27 May 2013
NEW SUPEREARTH A SUPERHERO TO ASTRONOMERS
Canada’s MOST space telescope uncovers
the hidden identity of a planet orbiting a nearby bright star
The new Superman movie, shot in Vancouver, Canada, opens next month. Soon after cameras stopped rolling on the story of the Daily Planet reporter whose true identity is Superman, the CCD camera aboard Canada’s first space telescope sent to astronomers in Vancouver the first clues that the true identity of a planet around the star HD 97658 is a superEarth that could help us better understand the history of our own world.
HD 97658 b may sound more innocuous than Clark Kent, but this planet – not much more than twice the Earth’s diameter and a little less than 8 times its mass – may be more exciting to astronomers than Superman’s fictional home planet Krypton could ever be. It’s a “superEarth” – a class of planet for which there is no example in our home Solar System.
This superEarth orbits a star about 70 light years away (“next door” to us by astronomical standards) that’s almost bright enough to see by eye. That means astronomers can study the planet HD 97658 b and its parent star in ways impossible for most of the exoplanet systems that have been discovered around fainter stars.
Superman can’t see through lead, but he could see through HD 97658 b, since its average density is about 4 grams per cubic centimetre, a third of the density of lead (but denser than most rocks).
Astronomers see great significance in that value, about 70% of the average density of Earth, since the surface gravity of HD 97658 b could hold onto a thick atmosphere. But there’s unlikely to be alien life breathing those gases. The planet orbits its sun every 9.5 days, at a distance a dozen times closer than we are from our Sun. Too close to be in the Habitable Zone, nicknamed “The Goldilocks Zone.” Why? Because if a planet is too close to its star, it’s too hot; if it’s too far away, it’s too cold; but if it’s in the Zone, it’s “just right” for liquid water oceans – one condition that was necessary for life here on Earth. Superman could survive there, but we likely couldn’t.
The story of HD 97658 b is not so much a comic book adventure as a detective story.
The planet was discovered in 2011 by a team of NASA and University of California
astronomers, using a technique (sometimes called “Doppler wobble”) pioneered at the
University of British Columbia (UBC) and the University of Victoria in the 1980’s.
But only a lower limit could be set on the planet’s mass, and nothing was known about
its size. As part of her PhD research at UBC, Diana Dragomir (now with the Las Cumbres
Observatory Global Telescope) looked for “transits” of this exoplanet with Canada’s
MOST (Microvariability & Oscillations of STars) space telescope, launched in 2003
to a pole-
Transits occur when a planet’s orbit carries it in front of its parent star and reduces
the amount of light we see from the star ever so slightly. Dips in brightness happen
every orbit, if the orbit happens to be almost exactly aligned with our line of sight
from Earth. For a planet not much bigger than our Earth around a star almost as big
as our Sun, the dip in light is tiny, but detectable by the ultra-
The first report of transits in the HD 97658 system turned out to be a false alarm, as proven by MOST observations in 2012. That might have been the end of the story, but Dr. Dragomir knew that the “ephemeris” of the planet’s orbit (a timetable to predict when the planet might pass in front of the star) was not exact. She convinced the MOST team to widen the search parameters and during the last possible observing window for this star last year, the data showed tantalizing signs of a transit. Tantalizing, but not certain beyond doubt. A year later, MOST ‘revisited’ HD 97658 and found clear evidence of the planet’s transits, allowing Dr. Dragomir and the MOST team to estimate the planet’s true size and mass for the first time.
Dr. Dragomir will announce these results during a talk at the annual meeting of CASCA (Canadian Astronomical Society / Societé d’astronomie canadienne) at the University of British Columbia, at 2:00 pm PDT on Tuesday, 28 May 2013. Details of the CASCA 2013 meeting can be found at http://casca2013.phas.ubc.ca/