11 October 2013

New Research Casts Doubt On Composition Of So-Called ‘Diamond Planet’

Image Caption: The smallest of several planets in the 55 Cancri system, the former 'diamond planet' is seen orbiting its host star at very close range in this artist's impression. A nearby brown dwarf with its own 'miniature' planetary system is pictured as well. Credit: NASA/JPL-Caltech 

What was believed to be the first planet discovered to be comprised largely of diamond most likely does not have a high content of the precious mineral after all, according to new research scheduled for publication in in the Astrophysical Journal.

The planet, which is located approximately 40 light years from our solar system, was first analyzed in 2010. At the time, it was concluded that the world had a surface of graphite surrounding a thick layer of diamond, instead of water and granite like on Earth. However, a new team of investigators, led by University of Arizona astronomy graduate student Johanna Teske, now believes otherwise.

Teske’s team revisited the original observations, which – together with simulations used by astronomers to model the interior of a planet, based on data such as its radius, mass and orbital velocity – had yielded a carbon-to-oxygen ratio of greater than one. In other words, those scientists believed that the planet was carbon-based instead of oxygen-based as are Earth and most other planets in the Milky Way.

However, Teske and her colleagues now report that the planet’s host star contains nearly one-fourth more oxygen than carbon, placing it about midway between the Sun and the levels suggested by the previous study. Those finds cast doubt on the original study’s conclusion, making it unlikely that the planet in question is actually diamond-rich.

“The 2010 paper found that ‘55 Cancri,’ a star that hosts five planets, has a carbon-to-oxygen ratio greater than one,” Teske said in a statement. “This observation helped motivate a paper last year about the innermost planet of the system, the ‘super-Earth’ 55 Cancri e. Using observations of the planet’s mass and radius to create models of its interior that assumed the same carbon-to-oxygen ratio of the star, the 2012 paper suggested the planet contains more carbon than oxygen.”

“However, our analysis makes this seem less likely because the host star doesn’t appear as carbon-rich as previously thought,” she added. “In theory, 55 Cancri e could still have a high carbon to oxygen ratio and be a diamond planet, but the host star does not have such a high ratio. So in terms of the two building blocks of information used for the initial ‘diamond-planet’ proposal – the measurements of the exoplanet and the measurements of the star – the measurements of the star no longer verify that.”

55 Cancri e is a super-Earth that is roughly twice the Earth’s diameter and eight times our planet’s mass, the researchers said. It is the smallest member of a five-planet solar system, located in the constellation Cancer. Furthermore, it is so close to its host star that it has a surface temperature topping 3,000 degrees Fahrenheit, and orbits at such a pace that one-year is only 18 hours long, they added.

“Since astronomers can’t probe the makeups of stars and planets directly, they rely on indirect observational methods such as absorption spectra; each chemical element absorbs light at different wavelengths, in a characteristic pattern that can be used as a fingerprint of that element,” the university said. “By analyzing the absorption spectra of starlight passing through a star’s atmosphere, it is possible to deduce what elements are present in the star’s atmosphere.”

“Instead of using the same absorption lines in the spectrum of the host star as the previous study of 55 Cancri, we looked at more lines of carbon and more lines of oxygen,” added Teske. “We find that because this particular host star is cooler than our sun and more metal-rich, the single oxygen line analyzed in the previous study to determine the star’s oxygen abundance is more prone to error.”

Teske and her associates instead opted to use alternative indicators of the oxygen abundance. By averaging all of those measurements together, she said that it provided a more complete picture of the amount of the element actually contained within the star. Furthermore, the researchers note that the “diamond planet” hypothesis was based on the presumption that a star’s composition has an impact on the composition of its planets.

“However, as astronomers discover more and more extrasolar systems, a one-size-fits-all formula becomes less likely,” the university said. Teske explained that we are currently uncertain whether or not our solar system is common or unusual in terms of the larger universe, as many of the systems astronomers have discovered have giant gas planets located closer to the star (versus our solar system, where rocky planets are closest to the sun).

“The compositions of planets and stars don’t always match,” she noted. “Depending on where 55 Cancri e formed in the protoplanetary disk, its carbon-to-oxygen ratio could differ from that of the host star. It could be higher or lower. But based on what we know at this point, 55 Cancri e is more of a ‘diamond in the rough.’”

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