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Squinting to Confirm a New 'Earth'

Does exoplanet Gliese 581 c exist? Could it support life? UBC team key to finding out.

By Monte Paulsen 11 May 2007 | TheTyee.ca

Monte Paulsen is a contributing editor at The Tyee. He welcomes your feedback via e-mail (monte@thetyee.ca), and invites you to participate in the online discussion happening below.

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Earth 2.0? Artist's rendition of Gliese 581 c

Canada's space telescope has spent the past two weeks straining for a glimpse of what an elite group of European astronomers claim is the first habitable planet discovered outside this solar system. The suitcase-sized Canadian satellite, called MOST, is the only instrument capable of quickly verifying the historic claim.

The extrasolar planet -- or "exoplanet" -- is named Gliese 581 c, and is thus far the only other place in the universe believed capable of supporting liquid water, and therefore extraterrestrial life. It was discovered using the HARP instrument at the European Southern Observatory's 3.6-meter telescope in La Sille, Chile.

But even before the April 25 announcement splashed across the pages newspapers, the European astronomers had quietly contacted the MOST mission control team at the University of British Columbia. The Europeans sought verification of their ground-based observation, because they hadn't actually "seen" the new exoplanet. Rather, they deduced its presence using the radial velocity method, in which the presence of a planet is deduced based on how its mass causes the orbit of its host star to wobble.

"The radial velocity signal is quite low, and there is a lot of scatter," said Jaymie Mark Matthews, principal investigator in charge of the MOST mission. "There is justifiable skepticism within the exoplanet community about whether this planet really exists."

To remove all doubt, astronomers need to catch a glimpse of the planet itself. MOST circles the Earth in a polar orbit, its 15-centimetre telescope unfettered by our murky atmosphere. Launched by the Canadian Space Agency in 2003 for a mere $10 million, MOST is the only space telescope sufficiently agile to re-point on relatively short notice.

Rather than watch for wobbles, MOST detects what astronomers call "transits." Just as a mosquito passing in front of a light bulb blocks the light ever so slightly, an exoplanet passing between its star and Earth dims the amount of light reaching MOST. By measuring the miniscule reduction in light, the MOST team can estimate the size of the transiting object.

"If we observe a transit, that will take away all ambiguity," Matthews said. "We'll know we're looking at a planet."

And MOST will secure its role in an international adventure the likes of which this world has not witnessed since the era of Drake and Magellan -- a race to determine whether or not we are alone in the universe.

To glimpse a passing planet

Matthews figures the odds of MOST catching a fleeting glimpse of Gliese 581 c at about one in 30. That's because for MOST to observe a transit, the exoplanet's orbit must pass directly between its host star and Earth. Astronomers do not know the inclination of the Gliese 581 c orbit, but assume that orbital planes are inclined randomly throughout the universe. Because Gliese 581 c orbits much closer to its dim star than Earth does to the Sun, the geometry improves MOST's odds.

MOST has already watched one potential transit period, and will observe several more before returning to its day job, stellar seismology.

"We had our first chance earlier this week," Matthews told The Tyee. "We'll have another intense stakeout in less than two weeks."

If MOST does catch a transit, astronomers will be able to combine MOST's data on the planet's size and speed with HARP's observations of mass. "We would be the first to measure the density of an Earth-like planet. No ones ever been able to do that," Matthews said. "We would be able to tell whether it was an ocean, or rocky."

Even more important to the scientists, MOST will return baseline information about the star itself. Gliese 581 a is a red dwarf, a type of star that tends to be more turbulent than stars like the Sun. MOST will likely determine whether or not that "variability" is sufficient to mislead the HARP instrument. This is precisely what MOST -- an acronym for Microvariability and Oscillations of STars -- was built to study.

Likewise, MOST could determine if the Gliese 581 system's habitable zone is actually habitable. If it splashes periodic waves of intense heat toward its companion planets, "that might not be all that great an environment for life to gain a foothold," Matthews said. "On the other hand, the star is remarkably quiescent for a red dwarf. That would be a good sign that that 581c could be a solid world that could support liquid water on the surface."

"We're lucky we live near such a boring star," Matthews added. "If the Sun's energy varied the way some red dwarfs do, we wouldn't be here. Our climate would be changing even more dramatically than the global warming everyone is currently concerned about."

Habitable, but not just like home

What makes the Gliese 581 c discovery so remarkable is that of the 233 planets discovered thus far, it is the only one that does not suffer from what some astronomers call "the Goldilocks paradox." All the other exoplanets discovered to date are either too hot (because they orbit too close to their stars) or too cold (they orbit too far out) for water to remain a liquid, which is regarded as a precondition for the existence of life as we know it.

Gliese 581 c is just right. Researchers predict its average temperature to be somewhere between 0 and 40 degrees Celsius. (The Earth's mean surface temperature is currently about 15 degrees Celsius, and is projected to increase between one and six degrees by 2100.)

But even if it does prove to be a wet, rocky planet, Gliese 581 c probably wouldn't feel much like home. It's about 50 per cent larger than Earth. It's also five times as massive, so its gravitational pull would be greater. And because it completes a full orbit of its dwarf star every 13 days, birthdays would be celebrated more or less every other weekend.

Gliese 581 c orbits 14 times closer to its host star than Earth orbits the Sun. To anyone standing on the surface of Gliese 581 c, the star would appear two or three times larger that than the sun does from Earth. "Red dwarfs are the Honda Civics of the Universe," Matthews said. "Being relatively dim, they don't use up their fuel at such a rapid rate. So they last a long time."

If there are creatures living on Gliese 581 c, they probably see things differently than we do. Our eyes have evolved to be most sensitive to the strongest light frequencies emitted by our Sun. The Gliesians, noted Matthews, "would likely possess vision skewed to red or infrared, because that's what their star emits."

The neighbourhood is bit different, too. Whereas Earth's solar system has eight full-patch planets -- plus Pluto, Xena and an icy crew of hang-arounds -- only three have been identified in the Gliese 581 system: The Neptune-sized 581b (15 times the mass of Earth) that orbits in only 5.4 days, the Earth-like 581c that orbits in 13 days, and 581d (eight times the mass of Earth) that orbits in 84 days.

Modern-day Magellans

If verified as a habitable planet, the discovery of Gliese 581 c will boost the reputation of the team led by Swiss astronomers Michel Mayor and Didier Queloz, who in 1995 became the first earthlings to identify an exoplanet. (See sidebar.)

Gliese 581 c was idenfitied with the HARPS (High Accuracy Radial Velocity for Planetary Searcher) instrument, a precise spectrograph operated by the European Organisation for Astronomical Research in the Southern Hemisphere.

"HARPS is a unique planet hunting machine," said Mayor, who serves as principal investigator. "We can say without doubt that HARPS has been very successful: Of the 13 known planets with a mass below 20 Earth masses, 11 were discovered with HARPS."

"The discovery of Gliese 581 c is an important stepping stone," said UBC's Matthews. "Even if it turns out that this isn't a planet, we anticipate that HARPS will uncover more Super-Earths. Once we've got 15 or 20 to observe, the odds are good that one of them is going to transit."

Matthews said MOST will likely release preliminary findings related to Gliese 581 c sometime next month.

If Canada's diminutive space telescope observes a transit of Gliese 581 c, a next step would be to use a larger colour space telescope to determine whether there is water vapour in the exoplanet's atmosphere. (MOST is a black-and-white instrument.) The first identification of interstellar water was recently deduced using infrared data from transits observed by the $3 billion Hubble Space Telescope.

"This might or might not be it," Matthews said. "I have no doubt that within the next five to 10 years, we will find another Earth-like planet."

Close, but still far away

Gliese 581a is among the 100 closest stars to Earth. It's located only 20.5 light-years away in the constellation Libra ("the Scales"). "You could observe this star with an amateur telescope," Matthews said. "You could probably see it with a decent set of binoculars."

Xavier Delfosse, a French member of the ESO team, released a statement declaring: "Because of its temperature and relative proximity, this planet will most probably be a very important target of the future space missions dedicated to the search for extra-terrestrial life."

Delfosse added, "On the treasure map of the Universe, one would be tempted to mark this planet with an X."

But getting there is another matter entirely.

In order to visualize 20.5 light years, imagine a model in which the Sun is about the size of a cherry. The Earth would be a grain of sand, revolving around the cherry at a distance of one metre. Pluto would be 40 metres away. If the cherry is in Vancouver, the next closest star (Proxima Centauri) would be just south of Seattle. And Gleise 581 would be in San Francisco.

These distances cripple fantasies of Star Trek–style travel. The fastest spacecraft ever launched left Earth last year. Travelling at 50,000 kilometres per hour, it will take nine and a half years to reach Pluto. That same spacecraft would require 350,000 years to reach Gliese 581 c. In order for that spacecraft to reach Gliese 581 c this decade, it would have had to have left Earth about the same time pygmy-sized hominids first stood erect.

Radio communication is more practical. The non-governmental Search for Extraterrestrial Intelligence (SETI) listened for signs of life in the Gliese 581 system in 1995 using the Parks Radio Telescope in Australia, and again in 1997 using the Green Bank Radio Telescope in West Virginia. No signal was detected. Senior astronomer Seth Shostak has announced that SETI will listen to the system again this summer when the new Allen Telescope Array begins operations.

Just as intriguing is the question of whether Gliese 581 c has been listening to Earth.

Radio signals travels at roughly the speed of light. So Gliese 581 c would only now be receiving radio and television programming broadcast in late 1986. "Walk Like an Egyptian" was the Bangles' hit single. Knight Rider was in its fourth (and final) season, and Alf had just premiered.

"Knight Rider," Matthews mused. "If there is intelligent life on Gliese 581 c, that probably explains why they haven't contacted us."

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