NASA's Kepler finds earth like planets may dominate galaxy

A few months ago NASA's Kepler mission made headlines when word leaked the plucky little spacecraft had found signs of hundreds of new planets circling distant stars. By contrast there are only about 500 exoplanets known now, after more than a decade of careful observation by some of the world's most sensitive land based instruments. But information on the most exciting class of potential planet, those more similar to the earth and less like the gas giants that orbit farther out in our own solar system, was frustratingly sparse. But one of the astronomers involved with Kepler may have just spilled the beans:

At 8:15 into his 18-minute talk, Sasselov showed a bar graph of planet size. Of the approximate 265 Kepler planets represented on the graph, about 140 were labeled "like Earth," that is, having a radius smaller than twice Earth's radius. "You can see here small planets dominate the picture," said Sasselov. Until now, astronomers' exoplanet finds had been more like gas giant Jupiter than rocky little Earth. Even Kepler investigators had refrained from discussing any Earth-size finds.

Earth-like in this context does not mean an oxygen-nitrogen watery world like our lovely blue-green familiar home. Earth-sized is probably a better way to say it, as in planets in our solar system seem to come in two main size types: gas giants like Jupiter, and terrestrial worlds like earth. The two categories can be further split up into true gas giants vs smaller ice giants (Jupiter vs. Neptune), and mineral vs. icy terrestrial worlds (Mercury vs. Pluto for example).

Some caution is in order: the reason astronomers in the know haven't released much detail on Kepler's terrestrial findings is because they're just tentative indications at this time. No one is sitting on a bunch of picturesque images of alien earths. The telescope, shown left courtesy of NASA, doesn't take those kinds of pictures. In fact it doesn't take pictures at all. Kepler is basically a horrendously accurate photometer married to a powerful wide view telescope designed to detect minute changes in stellar brightness as planets cross in front of a parent star. The accuracy of Kepler's photometer is so sensitive that it can detect a moth fluttering in front of a searchlight from hundreds of miles away. Only a tiny fraction of exosolar systems are likely to align in such a way that one or more planets conveniently eclipses its sun from our local perspective, but Kepler can look at one-hundred thousand stars at once!

If the changes in brightness Kepler is flagging turns out to be earth-sized planets passing in front of their star, and and not noise, or sunspots, or other non planet changes in stellar brightness of target stars, those planets are almost certainly different from earth in ways we can't imagine. Astronomers speculate on a menagerie of super-earths, water worlds, methane planets, gas dwarfs, and even diamond planets.

Super-earths as the name implies usually refers to terrestrial planets -- icy, rocky, liquid, or even gas dwarfs -- that are more massive than earth. A number of good super-earth candidates have been detected orbiting distant stars. The diagram right gives an idea of how large various earth-like planets might be at one earth mass and five earth masses, assuming specific compositions from dense iron to light hydrogen. But it's hard to say what kind of range they might have as far as actual composition mix or appearance: there's nothing like a super earth in our solar system.

A water world is a theoretical planet made up mostly of water. On such a world the global ocean would be hundreds to thousands of miles deep and would end not in rock or mud. Under immense pressure, water is transformed into a sort of global, red-hot glacier, exotic forms of ice that only exist in high pressure labs on earth. The Jovian satellite Europa is a decent representative of what a small, frigid water world would probably be like. A large, hot water world would be like nothing in our solar system. The nearest thing in gross external appearance would probably be something more like a gas giant.  

Pictured above and below are two hypothetical water-worlds representing known exoplanets orbiting the stars Gliese 581 and GJ1214 respectively. The first shows a cooler, smaller water world more massive than earth with ice-bergs visible in a clear atmosphere on a defined surface, the nearby red dwarf locked on the horizon, glowering. The second is the turbulent upper atmosphere of a much warmer and more massive version showing the effects of the steaming cauldron below, and the tremendous electric and magnetic effects in the atmosphere above produced by close orbit around the dwarf star. Both images were created for me by artist Karen Werhstein.

Methane worlds are similar to water worlds, except the primary constituent is natural gas instead of good old H2O! Titan, moon of Saturn, is the closest thing to a methane world in our solar system.

Carbon planets would be worlds made up mostly of carbon compounds instead of the iron, silica, and oxygen compounds that make up earth. Such a planet with liquid water would be a great candidate for life: all life we know is based on complex carbon molecules in a water solution. The surface of a carbon planet might resemble soot or carbon rubble, packed over a deeper strata of solid diamond.

Up to now, most exoplanets have been inferred by their effect on the parent star. Planets circle stars, but as illustrated in the image to the left, the planet also swings its star just a bit. We can't see the planets, yet, but we can see the wobbling stars! But the technique selects for relatively large planets that are tightly orbiting their comparatively small star. And so the list of exosolar planets to date is dominated by giant planets whizzing perilously close around modest stars in days or weeks. It's debatable if an earth-like world orbiting in the habitable "Goldilocks" zone could survive in such an inhospitable system.

The tentative findings now coming to light because of Kepler could completely change the picture. Earth-sized planets might make up roughly half or more of all planets in the galaxy, a proportion similar to that in our solar system where small worlds outnumber large ones. The implications are exciting: there are billions of stars in the Milky Way galaxy, many millions of them are as old or older than our sun. If Kepler's data holds up, that would mean there are probably hundreds of millions of earth-like planets that have been circling stars long enough for a lot of interesting things to happen, at least using our own planet's natural history as a guide. Best of all, the technology to go beyond Kepler, and get actual images or details of the chemical composition of some of these exosolar planets, is well within our grasp over the next 20 years.

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