Family portraits of two planetary systems: A simulation of the silhouette of planet 55 Cancri e passing in front of (“transiting”) its parent star, compared to the Earth and Jupiter transiting our Sun, as seen from outside the Solar System. The MOST space telescope detected the tiny dip in starlight caused when the super-Earth planet blocked a small portion of the disk of the star 55 Cancri A, which is nearly a twin to the Sun. (Credit: Jason Rowe, NASA Ames and SETI Institute and Prof. Jaymie Matthews, UBC) |
An
international team of astronomers today revealed details of a
“super-exotic” exoplanet that would make the planet Pandora in the movie
Avatar pale in comparison.
The
planet, named 55 Cancri e, is 60 per cent larger in diameter than Earth
but eight times as massive. Twice as dense as Earth – almost as dense
as lead – it is the densest solid planet known, according to a team led
by astronomers from the Massachusetts Institute of Technology (MIT), the
University of British Columbia (UBC), the Harvard-Smithsonian Center
for Astrophysics and the University of California at Santa Cruz (UCSC).
The
research, based on observations from Canada’s MOST (Microvariability
& Oscillations of STars) space telescope, was released online today
at arXiv.org and has been submitted for publication in The Astrophysical
Journal Letters. MOST is a Canadian Space Agency mission.
Approximately
40 light years from Earth, 55 Cancri e orbits a star – called 55 Cancri
A – so closely that its year is less than 18 hours long. “You could set
dates on this world by your wrist watch, not a calendar,” says UBC
astronomer Jaymie Matthews.
The
temperature on the planet’s surface could be as high as 2,700 degrees
Celsius. “Because of the infernal heat, it’s unlikely that 55 Cancri e
has an atmosphere,” says lead author Josh Winn of MIT. “So this is not
the type of place where exobiologists would look for life.”
|
However,
55 Cancri e is the type of place exoplanetary scientists will be eager
to “visit” with their telescopes, says Winn. “The brightness of the host
star makes many types of sensitive measurements possible, so 55 Cancri e
is the perfect laboratory to test theories of planet formation,
evolution and survival.”
While
the planet isn’t visible, even through a telescope, its host star, 55
Cancri A, can be observed with the naked eye for the next two months on a
clear dark night.
“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. By day, the sun would look 60 times bigger and shine 3,600
times brighter in the sky,” says Matthews, MOST Mission Scientist and
second author on the paper.
The
first planet discovered around 55 Cancri A – designated “b” – was found
by a California-based team in 1997. Over the next five years, two more
planets (“c” and “d”) were found by the same team around the star. In
2004, a Texas-based team found 55 Cancri e, the subject of the latest
paper. (A fifth planet, f, was discovered in 2008).
All
five 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.
Canada’s space telescope, MOST, which detected the transits of the super-Earth exoplanet 55 Cancri e. (Credit: Canadian Space Agency and the MOST Team.) |
Last
year, Rebekah Dawson, an astronomy PhD student at Harvard and Daniel
Fabrycky, a Hubble Fellow at UCSC, re-analyzed the data and proposed
that the orbital period of 55 Cancri e could be much shorter than others
had assumed.
MIT’s
Winn, along with Smithsonian astronomer Matt Holman, brought the
problem to Matthews, who ordered the astronomical equivalent of a police
stakeout using MOST, which was able to detect subtle dips in the
brightness of star 55 Cancri A as planet e passed in front of it during
each orbit.
The
research team found that these “transits” occur like clockwork every 17
hours and 41 minutes, just as Dawson and Fabrycky predicted. The
starlight is dimmed by only 1/50th of a per cent during each transit,
telling the astronomers that the planet’s diameter is about 21,000 km –
only 60 per cent larger than Earth.
“It’s
wonderful to be 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,”
says Winn.
Matthews
agrees. “That’s the kind of thing Captain Kirk would do in an old
episode of Star Trek, we’re finally catching up with – maybe starting to
surpass – the science fiction I dreamed about as a kid.”
Student’s prediction points the way to hot, dense super-Earth
