Nearby super-Earth likely a diamond planet
Illustration of the interior of 55 Cancri e — an extremely hot planet with a surface of mostly graphite surrounding
a thick layer of diamond, below which is a layer of silicon-based minerals and a molten iron core at the center.
(Image by Haven Giguere)
October 11, 2012
New research led by Yale University scientists suggests that a rocky planet twice Earth's size orbiting a nearby star is a
“This is our first glimpse of a rocky world with a fundamentally different chemistry from Earth,” said lead researcher
Nikku Madhusudhan, a Yale postdoctoral researcher in physics and astronomy. “The surface of this planet is likely covered
in graphite and diamond rather than water and granite.”
The paper reporting the findings has been accepted for publication in the journal Astrophysical Journal Letters.
The planet — called 55 Cancri e — has a radius twice Earth’s, and a mass eight times greater, making it a “super-Earth.”
It is one of five planets orbiting a sun-like star, 55 Cancri, that is located 40 light years from Earth yet visible to the
naked eye in the constellation of Cancer.
The planet orbits at hyper speed — its year lasts just 18 hours, in contrast to Earth’s 365 days. It is also blazingly hot,
with a temperature of about 3,900 degrees Fahrenheit, researchers said, a far cry from a habitable world.
The planet was first observed transiting its star last year, allowing astronomers to measure its radius for the first time.
This new information, combined with the most recent estimate of its mass, allowed Madhusudhan and colleagues to infer its
chemical composition using models of its interior and by computing all possible combinations of elements and compounds that
would yield those specific characteristics.
Astronomers had previously reported that the host star has more carbon than oxygen, and Madhusudhan and colleagues confirmed
that substantial amounts of carbon and silicon carbide, and a negligible amount of water ice, were available during the planet’s
Star map showing the planet-hosting star 55 Cancri in the constellation of Cancer. The star is visible to the naked eye, though better through binoculars.
(Image by Nikku Madhusudhan; created using Sky Map Online)
Astronomers also thought 55 Cancri e contained a substantial amount of super-heated water, based on the assumption that its
chemical makeup was similar to Earth’s, Madhusudhan said. But the new research suggests the planet has no water at all, and
appears to be composed primarily of carbon (as graphite and diamond), iron, silicon carbide, and, possibly, some silicates.
The study estimates that at least a third of the planet's mass — the equivalent of about three Earth masses — could be diamond.
“By contrast, Earth’s interior is rich in oxygen, but extremely poor in carbon — less than a part in thousand by mass,”
says co-author and Yale geophysicist Kanani Lee.
The identification of a carbon-rich super-Earth means that distant rocky planets can no longer be assumed to have chemical
constituents, interiors, atmospheres, or biologies similar to those of Earth, Madhusudhan said. The discovery also opens new
avenues for the study of geochemistry and geophysical processes in Earth-sized alien planets. A carbon-rich composition could
influence the planet’s thermal evolution and plate tectonics, for example, with implications for volcanism, seismic activity,
and mountain formation.
“Stars are simple — given a star's mass and age, you know its basic structure and history,” said David Spergel, professor of
astronomy and chair of astrophysical sciences at Princeton University, who is not a co-author of the study.
“Planets are much more complex. This ‘diamond-rich super-Earth’ is likely just one example of the rich sets of discoveries that
await us as we begin to explore planets around nearby stars."
In 2011, Madhusudhan led the first discovery of a carbon-rich atmosphere in a distant gas giant planet, opening the possibility
of long-theorized carbon-rich rocky planets (or “diamond planets”). The new research represents the first time that astronomers
have identified a likely diamond planet around a sun-like star and specified its chemical make-up. Follow-up observations of the
planet's atmosphere and additional estimates of the stellar composition would strengthen the findings about the planet’s chemical
The authors of the paper are Madhusudhan, Lee, and Olivier Mousis, a planetary scientist at the Institut de Recherche en
Astrophysique et Planetologie in Toulose, France.
The paper is titled “A Possible Carbon-rich Interior in Super-Earth 55 Cancri e.”
The research was supported by the Yale Center for Astronomy and Astrophysics (YCAA) in the Yale Department of Physics through
Madhusudhan’s YCAA postdoctoral prize fellowship.