100 billion planets, say astronomers

Less than two decades ago, there were exactly zero known planets orbiting sunlike stars in our Milky Way galaxy. Astronomers back then were engaged in a powerful struggle to seek out exoplanets, and they succeeded, so that today there are 861 confirmed exoplanets, according to on March 25, 2013. In the past year, astronomers have begun tossing around the word billion to describe how many planets might orbit Milky Way stars. Today (April 3, 2013), astronomers at The University of Auckland in New Zealand announced their new method for finding exoplanets. They say they anticipate 100 billion planets similar to our Earth, orbiting stars in the Milky Way. Their work will appear in the journal Monthly Notices of the Royal Astronomical Society.

Lead author of the New Zealand planet search – Dr. Phil Yock from the University of Auckland’s Department of Physics – said his team’s strategy is to use a gravitational microlensing technique. Yock said his team will use a combination of data from microlensing and NASA’s Kepler space telescope.

The Kepler space telescope, by the way, has single-handedly found 105 exoplanets and an astounding 2,740 planet candidates orbiting 2,036 stars (as of January 7, 2013). Yock said:

Kepler finds Earth-sized planets that are quite close to parent stars, and it estimates that there are 17 billion such planets in the Milky Way. These planets are generally hotter than Earth, although some could be of a similar temperature (and therefore habitable) if they’re orbiting a cool star called a red dwarf.

Our proposal is to measure the number of Earth-mass planets orbiting stars at distances typically twice the sun-Earth distance. Our planets will therefore be cooler than the Earth. By interpolating between the Kepler and MOA results, we should get a good estimate of the number of Earth-like, habitable planets in the galaxy. We anticipate a number in the order of 100 billion.

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But let’s back up a sec. The difficulty of detecting exoplanets from a distance has always been that planets – which are tiny in contrast to their parent stars and produce no light of their own – are extremely faint and hard to see in the glare of their stars. The first planet orbiting a sunlike star – 51 Pegasi b, discovered in 1995 – was found by what is called the radial velocity technique. That is, 51 Pegasi b was found through careful measurement of the motion of the star 51 Pegasi across the dome of night. Very detailed analysis of this motion revealed a slight wobble, revealing the presence of a small companion: a planet. This planet is called 51 Pegasi b according to the nomenclature of the International Astronomical Union.

The Kepler spacecraft finds planets in a slightly different way. It measures the loss of light from a star when a planet orbits between us and the star.

Read more about using microlensing to find exoplanets from NASA's Wise Observatory.
Read more about using microlensing to find exoplanets from NASA’s Wise Observatory.

Microlensing, used by the New Zealand astronomers, is a third technique for finding planets orbiting distant suns. It measures the deflection of light from a distant star that passes through a planetary system en route to Earth. This effect was predicted by Einstein in 1936 and has been used successfully not only to find exoplanets but also to study distant objects such as quasars. The April 3, 2013 press release from University of Aukland said:

In recent years, microlensing has been used to detect several planets as large as Neptune and Jupiter. Dr. Yock and colleagues have proposed a new microlensing strategy for detecting the tiny deflection caused by an Earth-sized planet. Simulations carried out by Dr. Yock and his colleagues – students and former students from The University of Auckland and France – showed that Earth-sized planets could be detected more easily if a worldwide network of moderate-sized, robotic telescopes was available to monitor them.

Their plan is to use just such a network, now being deployed by Las Cumbres Observatory Global Telescope Network (LCOGT) in collaboration with the Scottish Universities Physics Alliance. There are three telescopes in Chile, three in South Africa, three in Australia, and one each in Hawaii and Texas. In addition, they’ll use telescopes in the Canary Island and in Tasmania. But, as Yock pointed out:

Of course, it will be a long way from measuring this number to actually finding inhabited planets, but it will be a step along the way.

He’s just saying that Earth-like does not mean inhabited. And inhabited does not mean by an intelligent civilization. And why do we want to find Earth-like planets, anyway, when getting to even the closest known Earth-like planet – Alpha Centauri Bb, only four light-years away – would require hundreds of thousands of years of travel time, using conventional technologies?

Why? Because … aren’t you curious? I know I am.

Bottom line: Astronomers have begun to use the word “billion” or even “100 billion” to describe the possible number of Earth-like planets in our Milky Way galaxy. This post discusses the April 3, 2013 announcement by astronomers at the University of Auckland in New Zealand that they’ll contribute to the planet search using a gravitational microlensing technique.

How long would it take to get to Alpha Centauri?

Intelligent civilizations rarer than one in a million

Will the IAU democratize the way it names space objects?

April 3, 2013

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Deborah Byrd

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