Astronomers Reveal a “Super-Exotic Super Earth”

by Brian F. Ventrudo, Ph.D.

An international team of astronomers has used a suitcase-sized space telescope to help discover a planet around a nearby star in the constellation Cancer.  Measurements have revealed the planet, which revolves around the star 55 Cancri in less than one Earth day, to be a blazing hot “super-exotic” rocky world only slightly larger than Earth.  You can see the home star of this small, hot, and dense planet with your unaided eye or with a modest pair of binoculars in the night sky this month.

The planet, called 55 Cancri e, was found by a team of Texas astronomers in 2004. It orbits a sun-like star called 55 Cancri A in the constellation Cancer. The star, which is 40 light years from Earth, is known to have five planets labelled, in order of decreasing distance from the star, “d”, “f”, “c”, “b”, and “e”. The first four planets are likely massive gas giants without a solid surface.  But until now, the nature of the closest-in planet “e” was uncertain.

Last year, two American astronomers found a shorter orbital period for planet “e” than first assumed.  This implied the chance of the planet passing across the face of its home star as seen from Earth– an event called a transit– would be as high as 33%. And if a transit occurred, it would cause a small drop in brightness of 55 Cancri A which could be detected by a space-based telescope.

Artists impression of another hot rocky exo-planet CoRoT-7b, somewhat similar to 55 Cnc e

Determined to solve the mystery of planet “e”, astronomers Josh Winn of MIT and Matt Holman of Harvard approached Jaymie Matthews of the University of British Columbia. Matthews is the mission scientist for Canada’s MOST space telescope. MOST, sometimes called the “humble space telescope”, is a suitcased-sized instrument with a 6-inch mirror and sensitive CCD detectors to look for small fluctuations and oscillations in brightness of stars. While MOST was not intended for planet searches, it has the capability of detecting transit events.

Matthews ordered the astronomical equivalent of a police stakeout with MOST. And sure enough, the tiny space telescope detected small dips in the brightness of 55 Cancri as planet “e” passed in front of the star. The period of the planet’s orbit was found to be 17 hours and 41 minutes. And the planet appears to dim the light from the star by 1/50 of a percent, which means the radius of planet “e” is about 21,000 km, only 60% more than Earth’s.

With a mass eight times that of Earth, 55 Cancri “e” must be twice as dense as Earth, and nearly as dense as pure lead. Gravity at the surface is three times Earth’s. With such a short orbital period, the planet must be very close to the star, and the surface temperature of the planet could be as high at 2,700 ºC, too hot to support life. Yet with such strong gravity, the planet might hold an atmosphere. This offers the tantalizing chance for future measurements of the nature of the atmosphere as the planet passes in front of its star.

Here’s what the transit of planet 55 Cancri e would look like, compared to the transit of Earth and Jupiter across the face of our own Sun…

Of course, the planets around 55 Cancri A are not visible directly with any telescope. But you can see its host star with a pair of binoculars, or even with your unaided eye if you have dark sky. 55 Cancri A is a 6th-magnitude star just 1.5º east of the well-known double star iota Cancri.  Here are two maps to help you find the star, which is also known as rho-1 (ρ1) Cancri.  Look in the southwestern sky about 9 p.m.

Maps to help you find 55 Cancri in the southwestern sky at about 9 p.m. local time (click to enlarge).

Have a look for yourself at the star 55 Cancri over the next few days. Because, as Winn says, “There’s considerable pleasure in being able to point to a naked-eye star and know the mass and radius of one of its planets, especially a distinctive one like this.”

Matthews agrees. “We’re finally catching up with– and maybe starting to surpass– the science fiction I dreamed about as a kid”.