A Realm of Planets

More Exoplanet Discoveries

The search for exoplanets (also known as extrasolar planets) is a painstaking one. You have to tease out “signals” from the planets, and by that I mean you have to look for evidence of planets in the light from distant stars. If a planet orbiting a star passes in front of its “primary” (its star), that passage dims the light a tiny fraction of a percent. And so, you need a very sensitive light meter attached to your telescope to catch that signal. Likewise, as a planet orbits its primary, it can cause the apparent motion of the star to “wiggle” a bit, and that “signal” shows up in the light streaming from the star. To tease out the wiggles and signals, astronomers break starlight into its component wavelengths (i.e., they take a spectrum) and look for changes in that spectrum. Specific kinds of changes may mean that a planet is affecting the star’s orbit, or causing it to dim down a tiny bit.

There are, however other things that can cause a star to appear to dim down or wiggle in its orbit. For example, it could be a variable star whose light intensity varies a tiny but—just enough to make it seem like a planet is passing between us and the star. Or, the star could be experiencing gravitational perturbations from another nearby star, enough to make it wiggle in its motion. The data not only tell that a planet possibly exists at the star, but also gives astronomers a good idea of what its minimum mass could be.

So, looking for these distant worlds is a complex science. Along the way, astronomers have discovered many new variable stars and even the occasional stellar black holes that affect a star’s light as they pass by. But, often enough, astronomers also find planets. And, that’s when the fun begins. Ground-based observers using large telescopes (larger than you have in your backyard) train their instruments on distant stars with planet candidates and begin the lengthy (often years-long) observations it takes to confirm that a planet is actually orbiting those stars.

Just today, an international team of astronomers, led by Guillem Anglada-Escudé of the University of Göttingen, Germany, and Rory Barnes of the University of Washington announced that they’ve found three “Super-Earth” type planets in the s0-called “habitable zone” of a nearby star. Incredibly enough, this world was previously thought to host two or three planets already, so that brings its possible world count to six or seven. “Super-Earths” are planets that are what they sound like—not more than 10 times more massive than Earth. This is the first time that so many of them have been found orbiting in one system.

The name of the star is GJ 667C, and its actually part of a three-star system that lies in the S-shaped constellation Scorpius. GJ 667C is an M-dwarf star, somewhat fainter than the Sun, and has about a third of the mass that our star does. A star this faint has a habitable zone—the region around it where liquid water could exist on a world—that is very close.  Because the habitable zone is so close to the star, the each planet’s year is much shorter than Earth’s is. They range from 20 and 100 days to go once around the star.  The planets are also very likely “tidally locked,” which means the same hemisphere always faces the star. Luckily astronomers know that life can exist under such conditions.

Since such low-mass stars are inherently faint, their habitable zones—the swath of space that would allow an orbiting rocky planet to sustain liquid water on its surface—lie much closer to the star. The closeness of the habitable zone then makes it easier to find potentially habitable rocky planets around low-mass stars.

In the past few years, astronomers have confirmed at least two planets orbiting this star, and possibly a third one that is still being confirmed. More observations allowed the team to find the new planet candidates. The next steps are to do more observations and detailed study in order to prove that these really are planets. If they are, then there’s a very good chance that this realm of planets will feature worlds with solid surfaces and maybe even atmospheres similar to Earth’s.

That’s the exciting part about planetary searches around other stars. While no one has yet found an Earth analog where life is teeming in oceans or on the surface, the time is drawing near when such a world could be found. It surely does lie out there, somewhere, waiting to be discovered and confirmed.