28 April 2011

SUPER-EARTH EXOPLANET = SUPER-EXOTIC PLANET

55 Cancri e MAKES Pandora SEEM DULL

How’s this for a location for the next Avatar movie?  A planet less than twice the diameter of Earth but almost as dense as lead, so you’d weigh three times as much on its surface. That orbits a sun like ours but so closely that its sun looks 60 times bigger and shines 3600 times brighter in its sky. Circling that sun every 18 hours – the length of a year on this world!

 

This sounds like science fiction, but you don’t need to go to the movies to see the star 55 Cancri A. You can walk outside for the next two months on a clear dark night and see it without a telescope.  You can’t see the planet though, designated 55 Cancri e, even with a telescope, but a team led by astronomers at MIT, UBC, Harvard and UC Santa Cruz, using Canada’s MOST space telescope, are now able to paint a picture of this super-Earth.  Its exotic characteristics make it a unique laboratory to investigate the story of how planets form and evolve.

The story of how 55 Cancri e was tracked down is more like a detective mystery than a sci-fi fantasy. And the star sleuth is Rebekah Dawson, an astronomy PhD student at Harvard.  In 1997, a planet (designated b) was detected orbiting the star 55 Cancri A by a California team, and in the next five years, two more (c and d) were found by the same team around the same star.  In 2004, a Texas-led team found a fourth planet, 55 Cancri e.  The planets were detected using the Doppler technique, where a star’s ‘wobbles’ due to the gravities of its unseen planets are measured in the shifting wavelengths of the spectra of the starlight.  

Rebekah and her collaborator, Dr. Daniel Fabrycky (University of California, Santa Cruz) re-analyzed the clues and argued last year that the true orbital period of planet 55 Cancri e is much shorter than others had assumed.  That it should be measured, not in days, but in hours. To keep track of its year, you wouldn’t need a calendar, but a wrist watch!

Dr. Josh Winn of MIT (Massachusetts Institute of Technology) and Dr. Matt Holman (Harvard-Smithsonian Center for Astrophysics) then took the case to Dr. Jaymie Matthews (University of British Columbia), Mission Scientist for a space telescope called MOST that could settle the mystery once and for all.  They, Rebekah, Daniel, and the rest of the MOST team put the star 55 Cancri A under the astronomical equivalent of a police stakeout, monitoring it continuously for two weeks.

The MOST (Microvariability & Oscillations of STars) satellite – a Canadian Space Agency mission – detected subtle dips in the star’s brightness, as the planet passed in front of it during each orbit.  These “transits” occur like clockwork every 17 hours and 41 minutes, just as Rebekah and Daniel predicted for planet 55 Cancri e.  The starlight is dimmed by only 1/50th of a percent during each transit, telling astronomers that the planet’s diameter is only 60% larger than Earth’s.  Combined with the spectroscopic data, we now know that 55 Cancri e is a “super-Earth” – 8 times more massive than our home world, and twice as dense.

In fact, 55 Cancri e is the densest solid planet known, anywhere.

In a tight orbit around a star that’s almost a twin to the Sun, the temperature at the surface of 55 Cancri e could theoretically be as high as 2,700 degrees Celsius.  Despite this infernal heat, but thanks to its strong gravity, the planet might be able to retain an atmosphere.  But even if it did, this is not the type of place where exobiologists would look for life.  It is, however, the type of place where exoplanetary scientists will look for clues to test theories of planet formation, evolution and survival.  The brightness of the host star makes many types of sensitive measurements possible.

“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,” enthuses lead author Dr. Winn.

Dr. Matthews, second author on the paper, agrees: “Yeah, that’s the kind of thing Captain Kirk would do in an old episode of Star Trek. We’re catching up with, and even starting to surpass, the science fiction I dreamed about as a kid.”