'Exotic' planet is densest of its kind

A rocky planet that is as dense as lead and where a year lasts less than 18 hours has been described by a team of U.S. and Canadian scientists.

55 Cancri e as dense as lead and has year less than 18 hours long

55 Cancri e is a super-Earth located in a very tight, short orbit around a yellow dwarf star similar to Corot-7b, above. Corot 7b was confirmed in 2009 to be the first rocky extrasolar planet. ((European Southern Observatory/Associated Press))
A rocky planet that is as dense as lead and where a year lasts less than 18 hours has been described by a team of U.S. and Canadian scientists.

"On this world — the densest solid planet found anywhere so far, in the solar system or beyond — you would weigh three times heavier than you do on Earth," said University of British Columbia astronomer Jaymie Matthews in a statement.

"By day, the sun would look 60 times bigger and shine 3,600 times brighter in the sky."

Of course, it's unlikely you would ever have a chance to get that view, because the planet, named 55 Cancri e for the star it orbits (the star is called 55 Cancri A), is 40 light years from Earth, and the temperature on its surface is estimated to be as high as 2,700 C.


Planets such as 55 Cancri e that are larger than the Earth but less than 10 times larger are known as "super-Earths."  Astronomers are interested in such planets because:

  • There is nothing like them in our solar system.
  • They have the potential to be solid or have liquid oceans. That means if other conditions, such as temperature, are right, they may be a potential home for life as we know it.

The planet is described in a paper co-authored by Matthews and posted this week on the online archive for papers in the physical and mathematical sciences.

The article, whose lead author is Joshua Winn, an astrophysicist at the Massachusetts Institute of Technology in Cambridge, Mass., has been submitted for formal publication in the Astrophysical Journal Letters but has not yet undergone peer review.

Matthews told CBC News that he and his team rushed to get the data out because 55 Cancri A will only be visible until June this year, and they wanted to give other researchers the chance to do follow-up research on what he called the "very exciting" results. 

After June, the 55 Cancri A star will be visible above the horizon at the same time as the sun and therefore impossible to observe again until January 2012. Matthews said he is very confident in the data and in the scrutiny of scientists who read the paper critically after seeing it posted online.

The planet, described by Matthews as "exotic" because it is so different from other known planets, was first discovered around a star in the constellation Cancer in 2004 using a ground-based telescope. However, because such telescopes can't collect data about stars during the day, the Texas-based team that discovered it appears to have misinterpreted information about its orbit, Matthews said.

The new paper estimates the planet's mass, radius and density and the length of its year based on measurements by the Canadian MOST satellite. The microsatellite, which orbits the Earth as part of a Canadian Space Agency mission, carries a telescope that feeds into a photometer, an instrument that measures the intensity of light from distant stars. Matthews is the mission scientist who leads the MOST team.

Based on the satellite's measurements, Cancri 55 e has a diameter 60 per cent larger and a mass eight times larger than Earth's, making it twice as dense as the Earth. It orbits its star every 17 hours and 41 minutes.

Matthews and his team managed to figure that out by using MOST to observe small changes in the brightness of Cancri caused by the planet passing in front of the star, which happens each time it completes an orbit, because of its orientation relative to the Earth. Larger planets cause more dimming, allowing for an estimate of its size.

The MOST data also allowed the researchers to figure out the tilt of the planet's orbit, which was needed to calculate its mass using previously published data.

The star 55 Cancri is a G-type star, or yellow dwarf, like our own sun that is visible to the naked eye from Earth. That star is orbited by four other planets and another star — a small, cool, dim one called a red dwarf, or M-dwarf.

Winn, who led the study, approached the MOST team and asked them to turn their sights on 55 Cancri e after a paper came out suggesting that the original data about the planet might have been analyzed wrong. That paper was done by Rebekah Dawson, an astronomy PhD student at Harvard University, and Daniel Fabrycky, a Hubble Fellow at the University of California Santa Cruz. 

Dawson and Fabrycky's own analysis suggested the planet could be far more unusual than previously believed.

The star 55 Cancri is visible to the naked eye. To find it, look southwest, below the Big Dipper, near the northern tip of the brightest stars in the constellation Cancer (the Crab). ((Stellarium and Prof. Jaymie Matthews/University of British Columbia))


Emily Chung

Science, climate, environment reporter

Emily Chung covers science, the environment and climate for CBC News. She has previously worked as a digital journalist for CBC Ottawa and as an occasional producer at CBC's Quirks & Quarks. She has a PhD in chemistry from the University of British Columbia. In 2019, she was part of the team that won a Digital Publishing Award for best newsletter for "What on Earth." You can email story ideas to