Illustration courtesy Jason Rowe/NASA/SETI and Jaymie Matthews/UBC

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A simulation of 55 Cancri e transiting its parent star compared to Earth and Jupiter transiting our sun.

Illustration courtesy Jason Rowe/NASA/SETI and Jaymie Matthews/UBC

Dense, Hot Super-Earth Is "New Class of Planet"

Exotic world is "a Sumo wrestler in a supermodel body," expert says.

It may be close to Earth's size and orbiting a sunlike star—but that's where the similarities end between our world and 55 Cancri e.

New observations show that the so-called super-Earth is the densest planet yet found outside our solar system—twice as dense as Earth—and the hottest known rocky world.

"With a year that lasts less than 18 hours, surface temperatures reaching 2,700 degrees Celsius [4,892 degrees Fahrenheit], and having the density of lead, this is hands down the most exotic world we have ever seen," said study co-author Jaymie Matthews, an astronomer at the University of British Columbia in Canada.

Nestled in the constellation Cancer, 55 Cancri e is part of a four-planet system that's just 42 light-years away.

The unusual world was discovered in 2004 via ground-based telescopes using a technique called radial velocity. This method measures how much a star "wobbles" due to the gravitational tug of orbiting planets. Astronomers at the time estimated that the planet took about 2.8 days to orbit its parent star.

In 2010, however, scientists flagged possible errors in the initial measurements. The corrected figures suggested the planet hugged its star much closer than previously believed.

Using Canada's suitcase-size MOST (Microvariability and Oscillations of STars) space telescope, astronomers have now made measurements of the tiny dips in starlight as 55 Cancri e transits—or passes in front of—its star, as seen from Earth.

The new data show that 55 Cancri e orbits once every 17 hours and 41 minutes, which means it's about 20 times closer to its star than Mercury is to our sun.

"If you looked up in the sky, their sun would appear 60 times bigger and shine more than 3,600 times brighter than on Earth," Matthews said. "To block out the sun you would need to hold up a dinner plate at arm's length."

By contrast, a thumb held at arm's length would block out the midday sun on Earth.

Densest Planet Is a Sumo in Supermodel Form

Because the host star is so bright, MOST was able to make many types of measurements, Matthews said.

In addition to the planet's orbital period, the satellite was able to determine 55 Cancri e's size and mass, giving its density. The research was published online last week on and has been submitted to the Astrophysical Journal Letters.

The team found that the planet is 60 percent larger and eight times more massive than Earth, making it a super-Earth—a planet that's up to ten times Earth's mass but with the potential for solid surfaces and liquid oceans.

Given its size and mass, Matthews said, "you can liken 55 Cancri e to a Sumo wrestler in a supermodel body."

And the scrutiny surrounding this exotic superEarth may continue: This week a second team of astronomers independently reported on that 55 Cancri e may be larger than thought, and so perhaps not as dense.

Using NASA's Spitzer Space Telescope to watch the planet transit, MIT planet hunter Sara Seager and her team determined that 55 Cancri e may be as much as a third bigger than what the MOST team is reporting.

"We looked at the planet at a different wavelength, and it definitely appears to have a different radius," Seager said. While MOST examined the planet in visible light, Spitzer sees the cosmos in infrared wavelengths.

Exotic Super-Earth May Be New Planet Class

One possible explanation for the differences in the size measurements, Seager said, is that the Spitzer's infrared data might be including an atmosphere in the planet's radius.

"We may be measuring the edge of a thick atmosphere like those found on Uranus or Neptune, or an exosphere—a tenuous atmosphere—like that found on Mercury. We just don't know."

"At this point we are just reporting what Spitzer has found, motivated by what MOST has seen, and we don't really understand why there is this discrepancy in the measured radius of Cancri e," Seager said. "Basically we are in the middle of a mystery that is unsolved."

The next steps will be to collect more data on 55 Cancri e with Spitzer and to enlist the help of the Hubble Space Telescope, which has a larger mirror than MOST and can also collect data in visual wavelengths.

In general, though, Seager agrees with study co-author Matthews that, whatever size it ends up being, 55 Cancri e is definitely a new type of world.

"This is basically a new class of super-Earths that live close to their stars," MIT's Seager said. "Regardless of what the radius actually is, this definitely is a new class of planet we are seeing."