Friday, August 26, 2016

How many habitable worlds are out there?

In all, astronomers have counted 2,000 exoplanets in our galactic neighborhood. Starting in 1988, and as of August 25, 2016, there have been 3,375 exoplanets in 570 planetary systems and 592 multiple planetary systems confirmed. The most recent find was just next door with in a discovery that has been years in the making, researchers have confirmed the existence of a rocky planet named Proxima b orbiting Proxima Centauri, the closest star to our sun.

Proxima b is a mere 4.2 light-years away from our solar system, or 266,000 times the distance between the Earth and the sun, which are 92.96 million miles apart. Previous rocky exoplanet discoveries, like those orbiting ultracool red dwarf star TRAPPIST-1, were previously described as "close" at 40 light-years away.

Until recently this question was impossible to answer, and even now astronomers are just beginning to make educated estimates. Extrasolar planets, or exoplanets, are too small, relative to their vast distance from us, and too outshined by their host suns to be seen directly by most observatories on Earth. The great majority of exoplanets can be detected only indirectly by their minute gravitational or eclipsing effects on the stars they orbit.

Exoplanets tug on their parent stars, causing a barely perceptible "wobble" in the star. By breaking a star's light into a spectrum of colors, astronomers can track this wobble by watching how the light it produces shifts back and forth in wavelength. It's an incredibly small effect, measurable only since technological advances of the late 20th and early 21st centuries. Astronomers are also able to "see" exoplanets eclipse a minuscule amount of starlight as they pass in front of their host star (relative to our line of sight). Through these observations and subsequent calculations, we can infer the sizes, masses, and orbital properties of the exoplanets. Their distances from their host suns provide us a good estimate of their surface temperatures as well.


For years many of these observations had to be made from the ground. That can be tricky—our atmosphere has the tendency to block some starlight and make the light that we do see too blurry for extremely precise spectroscopic measurements. The Kepler Space Telescope helped alleviate that problem. After its launch in 2009, the telescope focused on a subset of stars near the constellation Cygnus—the swan—tracking their light and watching for the tiny dips in their apparent brightness that would be evidence of eclipsing exoplanets.

Kepler made many exciting discoveries, including potentially habitable planets circling other stars. In all, astronomers have counted nearly 2,000 exoplanets in our galactic neighborhood. That's nearly 2,000 more planets than the eight we knew of—those in our solar system—in the early 1990s. Judging from the sample of stars that Kepler surveyed, we can safely guess that, on average, there should be at least one planet circling each of the roughly 200 billion stars in the galaxy. The number and types of exoplanets Kepler scientists discovered indicate that about one in five that orbit stars similar to our sun could be Earthlike in size and mass. That's important because if Earth is any indication, only stars that are sun-size or smaller should live long enough and have the right properties for their planets to be thoroughly and sustainably habitable. That means there should be more than ten billion habit-able planets in the Milky Way. And if we include the potentially habitable moons of Jupiter-like planets as well as planets around more numerous red dwarf stars, the number becomes tens of billions.

It's only an estimate, but it's a great start. Among the hundreds of billions of galaxies in the visible universe beyond the Milky Way, there might be a sextillion habitable planets! We have reason to hope our cosmos isn't so lonely after all.

Portions of this article is from National Geographic's "Are We Alone?" special magazine and CNN.

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