The Hottest, Densest Super-Earth Yet

On a very dark and clear night far from city lights, you can just make out 55 Cancri as a 6.0-magnitude pinprick in northern Cancer. Shining from 41 light-years away, it’s a main-sequence G8 star a little cooler and dimmer than the Sun. Last Friday it became the brightest star in the sky known to have a planet transiting across its face.

The planet is 55 Cancri e, the closest of the star’s five known worlds. All were discovered by the radial-velocity wobbles that they induce in the star. The wobbling due to planet e was teased out of complex radial-velocity patterns in 2004. Its strength tells that planet e has only 8.6 ± 0.6 Earth masses — a super-Earth.

Astronomers had assumed that 55 Cnc e revolves around the star every 2.8 days. But in late 2010 a paper by Rebekah Dawson, a graduate student at Harvard, and her colleague Dan Fabrycky, now at the University of California in Santa Cruz, analyzed the data differently. They proposed that its orbital period is four times faster: that it circles the star in 17 hours 41 minutes. This would put the planet even closer to the star, increasing the likelihood that it might cross the star’s face as seen from Earth.

Now a team led by Joshua Winn (MIT) confirms that 55 Cnc e indeed transits its sun. The team has observed 17 transits using Canada's MOST (Microvariability and Oscillations of Stars) satellite, the world’s first astroseismology space telescope. MOST studies the internal pulsations of stars by tracking tiny variations in their brightness).

"I'm excited that by calculating the planet's true orbital period, we were able to detect transits, which tell us so much more about it," said Dawson.

In particular, the amount of dimming during the transits tells that the object has a diameter 60% larger than Earth’s. Combined with its mass, this yields an average density of 11 ± 3 grams per cubic centimeter — higher than Earth’s value of 5.5 g/cm³, and implying a large iron core overlain by rock. The surface gravity must be about 3 gs.

Dawson and Fabrycky were inspired to reexamine the wobble data by a paper that MIT astronomer Jack Wisdom circulated informally in 2005, pointing out the possible ambiguity of the orbits in the 55 Cancri system. While radial-velocity tracking has been the mainstay of exoplanet hunters, it has its limitations. Daylight, clouds, and scheduling cause periodic gaps in the observations (which must be done at a large observatory on the ground), creating spurious wobble periods, called aliases, that would not occur if the observations were continuous. Dawson and Fabrycky studied the patterns of these aliases and devised a method to set them apart from true wobble periods. This led to the discovery of the planet’s correct orbit.

Planet e not only holds the record for transiting a naked-eye star, but it also has the fastest orbital period of any exoplanet known. It’s even closer to its star than CoRoT-7b (the “Planet from Hell” featured in Sky & Telescope in May 2009) and the similar Kepler-10b (April 2011 issue, page 12). Its star-facing side should be as hot as 4890° F (2970 K), making this also the hottest known world. Being so close to its star it must be tidally locked, so that one side always faces the star and the other always faces away.

All three of these roasted super-Earths have similar high densities and seem to form a class of their own. But since it orbits a star as bright as 6th magnitude, 55 Cancri e holds special promise for future study. “It is very exciting that we have a planet around a bright star,” said Sara Seager, an MIT expert on exoplanets who has contributed to Sky & Telescope.. “We can study it in a way Kepler-10b and CoRoT-7b cannot be observed.” Their stars are magnitudes 11.0 and 11.7, respectively.

A super-Earth so close to its star should be stripped of most of its atmosphere due to both heat and furious stellar wind, according to an analysis by Joshua Winn’s team. On the other hand, a paper by Brice-Olivier Demory (MIT) in May 3rd's Astrophysical Journal suggests that maybe this is wrong. The Spitzer Space Telescope’s infrared cameras recorded a single transit in January 2011, and it found the planet’s radius to be 30% larger than that deduced from MOST's observations. This disparity could be caused by thin gases in a very extended outer atmosphere blocking infrared but not visible wavelengths. Seager says that further observations with space telescopes like Spitzer and Hubble will be necessary before coming to any conclusion about an atmosphere.

Could amateurs possibly get in the game? The transits dim 55 Cancri by only 0.0002 magnitude, or two parts in 10,000, much less than even the best amateur photometrists can normally detect. But Greg Laughlin (University of California, Santa Cruz) thinks it could be done nonetheless. “The transit depth is small enough that you would definitely need to observe multiple transits and then stack the data,” he says, “but I think that some of the top amateurs could do it.”

Take that as a challenge.

Shweta Krishnan, an editorial intern at Sky & Telescope this summer, is a recent graduate of the Science and Medical Journalism Program at Boston University. She has a background in clinical medicine and strong interests in astronomy and physical sciences.