In the past decade we’ve discovered thousands of planets around other stars, but we’re only able to observe a few of the larger ones directly. This means that while we can determine some of an exoplanet’s properties such as its size and mass, it’s far more difficult to determine other properties such as the composition of its atmosphere. 

If a planet passes in front of its star from our vantage point, then it is theoretically possible to determine some of the compounds that make up its atmosphere. When a planet passes in front of its star, the atmosphere absorbs certain wavelengths of light, and those wavelengths depend upon the types of molecules in the atmosphere. So far we’ve been able to study the composition of gas giant atmospheres, but recently a team of astronomers observed the atmosphere of a “super-Earth” sized planet.

55 Cancri e has a mass about 8 times that of Earth, or about half that of Neptune. We don’t have a similar planet in our own solar system, so understanding this type of planet is a big goal of astronomy. This particular planet orbits it star at about 1/20th the distance of Mercury from the Sun. It’s star-facing side is estimated to have a temperature of about 2000°C, so there was some question as to whether it would have an atmosphere at all. Using observations from the Hubble’s Wide Field Camera 3, the team found not only that the planet has an atmosphere, but that the atmosphere contains significant amounts of hydrogen cyanide (HCN).

An atmosphere with significant HCN means that 55 Cancri e is likely a carbon planet, meaning that it would likely have an iron core like Earth, but instead of silicates its crust would consist of oxygen-carbon compounds. So it would likely have a crust of graphite, diamond and carbonate minerals. Carbon planets would also likely lack water, since carbon bonds so well with oxygen there would likely be little left to bond with hydrogen to form water. It was suspected that 55 Cancri e was a carbon planet because its parent star contains much more carbon than our Sun. Now it seems our suspicions were correct.

Paper: A. Tsiaras, et al. Detection of an atmosphere around the super-Earth 55 Cancri e. arXiv:1511.08901 [astro-ph.EP]

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Yesterday I talked about the 55 Cancri system, a Sun-like star which has at least four gas giant planets, two of which orbit the star much closer than Mercury in our own system.  What I didn’t talk much about is the other known planet in the system,  known as 55 Cancri e. It is a super-Earth with a mass about 9 times that of Earth’s.  One of the most surprising aspect of this world is that it orbits its star at a distance of about 0.016 AU, which is 1/20th the distance of Mercury from the Sun.  It is so close to the star that it is tidally locked, and its star-facing side has a temperature of more than 2000°C. Given its high temperature and proximity to the star, one would expect much of the lighter elements to have boiled off, leaving a rather dense iron-rich world.  What we find is that its density is only slightly higher than Earth’s. This means it is likely a new kind of planet.

At the moment we can’t observe any spectra from the planet, so we don’t know for sure what it is made of, or if it has an atmosphere.  All we know is its mass, size and orbital characteristics. But a hint lies in an unusual property of the star, specifically that it has much more carbon than our Sun. Assuming the planets have a similar composition, this could mean that this particular super-Earth could be a carbon planet. Such planets are sometimes referred to as diamond planets, partly because they could have diamond-laden mantles, but mainly because “diamond planet” sounds cooler than “carbon planet”.

Calculated sizes for different types of planets by mass.
Credit: Marc Kuchner/NASA GSFC

Given that Earth is teeming with carbon-based life, you might think Earth is a carbon planet.  But in fact, carbon makes up less than a tenth of a percent of Earth’s bulk.  Most of the planet is oxygen and silicon, which is evidenced by the fact silicates make up about 90% of the Earth’s crust.  A carbon planet would likely have an iron core like Earth, but instead of silicates it would consist of oxygen-carbon compounds.  So it would likely have a crust of graphite, diamond and carbonate minerals.  Carbon planets would also likely lack water, since carbon bonds so well with oxygen there would likely be little left to bond with hydrogen to form water.

The density of 55 Cancri e matches theoretical calculations for a carbon planet.  So that seems like a good possibility.  The planet is not dense enough to be a silicate planet like Earth, and other solutions such as water-ice seem unlikely given its high temperature.

Planets like 55 Cancri e are among the first to demonstrate that the composition of a planetary system depends critically on the composition of its star.  We like to imagine that exoplanets are similar to planets in our own system.  We even categorize them as “Earth-like” or “Jupiter-like”.  But it is very likely that these worlds are far more alien than we expect.

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The 55 Cancri system is in some ways very similar to our own.  The star is about the same mass and size as our Sun, and it has a Jupiter-sized planet at a distance of 5.7 AU, which is just slightly more distant than Jupiter’s 5.2 AU distance. It has three other gas planets, as well as a super-earth. One would think that the system might be similar in other ways, such as having rocky planets close to the star and gas planets more distant.  But this is not the case.  

Inner planets of 55 Cancri compared to our solar system. Credit: Center for Exoplanets and Habitable Worlds

The Jupiter-type planet is actually the most distant known planet.  It was first discovered by observing the motions of the closer planets, and noticing that their orbits drift slightly due to the outer planet.  The three other gas planets are remarkably close to the star.  One orbits at about the distance of Venus in our own system, while the other two are much closer than Mercury is in our own.  The closest gas planet orbits the star every 15 days.

It’s surprising that two gas giants would be so close to their star, as well as so close to each other.  In fact, there has been some debate as to whether the planets could be stable over billions of years.  But a recent article in the Monthly Notices of the Royal Astronomical Society shows they likely are.  The paper compares detailed observations of the planetary motions with a statistical analysis of their orbital dynamics.  It was found that while the two gas giants do affect each other’s orbit in measurable ways, the pairing is stable over a long time range.  So this planetary arrangement is not just a fluke of unstable orbits.

What 55 Cancri shows is that planetary systems can vary widely and still be stable.  Our solar system with rocky inner planets and gas giant outer planets is just one possibility among many.

Paper: Nelson BE, Ford EB, Wright JT, et al. The 55 Cancri planetary system: fully self-consistent N-body constraints and a dynamical analysis. Monthly Notices of the Royal Astronomical Society. doi: 10.1093/mnras/stu450

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