Newest Exoplanet Is the Lightest One Yet

by Eric on April 21, 2009

Gliese 581 Artist Conception

Rack up another exoplanet find for Michel Mayor of the University of Geneva. At today’s session of the JENAM conference at the University of Hertfordshire, UK, Mayor announced that yet another planet has been found orbiting that exoplanet superstar Gliese 581.

The addition of planet “e” brings the total number of exoplanets thought to be orbiting Gl 581 to 4: b, c, d, e. Let’s meet e shall we?

  • 1.9 Earth masses
  • Probably a rocky body due to its mass
  • Orbital period of 3.15 Earth-days puts it too close to fall inside the so-called “habitability zone.”

Oh well, but thanks for playing!

Our next contestant is planet d, which originally looked to be at the edge of the habitability zone, but with refinement to the orbital calculations, now turns out to be well within the habitability zone. Yay!

Mayor and collaborators credit the ever decreasing size of discovered exoplanets to the HARPS, a precision spectrograph attached to the 3.6 meter telescope at the European Southern Observatory’s La Silla Observatory. Mayor’s team uses the HARPS spectrograph to specifically track the wobble of M-dwarf type stars like Gl 581 as their orbiting planets tug on them. Using a technique called Doppler spectroscopy, changes in a star’s redshift reveals the relative motion of the star itself.

Why pick on such small stars? Since Gl 581 is relatively close in mass to its planets (at least compared to our solar system), these wobbles are going to be easier to detect. To give you an idea of how fine a motion we’re talking about, the noise floor caused by “jitter” in the observations is 1.2 m/sec. According to the paper, the quality of the technique is so good, with that level of jitter, they would have still been able to find an exoplanet only twice as massive as the earth with orbital period of 25 days, or the equivalent of an Earth-like planet in Gl 581’s habitable zone.

Now, looking for Earth-sized exoplanets or “superearths” inside the habitability zone of a host star is pretty sexy stuff. Liquid water on an rocky planet only a couple dozen or so light years away sure does sound cool, but we’re still a long way off from being able to determine whether these planets harbor life. In the meantime, astronomers have found so many exoplanets in recent years that they’re starting to see patterns in the distribution of exoplanets. According to the paper:

  • Exoplanets tend to fall into either one of two categories: gas giants or superearths
  • Superearths live with gas giants in multiplanetary systems
  • Rocky planets aren’t any more frequent around metal-rich star, as opposed to gas giants 
  • Around 30% of the low-mass planets orbit close to their star

That last one has implications for determining whether these lower-mass planets form as part of a large accretion disk, or form due to a migration of heavier elements toward the star. Future models will need to take into account these observations. Astronomers may not find Earth’s twin, but will come that much closer to figuring out how Earth got here in the first place.


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