A Wet World Far Away?

I was just hopping over to Bad Astronomy to check out Phil Plait’s site layout. Focusing on the margin widths, I didn’t see the text for several heartbeats. When I did, my heart stopped.

The European Southern Observatory has let forth a yawp over the rooftops of the world, announcing the most Earthlike extrasolar planet yet discovered. It’s about five times the Earth’s mass, it orbits the red dwarf Gliese 581 once every thirteen days. . . and it just might have liquid water on its surface.

The name and number of Gliese 581 may tug at your memory, if you’ve got a head full of trivia like mine. Back in November 2005, a team of French and Swiss astronomers revealed a planet orbiting that same star. Now known as Gliese 581 b, this planet is about 16.6 times as massive as the Earth and orbits only 6 million kilometers from Gliese 581 itself, making its year only 5.366 days. Today, in addition to Gliese 581 b, we have identified Gliese 581 c — the first rocky exoplanet ever found within a star’s habitable zone — along with a third planet, eight times the Earth’s mass and orbiting outsize the habitable zone.

(For the densest concentration of technical details, see the Astronomy and Astrophysics preprint.)

We now know of many more planets outside our solar system than in it. We got into this game in 1992, when Aleksander Wolszczan and Dale Frail spotted planets orbiting the pulsar PSR 1257+12. Then in 1995 came Michel Mayor and Didier Queloz‘s “hot Jupiter” circling madly around 51 Pegasi. As of today, the Extrasolar Planets Encyclopaedia gives a total of 227 planets orbiting suns other than our own. Think of it: a world for every country on Earth. If you made a whirlwind tour of a world each day, you would occupy yourself from New Year’s until the middle of August. And now, we know one of these worlds just might have weather like home.

As Phil Plait says,

There is much more to learn about this planet. Getting an image of it is currently not possible: at a distance of 20 or so light years, Gliese 581 one of the closest stars in the sky, but still far too distant to separate the planet from the star. So I’m left wondering about this planet. Does it rotate once every orbit due to the gravitational interaction with its star? This is what has happened to every moon in the solar system; they spin at the same rate they go around their parent bodies, so they always show one face to their parent (which is why the Moon always has the same face toward us here on Earth). If so, how does this affect the atmosphere? Models indicate that the air should carry the warmth of the star around the planet, so the temperatures should actually be fairly moderate on both the day and night sides of such a world. But if it’s covered by an ocean, how does having one side of the planet eternally locked into daylight affect it?

Criminy, what would life be like on a tidally-locked ocean world?


Assorted reading:

Blagnet roundup:

UPDATE: Over at Bad Astronomy, a commenter asked, “How long would it take to get there?” Well, of course this depends upon how fast you can go. If you want to get to a star 20 light-years away in a reasonable amount of time, you have to travel at relativistic speeds, which means that (duh) Einstein’s Relativity comes into play. This is such a fun problem that the “relativistic rocket” ends up being worked out time and time again. The time measured by a clock on your spaceship will differ from that measured on Earth; I’ll use T to denote the former (which we call “proper time“) and a lowercase t to denote the latter. The answer involves the hyperbolic trigonometric functions, and in particular the hyperbolic cosine:

[tex]\cosh x = \frac{e^x + e^{-x}}{2}.[/tex]

This function takes in a number x and spits out the corresponding hyperbolic cosine, which is like the cosine we saw in high school but slightly stranger (it’s defined in terms of hyperbolas instead of circles). The proper time — the time measured by clocks and people on the ship — can be found using the inverse cosh. Writing d for the distance traveled, c for the speed of light and a for the acceleration the people in the ship feel, the formula is

[tex]T = \frac{c}{a} \cosh^{-1}\left(\frac{ad}{c^2} + 1\right).[/tex]

What’s a good value for a? Well, a comfortable acceleration would be 1 g, what we feel sitting on Earth. Plugging in 9.8 m/s2 for a gives (somebody should double-check my arithmetic) the result that T is 3.6 years.

That’s for people on the ship. What about the folks back home? Well, the formula for little t is

[tex] t = \sqrt{\left(\frac{d}{c}\right)^2 + \frac{2d}{a}}, [/tex]

which with the same values for d, c and a as before gives t = 20.5 years. Remember, the destination star is 20 light years away (meaning, of course, that light gets there in just 20 years). Accelerating at 1 g for three years, ship time, you pick up so much speed the people back on Earth see you going almost the speed of light!

If you want to slow down so that you stop at Gliese 581 instead of whizzing by, the simplest solution is to turn your relativistic rocket around at the midpoint of your journey and start blasting in the opposite direction. This trip will take just about twice as long, ship time, but because so much of the journey takes place at near light-speed, the people on Earth won’t see much of a difference.

The details of building a vessel which can travel this fast for this long and keep everybody alive inside are left as an exercise to the interested reader.

UPDATE THE SECOND: Quoting from Dennis Overbye’s New York Times piece, with added emphasis:

The planet, officially known as Gliese 581c, circles the star every 13 days at a distance of about 7 million miles. According to models of planet formation developed by Dr. [Dimitar] Sasselov and his colleagues, such a planet should be about half again as large as the Earth and be composed of rock and water, what the astronomers now call a “super Earth.”

The most exciting part of the new find, Dr. Sasselov said, is that it “basically tells you these kinds of planets are very common.” Because they could stay geologically active for billions and billion of years, Dr. Sasselov said he suspects that such planets could be even more congenial for life than the Earth. NASA’s coming Kepler mission, he added, would probably find hundreds of these super Earths.

Trouble for the Anthropic Principle? Discuss.

(Links for Kepler Mission, budget troubles for planet hunters, NASA’s FY2008 budget.)

6 thoughts on “A Wet World Far Away?”

  1. There’s no indicator (as far as I know) of how constraining the evolution of observant life is. Evolution of observant life could be common among planets with life, or very rare – we have no way to know at this point.
    Furthermore, the existance of this planet is at best a very weak constraint on estimations of the frequency of planets that harbor life.

    Ultimately, I don’t think we can say much about the Anthropic principle (with respect to the suitability of earth for life) until we know of a selection of planets with life, and some notion of the kind of life present (particularly the abilities of said life to observe the world around it).

    Auggg! How can I write posts with acceptable faux-html or formatting without preview?

  2. NB: It’s not actually the first planet found in the habitable zone, it’s just the first rocky planet found in the habitable zone. (The other gaseous planets could conceivably have rocky moons orbitting them which could sustain life.)

  3. I’m not sure how the Anthropic principle is affected at all. After all, the idea is that the universe itself must have the properties it actually has in order for life to exist. (duh!) If life exists in one or a zillion places, how does that affect that principle?

  4. Sometimes I hear anthropic arguments phrased in terms of the Universe as a whole, and sometimes about the Earth in particular. After hearing the same tired refrain so often about how our own planet is “perfect for life” or even “engineered for discovery”, I like the idea of a planet even, ahem, more perfect than ours (even if it is just happy speculation). That’s all.

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