Tuesday, April 21, 2009

Gliese 581: Lightest Known Expoplanet (Caution! Geeky Content!)

Update (December 14, 2009)
"Lightest Known Exoplanet Discovered"
Space.com (April 21, 2009)

"The lightest exoplanet yet discovered — only about twice the mass of Earth — has been detected, astronomers announced today.

" 'With only 1.9 Earth-masses, it is the least massive exoplanet ever detected and is, very likely, a rocky planet,' said Xavier Bonfils of Grenoble Observatory in France, a member of the team that made the discovery, which was announced at the European Week of Astronomy and Space Science at the University of Hertfordshire in the United Kingdom.

"The planet was found in the famous system Gliese 581 and has been dubbed 'Gliese 581 e.' It was detected using the low-mass-exoplanet hunter HARPS spectrograph attached to the 3.6-metre ESO telescope at La Silla, Chile. ..."

The Gliese 581 system is a very exciting place for people interested in exoplanets. The most recently-discovered planet there is very close to Earth's diameter, and another might - just might - have liquid water on its surface.

More about that later. First:

Not All That Long Ago, in a Galaxy That We're In - - -

One thing I appreciate about Space.com is that its writers generally are fairly familiar with astronomy. Another science-themed website wrote about Gliese 581 e too, informing readers "...that a planet less than twice the size of Earth has been located in a galaxy outside our solar system...." [emphasis mine]

Technically, the statement is correct. Gliese 581 and its planets are about 20.5 light years away. That's very definitely outside our solar system. And, Gliese 581 is in a galaxy. So is our sun. We're both in the Milky Way galaxy. and, since the vast bulk of the Milky Way galaxy is outside our solar system, yes: Gliese 581 is "in a galaxy outside our solar system." Using that website's definitions, every star other than the sun is "in a galaxy outside our solar system."

A Sense of Scale

Space is really, really, big.



Although there's still some debate about exactly how big the Milky Way galaxy is, the last I checked, astronomers were hovering around 100,000 light years for the diameter of that spiraling disk of stars we call home. We're roughly 25,000 light years from the center. Think of this diagram as a sort of "you are here" map of the Milky Way:

(from Department of Physics, University of Oregon, used w/o permission)

On this scale, Sun and Gliese 581 are in the same pixel. The graphic is 648 pixels across, which makes each pixel well over 100 light years across.

The "in a galaxy outside our solar system" gaffe isn't all that uncommon - although I hope it shows up more often in science fiction, than in well-intentioned science articles. " Scifi Writers Have No Sense Of Scale" (tvtropes) is a pretty good discussion of the troubles many science fiction writers have, dealing with stellar, galactic, and cosmic distances.

Gliese 581's Planetary System: Almost Like Home

Emphasis on "almost." Space.com's article includes a diagram of Gliese 581 and its known planets, with our sun and planets for comparison.

(from Space.com, used w/o permission)

Gliese 581 is smaller, and cooler, than our sun, so a planet would have to have a tighter orbit to be at a comfortable temperature. Gliese 581 e is too close for life-as-we-know it. G. 581 d, however, just might have liquid water on its surface. It's near the outer edge of the 'habitable zone,' where temperatures allow water to exist in all three states: gas, liquid, and solid.

" 'Gliese 581 d, which orbits the host star in 66.8 days, is probably too massive to be made only of rocky material, but we can speculate that it is an icy planet that has migrated closer to the star,' said team member Stephane Udry of Geneva University in Switzerland. ' "D" could even be covered by a large and deep ocean — it is the first serious "water world" candidate.' "

Isn't "Mass" the Same as "Size?"

Yes and no. "Size" has quite a few meanings. "Mass" in this context means "the property of a body that causes it to have weight in a gravitational field."

Gliese 581 e is close to being 1.9 Earth-masses, or 1 and nine tenths times the mass of Earth. If it packs as much mass into each cubic mile as Earth does, it would be very roughly 1.238 times Earth's diameter. (The cube root of 1.9 is just over 1.238 - I didn't do the math myself - there's a pretty good Cube And Cube Root Calculator on the CSG (Computer Support Group) website.)

Think about it this way: if you have 8 cubical blocks, each weighing a pound, and stack them so that they make a bigger cube, the cube will be 2 blocks on a side, and weigh 8 pounds. So, if you know the volume There's a decent explanation of the square-cube law at Boston University's website.

at same density as Earth, would make a sphere roughly 1.238 times the diameter of Earth. That's very similar to Earth: although I wouldn't want to try walking there. Aside from higher gravity, Gliese 581 e is probably a very, very cold place.

Life in the Gliese 581 system?

Odds are that there isn't anything living there - if Gliese 581 d is a 'water world,' made mostly of water. Water's important, but life needs other materials, too - which is another topic.

Even if there isn't life there, the presence of one planet that's less than twice the mass of Earth - and another one that quite likely may have liquid water - hints that conditions for life are so common, that we've got almost-good-enough conditions right next door. On a galactic scale, that is.

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