without oxygen, high energy organisms, i.e. any form of mobile animals, are unlikely to be possible, just as we don’t have plants walking around on Earth.

As on Earth, carbon dioxide would likely be the limiting feedstock and would be completely sequestered away into biomass. An absence of CO2 in an atmosphere could then mean 2 things: 1) There is no carbon and thus no carbon-based life, 2) there is carbon-based life which has sequestered all the CO2. Turning this around, the presence of CO2 would be a strong indicator of the absence of carbon-based life, as on Mars and Venus.

Super-Earths of a certain size (anyone know that size?) will retain hydrogen due to their deeper gravity well and thus should pretty much always have the reducing conditions which would prohibit oxygen from forming and animals from evolving. This ought to be a strict limit on the size of any inhabited (or inhabitable) planets we might find. Is this true? Has it been recognized?

Alex December 17, 2009 at 12:22

Ahh-The discovery of another Super-Earth. It Brings joy.
The size of this Red Dwarf is interestingly small, hmm? The Habitable zone so close-b’s orbital period being 38 hours-hmm?

Well, it’s nice to see Astronomy unfurling as we enter a new decade. Especially of this sort-Super-Earth Habitability will surely attract attention in Media =]

Good luck for anything, Alex

Christopher L. Bennett December 17, 2009 at 17:03

If this is a Leger-type Ocean planet as it seems, made mostly of water and volatiles with a small rocky core, then I doubt there’d be a significant amount of mineral compounds in the ocean (if there is an ocean rather than just a bunch of supercritical water/steam). The bulk of the planet’s volume would be allotropic ices, thousands of kilometers’ worth separating the ocean from the rock and metal. Anything that fell in from space would probably sink to the bottom in time. I used an Ocean planet as the setting of my latest novel, Star Trek Titan: Over a Torrent Sea, and to justify it being inhabited I needed to put it in a cluttered system with frequent asteroid bombardment and concoct an elaborate ecological mechanism for recirculating sunken minerals back to the shallows.

Adam December 18, 2009 at 0:22

Hi All

Chris, that ocean planet sounds interesting, but the depth of the ices can be deeper or even absent given the right thermal profile and the presence of dissolved minerals, which depress the melting points of all the high-pressure phases that can be studied – Ice VI & VII, for example. There’s no reason why some kind of high pressure/temperature cryovolcanism can’t occur either, ferrying minerals up from the silicates below, given a decent heat supply.

ljk January 10, 2010 at 1:26

The Nature of the Atmosphere of the Transiting Super-Earth GJ 1214b

Authors: E. Miller-Ricci, J. J. Fortney

(Submitted on 6 Jan 2010)

Abstract: The newly discovered planet GJ 1214b is the first known transiting super-Earth requiring a significant atmosphere to explain its observed mass and radius. Models for the structure of this planet predict that it likely possesses a H-He envelope of at least 0.05% of the total mass of the planet.
However, models without a significant H-He atmosphere are not entirely ruled out by the available data.

Here we explore a range of possible atmospheres for the planet, ranging from solar composition gas, to pure CO_2 or water (steam). We present transmission and emission spectra for each of these cases.

We find that, if GJ 1214b possesses a hydrogen-rich atmosphere as expected, then the primary transit depth for such an atmosphere would vary at a level of up to 0.3% as a function of wavelength, relative to the background light of its M-dwarf host star. Observations at this level of precision are potentially obtainable with current space-based instrumentation. Successful detection of the transmission signature of this planet at the ~0.1% level would therefore provide confirmation of the hydrogen-rich nature of the planetary atmosphere.

It follows that transmission spectroscopy at this level of precision could provide a first glimpse into answering the question of whether planets in the super-Earth mass regime (1 – 10 M_Earth) more closely resemble large terrestrial planets or small gas giant planets.

Comments: submitted to ApJ Letters

Subjects: Earth and Planetary Astrophysics (astro-ph.EP)

Cite as: arXiv:1001.0976v1 [astro-ph.EP]

Submission history

From: Eliza Miller-Ricci [view email]

[v1] Wed, 6 Jan 2010 21:07:12 GMT (141kb)

http://arxiv.org/abs/1001.0976

Ramkumar February 7, 2010 at 2:09

Its is near to hear to discovery of a super earth with atmosphere ansd water .Yet the conditions will be similar to that of Venus ….

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