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Looking Hard at Gliese 581

We’d all like to know more about Gliese 581 c, the most talked about exoplanet of them all because of the possibility — however controversial — that it may be habitable. One way to learn more would be to observe a transit, which is what the Canadian space telescope called MOST is now attempting to do. The odds are roughly one in thirty, according to MOST principal investigator Jaymie Mark Matthews, but even the few observations ahead for MOST will tell us more about the star in question.

Matthews’ thoughts are reported in an article in the British Columbian alternative daily The Tyee, along with a nice backgrounder on the planet by writer Monte Paulsen. Evidently the Swiss team behind the Gliese 581 c announcement, which includes Michel Mayor and Didier Queloz (the first to identify an exoplanet, in 1995), had contacted the MOST controllers at the University of British Columbia before going public with their latest work. They hoped a transit could verify the existence of the new planet and sharpen up our knowledge of its parameters.

From the story:

“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 one’s ever been able to do that,” Matthews said. “We would be able to tell whether it was an ocean, or rocky.”

But in the absence of a transit, MOST can still be helpful in looking at how active Gliese 581 actually is. The mission — Microvariability and Oscillations of STars — is uniquely qualified for that project, being designed to conduct seismic probes of stellar structures and ages. A closer look at the star’s flare activity may tell us something about conditions in its habitable zone, whether or not Gliese 581 c is actually within that zone. According to Matthews, MOST will release preliminary findings on the investigation some time next month.

Comments on this entry are closed.

  • andy May 14, 2007, 15:03

    Are transits of the outer planet also being searched for, or are the transit windows being observed with MOST just those for planet c? It seems that the innermost planet does not transit, but as yet we do not know whether the exoplanetary systems we have discovered are coplanar. A detection of either of the outer two planets of Gliese 581 would be very valuable.

    To bear in mind: while radius measurements should be able to distinguish between terrestrial and icy (“ocean”) planet possibilities, it will not reveal what surface conditions are like – a hot icy planet will not necessarily have liquid water at its surface, and a supercritical atmosphere is a definite possibility, detailed modelling of the structure and evolution of an ice planet at that distance from its star would be required to determine whether liquid oceans are a possibility.

    If the planet is terrestrial, it likely has a relatively thin atmosphere, and the question becomes whether an atmosphere/hydrosphere system could avoid runaway greenhouse or atmospheric freezeout on a tidally-locked world.

    What would be really bizarre would be if one of these planets were some kind of super-Mercury, a gigantic ball of iron with a thin silicate mantle. That would be a major challenge for planet-formation theories to explain…

  • Administrator May 14, 2007, 19:12

    Although I haven’t been able to confirm this, I believe that c is the only planet being checked for transits. I’ll post here when I get further info.

  • David May 15, 2007, 6:28


    They probably are not looking for transits from the outer planet because probability of a transit scales to the inverse of the semi-major axis for circular orbits so the chance go down here. The MOST mission may well have a finite lifetime so they can’t spend too much time looking for events that have a small chance. The payoof from detecting a transit from planet c justifies the time, less so for the outer planet.

  • andy May 15, 2007, 11:08

    I guess you’re right there. Given that the timescales they are talking about is about a month or so, and the orbit of planet d is around 80 days, it would take much longer than this baseline to determine whether planet d is transiting.

  • Athena May 15, 2007, 11:48

    The possibilities of Gliese 581c are as intriguing and exciting to biologists as to astrophysicists and planetary scientists. I wrote a brief essay from that angle, you can find it here: http://www.starshipnivan.com/blog/?p=45

  • andy May 16, 2007, 7:02

    We have a transiting hot Neptune! Detection of transits of the nearby hot Neptune GJ 436 b. The planet is probably mainly icy with a hydrogen-helium envelope.

    Gliese 436 b is more massive and orbits closer to its star than the innermost planet of Gliese 581 does.

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