Thursday, December 17, 2009

Water world, water world

An artists conception of the red dwarf star GJ 1214 and its planet
Image Credit: David A. Aguilar, CfA

Yesterday, astronomers announced that they had found a new "super-earth" planet around another star.  The team also claims that the planet must be made of a substantial amount of water.  Perhaps as interesting as the discovery is the story of how it was made and the timing of the announcement.

Let's start with the discovery.
The star, named with the typically inscrutable catalog  GJ 1214 is a red dwarf star in the constellation Ophiuchus.  Red dwarf stars are the most common type of star in our Milky Way galaxy.  Red dwarf stars are much fainter than our sun, but they can produce light for trillions of years, while our sun will last a measly 10 billion years.

The planet around GJ 1214 was not discovered with one of the world's largest telescopes, but with a telescope only 16-inches in diameter.  Such telescopes are owned by many amateur astronomers, though this telescope is part of an array of 8 identical small telescopes called MEarth (pronounced "mirth").  MEarth's telescopes look for planets around red dwarf stars.  They look for planets by the transit method -- as seen from Earth, a planet around another star has a small chance of passing in front of its parent star, blocking a little bit of light.  This blocking of light is called a transit.

Transit methods have been used to find Neptune- and Jupiter-sized planets around sun-like stars, but Earth-sized planets around sun-like stars are impossible to detect from the ground.  The planets are too small, and their parent stars too big, so the amount of light that gets blocked is tiny, too tiny to detect with Earth's atmosphere in the way.  NASA's Kepler satellite and the European CoRoT satellite are able to detect smaller planets, but Kepler will take a few years to find Earth-like planets in Earth-like orbits around sun-like stars, and CoRoT looks at relatively few stars.

MEarth tries to find small planets by looking around small stars.  Red dwarf stars are small enough that an Earth-sized planet blocks almost as much light as a Jupiter-sized planet blocks around a sun-like star.  So, the same ground-based telescopes that have discovered Jupiter-sized planets transiting sun-like stars should also be able to detect Earth-sized planets around red dwarfs.  MEarth is taking advantage of this by looking at as many red dwarf stars as possible.  And, with GJ 1214, they've succeeded.

Every 38 hours, the light we see coming from GJ 1214 dips a little bit.  By measuring the amount that the light dips and using some high-school geometry, the astronomers studying this planet were able to determine that the planet's diameter is 2.7 times larger than the Earth's.  Only now did they bring in the big telescopes.  Using sensitive equipment, the MEarth team was able to measure the slight back and forth wobble of the parent star due to the gentle tug of gravity from the planet.  This is quite a technological achievement -- the star moves at a maximum of 44 kilometers per hour, or 27 miles per hour!  We're able to determine that a star 40 light years away is moving at speeds slower than most automobiles!  More amazingly, this measurement is considered "easy" these days.

Anyway, in order to move this star to move at that speed using gravity, we can calculate the planet must have a mass 6.6 times that of the Earth.  Just to state -- this is very basic orbital math.  Isaac Newton could have calculated this back in 1687.  With the mass of the planet, the mass of its parent star, and the orbital period, we find that the planet orbits only 1.3 million miles away from GJ 1214 (whereas the Earth is, on average, 93 million miles away from the sun).

Now, if you do some more basic geometry, you can find the average density of the planet.  And you'll get that it is a little less than 2 grams per cubic centimeter.  As a comparison, the density of water is 1 gram per cubic centimeter, the density of granite is 3 grams per cubic centimeter, and the average density of the Earth (which has a lot of iron in its center), is 5.5 grams per cubic centimeter.  So, in short, the planet is less dense than the Earth, and is sort of an average between water and rock.  So, the planet could be half water and half granite.  Or it could be made of 100% red brick, which has a density of about 2 grams per cubic centimeter.  We don't really know.

So, if we don't know what the planet around GJ 1214 is made out of, how can astronomers get away with saying it must be a world made of at least half water?  When we look in our own solar system, or look at new solar systems forming around other stars in the Orion Nebula and elsewhere, we can get a pretty good idea of what materials are common.  Hydrogen, helium, oxygen and carbon are the most common materials.  Water (a combination of hydrogen and oxygen) is also very common.  We also see a lot of silicates ("rocks").  So, if you start making planets on your computer and you make a few basic requirements, like the planets must be made out of common materials, must be bound together by gravity, and must be able to survive for billions of years, and if you require your planet to have the diameter and mass of the planet around GJ 1214, you find that a mix of water and rock works pretty well, while other things, like a Jupiter-type construction of rock surrounded by hydrogen gas, seem less likely.

However, assuming the planet really is half water, don't get in your mind that it looks like a souped-up version of the Earth's oceans, complete with alien dolphins porpoising and wondering if other planets with dolphins may be out there.  The "surface" of this planet likely has a temperature of 400 degrees Fahrenheit!  More realistically, a water world around GJ 1214 would probably have a steam atmosphere that gets hotter and denser as you go down, until the pressure gets so high that the steam merges into a weird form of hot, solid water called Ice VII that doesn't exist on Earth outside of laboratories.  It would not be a very pleasant environment for life as we know it.

Of course, we could be wrong about the structure of this planet.  The steam/hot-ice and rock idea is just our best guess.  If the planet does somehow have a solid surface with liquid water (at 400 degrees?  seems doubtful, but not impossible -- after all, even the planet Mercury has ice in some shaded craters, even though in direct sun it is much hotter than 400 degrees), then, perhaps, there might be something alive on the planet.  (And, since it is 40 light-years away, the first episodes of Sesame Street are just starting to arrive.  In 40 more years, we might want to listen for a return broadcast of "Yip Yip Yip Yip Uh-huh Uh-huh" acknowledging that our transmissions have been received).

Last, a comment on the interesting timing of this announcement.  In just a few weeks (during the week of January 3), NASA will be releasing the first batch of planet discoveries from the Kepler mission.  I don't know what will be announced, but it is quite possible than many planets like GJ 1214 have been found by Kepler.  The discoveries of new and ever-more-peculiar extrasolar planets are likely to start coming as a steady stream, if not a flood, instead of the current trickle we have now.  I strongly suspect that "super-Earth" planets like the one around GJ 1214 are quite common, and in a few weeks, we will know how common they really are.

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