Detecting Exoplanet Atmospheres

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Studying exoplanet atmospheres is one of the best ways to determine whether or not a planet is habitable  because it gives insight into temperatures, biosignature gases, and other habitability indicators.

There are several techniques used to search for exoplanets, but only two can be used for analyzing exoplanet atmospheres.

Direct Imaging Method

Direct imaging is used to analyze the planets and moons within our Solar System, such as the planet Mars and the moon Titan. However, measuring spectra for extrasolar planets is difficult. The contrast in light emitted or reflected between a star and it's orbiting planet is on the order of about ten million to ten billion times. This contrast ratio is what makes analyzing atmospheres directly exceedingly difficult, particularly for small planets that are close to their star.

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Transit Method

During primary and secondary eclipses, atmospheres can be analyzed by studying differences in the emitted specta before, during, and after a transit. During a primary eclipse, the light from the star passes through the planet's atmosphere, allowing chemical species in the atmosphere to absorb some of the light being emitted by the star. This absorbtion will be visible in the detected spectrum, and from this pattern elements or compounds that are present in the exoplanet atmosphere can be inferred.

 

 

 

Types of Atmospheres

Sara Seager, Drake Deming. Exoplanet Atmospheres. Annu. Rev. Astron. Astrophys. 2010. 48:631–72

In the Solar System there is as apparent relationship between the relative abundance of ice, rock, or gas and planet mass. This can be seen in the image to the left. Given these different compositions, scientists (such as professor Sara Seager) can begin to categorize the different types of atmospheres, and predict what we might expect from an atmosphere on a given type of planet. Although the relationship between mass and composition is quite strong in our solar system, scientists are finding that the composition and mass distribution of planets likely fills the entire range of possibilities shown in the image to the left. Still, exoplanet atmospheres can be roughly grouped in the following categories:

1) Hydrogen and Helium dominated

Throughout the Milky Way galaxy, the average composition of all visible matter is roughly 75% hydrogen and 25% helium (1-3% comes from other elements). Planets which are dominated by hydrogen and helium, similar to the gas giants in our solar system, will likely have these same elemental composition ratios, which would indicate that they are formed primarily by capture from the protoplanetary nebula (or by planet formation from gravitational collapse).

2) Hydrogen-rich Outgassed Atmospheres

žOther planets will have atmospheres that are made of up of something other than captured cosmic material; some of these planets will have outgassed material from an icy or rocky surface. If the planet is massive enough, outgassed hydrogen can become trapped in the planet's atmosphere, and the atmosphere itself can be composed of up to 50% hydrogen by weight. These hydrogen rich atmopheres will likely contain Hydrogen gas (H2), naturally occurring water (H2O), and methane (CH4) or carbon monoxide (CO),  žbut not the carbon dioxide (CO2) or nitrogen gas (N2) that is seen in the terrestrial planets in our Solar System. Helium would also not likely be present, since this element is not trapped in rock and therefore would not be outgassed into the atmposphere.

3) Carbon Dioxide-rich Outgassed Atmospheres

žThese outgassed atmospheres are similar to those above, except that they have lost their hydrogen and helium and are dominated by carbon dioxide (CO2). žOn Earth, our atmosphere could potentially be in this category except that CO2 became dissolved in our oceans and sequestered in limestone sedimentary rocks, leaving nitrogen (N2)as the dominant atmospheric gas. In these atmospheres, signs of water molecules (H2O) may indicate a liquid water ocean. 

4) Hot Super-Earths Lacking Volatiles

žThis category is reserved for atmospheres that are very hot. When atmospheric temperatures reach 1,500 degrees Kelvin, these hot Super Earths will lose their hydrogen and other volatiles such as carbon, nitrogen, oxygen, and sulphur. Instead, these atmospheres would likely be composed of silicates and other refractory elements such as calcium, aluminum, and titanium.

5) Atmosphereless Planets

The final category includes planets that have been unable to retain any significant atmosphere. Less massive planets, very hot planets, planets without active outgassing, and planets subject to strong solar winds can potentially find themselves in this category.

In the end, the actual planet atmospheric composition depends on the interior composition of the planet, the mass and temperature of the planet, and the planet evolution.

Case Study: Origins of Gas Layer on GJ 1214b

THREE POSSIBLE ORIGINS FOR THE GAS LAYER ON GJ 1214b. L. A. Rogers and S. Seager. The Astrophysical Journal, 716:1208–1216, 2010 June 20.

GJ 1214b is a Super Earth type exoplanet found orbiting the star GJ 1214. The bulk composition of the planet has been analyzed using interior structure models which fit the measured planet properties (click here). One main conclusion of the analysis is that, given the planet's low density, there must be a significant atmosphere.

Atmosphere Origin

GJ 1214b may have formed from a variety of primordial materials in its protoplanetary disk. If gas accreted directly from the protoplanetary nebula and was retained in the atmosphere, then the atmosphere would be predominantly composed of hydrogen and helium. In this scenario, a hydrogen and helium layer surrounding an interior of iron, silicates, and ice would need to contain between 0.01% and 5% of the planet mass in order to account for the transit radius. This is interesting because the gas envelope would be less massive than Uranus’ and Neptune’s envelopes (which account for 5%–15% of the planet mass), yet greater than Earth’s or Venus’ atmospheres (which contribute 0.0001% and 0.01% of the planet mass, respectively).

Alternatively, the atmosphere could be outgassed via sublimation of ice-forming materials, which would release mostly water, carbon monoxide, carbon dioxide, methane, and ammonia. If sublimated ices dominate the gas layer, a massive water envelope comprising at least 47% of the planet mass could account for GJ 1214b’s observed parameters. In this case, a hydrogen/helium layer would not be required to explain the measured mass and radius.

The third scenario is that gas was sublimated from rocks and/or refractory materials. This would produce mostly iron, silicates, and sulfides. This outgassing could occur during the planet formation and/or from future techtonic activity. If outgassing was the dominant cause of the atmosphere, and not the retention of primordial hydrogen/helium, then the outgassed atmosphere would need to be hydrogen rich in order to account for the measured radius and density. This then constrains the composition of the primordial rocky material from which GJ 1214b was formed.

To understand the composition of GJ 1214b, models are constructed to find equations for the radius and pressure within the atmosphere based on interior mass, radius, and solar irradiation. These models are then compared to measurements of the planet's atmosphere during transits, and from there conclusions can be drawn about the true composition of the planet interior.

Data Analysis

The transit spectrum of GJ 1214b is shown below. Of the models proposed above, only the predicted spectra from a cloud-free atmosphere composed of roughly 70% water vapour by mass agree with the measured transmission spectrum. This indicates that the atmosphere was formed from the sublimation of icy materials which dominate the planet's interior composition.

Alternatively, high altitude clouds or hazes would be indistinguishable from a steam atmosphere, so the observed spectrum could also be indicative of clouds or hazes in the high atmosphere rather than low altitude water vapor.

A ground-based transmission spectrum of the super-Earth exoplanet GJ 1214b. Jacob L. Bean, Eliza Miller-Ricci Kempton & Derek Homeier. Nature, Vol 468, 2 Dec. 2010.

Reference material:

L. A. Rogers and S. Seager. Three possible origins for the gas layer on GJ 1214b. The Astrophysical Journal, 716:1208–1216, 2010 June 20.

Jacob L. Bean, Eliza Miller-Ricci Kempton & Derek Homeier. A ground-based transmission spectrum of the super-Earth exoplanet GJ 1214b. Nature, Vol 468, 2 Dec. 2010.

Sara Seager, Drake Deming. Exoplanet Atmospheres. Annu. Rev. Astron. Astrophys. 2010. 48:631–72