Scientists believe most exoplanets, which are located far beyond our solar system, are composed of elements similar to those found on Earth. But astronomers say there may be planets made largely out of diamonds or graphite, and they could be more common than previously thought.
“Despite the relatively small amount of carbon on Earth, carbon has been critical for the emergence of life and the regulation of our climate through the carbon-silicate cycle,” says John Moriarty, a doctoral candidate at Yale University.
“It’s an open question as to how carbon-rich chemistry will affect the habitability of exoplanets. We hope our findings will spark interest in research to help answer these questions.”
Moriarty collaborated with Debra Fischer, astronomy professor, and Nikku Madhusudhan, a former postdoctoral researcher now at Cambridge University.
Astronomers generally believe that rocky exoplanets are composed—as Earth is—largely of iron, oxygen, magnesium, and silicon, with only a small fraction of carbon. In contrast, carbon-rich planets could have between a small percentage and three-quarters of their mass in carbon. Earth has 0.005 percent.
‘Drastically Different’ From Earth
For the study, published in Astrophysical Journal, researchers developed an advanced model for estimating exoplanet composition. Previous models were based on static snapshots of the gaseous pools (or disks) in which planets form. Their new model tracks changes in the composition of the disk as it ages.
Disks with carbon-oxygen ratios greater than 0.8, carbon-rich planets can form farther from the center of the disk than previously understood. They also found that carbon-rich planets can form in disks with a carbon-oxygen ratio as low as 0.65 if those planets form close to their host star.
Previous models predicted carbon-rich planets could only form in disks with carbon-oxygen ratios higher than 0.8. This is important, the researchers say, because there are many more stars with carbon-oxygen ratios greater than 0.65 than there are with carbon-oxygen ratios greater than 0.8.
“Our study shows that extraterrestrial worlds can be extremely diverse in their chemical compositions, including many that are drastically different from our earthly experience,” Madhusudhan says.
There are more than 1,000 confirmed exoplanets and more than 3,000 exoplanet candidates.
“An important question is whether or not our Earth is a typical rocky planet,” Fischer says. “Despite the growing number of exoplanet discoveries, we still don’t have an answer to this question. This work further expands the range of factors that may bear on the habitability of other worlds.”
The Yale Center for Astronomy and Astrophysics provided support for the research.