Although there appears to be a mysterious dearth of exoplanets smaller than Earth, astronomers using data from NASA's Kepler space telescope have estimated that nearly a quarter of all sun-like stars in our galaxy play host to worlds 1-3 times the size of our planet.
These astonishing results were discussed by Geoff Marcy, professor of astronomy at the University of California, Berkeley, during a talk the W. M. Keck Observatory 20th Anniversary Science Meeting at The Fairmont Orchid, Hawaii, the Big Island, on Thursday.
"23 percent of sun-like stars have a planet within (1-2.8 Earth radii) just within Mercury's orbit," said Marcy.
"I'll say that again, because that number really surprised me: 23 percent of sun-like stars have a nearly-Earth-sized planet orbiting in tight orbits within 0.25 AU of the host stars," he continued. One Astronomical Unit (AU) is the equivalent distance to the average orbital distance of the Earth around the sun.
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"And of course, this begs the question, what about farther out? Out to 0.5 AU? Or an Earth-sun distance of 1 AU? So you get the feeling that most (sun-like) stars have an Earth-sized planet around them out to at least the Earth-sun distance of 1AU. It's a remarkable reality."
Kepler detects distant exoplanets when they orbit in front of their host star, blocking a tiny amount of light, an event known as a "transit." When Kepler takes more and more transit observations, the signal-to-noise ratio will reduce and small planets orbiting at 1 AU around sun-like stars will start to pop-up in the data. It's all a matter of time.
Since Kepler's launch in 2009, the prolific planet hunter has discovered 2,740 candidate exoplanets and, earlier this year, astronomers used Kepler data to estimate that there were a minimum of 17 billion Earth-sized exoplanets in our galaxy. Marcy specifically singles-out the sun-like stars in the Milky Way - an incredible twist to the question of whether or not other worlds could play host to life.
Interestingly, Kepler data also appears to demonstrate a noticeable lack of exoplanets smaller than Earth. That's not to say the mission hasn't found them - for example, Kepler-37b, the tiniest exoplanet discovered, is smaller than Mercury - there just seems to be fewer sub-Earth-sized worlds than exoplanets bigger than Earth.
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But Marcy doubts that this finding is indicative of a bizarre small-world destroying mechanism around stars and is more likely an instrumental limitation.
"This is a guess, but they're just harder to detect," Marcy told Discovery News. "Small planets dim the star less - the dimming has to be greater than the noise to detect the planet.
"Kepler was designed to detect Earth-sized planets, but not half-Earth-sized planets. However, it can do that by recording enough dimmings. It can happen but the bottom line is that small planets are harder to detect and that's probably why there's few of them.
"But having said that, it's just a guess. The alternative is that planets smaller than the Earth are rarer and rarer and, basically, planets half the size of Earth aren't that common."
However, he pointed out that there probably isn't a preferential small-world killing mechanism and, in fact, one would expect more of the smaller worlds to be out there - large planets, after all, are thought to be agglomerations of asteroids and smaller planets.
But Kepler observations of exoplanets - regardless of whether they're larger or smaller than Earth - can do only so much. Kepler observations are being complemented by the powerful Keck Observatory so these exoplanets can be properly characterized.
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"Keck does three very important things. First, you need the spectrum of the star so you can find out the size of the star and to find out what kind of star it is," Marcy continued. "Only by knowing the size of the star can you then interpret the fractional dimming (of starlight) as measured by Kepler to give you the size of the planet."
"The second thing is that the Keck Observatory takes the Doppler shift of the star, caused by the planet yanking on it gravitationally, to validate the existence of that planet."
Doppler observations are possible by using Keck's immensely sensitive spectrometers that measure the light emitted by the target star. When the star wobbles away slightly (as the exoplanet gravitationally tugs it away from us), the wavelength of light is stretched, or red-shifted. As it wobbles toward us, the light is blue-shifted. This method - known as the "radial velocity method" - is an independent verification that a world detected by Kepler actually exists.
"The Doppler shift of the star tells us that the star is wobbling, which in-turn tells us the mass of the planet. Now this is very important as we want to know the density (of the exoplanet) to find out if it's made of rock," he added.
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This point emphasizes the important collaborations with ground based observatories, critically Keck, in conjunction with the Kepler space telescope. Kepler can spot the slight dimming of a star and Keck's sophisticated suite of instrumentation follows up to provide information on the mass of Kepler's worlds, ultimately helping us understand their densities, thereby providing information on whether we are looking at small rocky worlds.
Small worlds appear to dominate the galaxy and if they are rocky and if they have liquid water on their surface (worlds orbiting within their star's "habitable zone"), this whole effort could identify worlds with life-giving potential.
The 20th Anniversary Science Meeting is part of "Keck Week," the first meeting of its kind on Hawaii, the Big Island - near famous Mauna Kea that hosts a number of observatories at its summit - celebrating a rich two decades of science to come from the twin Keck telescopes. I'm fortunate enough to be here, getting a taste for astronomy in a tropical paradise.
Image credit: NASA