Checking Out the Stellar Neighborhood

This top image is of one-half of the Hubble Space Telescope field of view in the Sagittarius Window Eclipsing Extrasolar Planet Search (SWEEPS). The green circles identify 9 stars that are orbited by planets with periods of a few days. The bottom frame identifies one of two stars in the field where astronomers were able to spectroscopically measure the star’s back-and-forth wobble due to the pull of the planet.
Credit: NASA, ESA, K. Sahu (STScI) and the SWEEPS science team

Ancient astronomers logically focused their efforts on the brightest stars, the ones they could see in the night sky. Many modern astronomers, aided by ever-more-sensitive telescopes, have investigated objects that are unimaginably distant and ancient, phenomenally bright and powerful. Fewer astronomers have concerned themselves with dimmer red dwarfs. Although these cool, dim stars dominate our galactic neighborhood, it is only recently that astronomers have begun to look in earnest for them.

Interest in red dwarfs is growing, in large part because scientists have realized that these stars may host planets suitable for life. In December, Todd Henry, a professor of physics and astronomy at Georgia State University, published a study in The Astronomical Journal that identified and described 20 newly discovered star systems within 33 light years of Earth. The systems include three binary and two triple star systems. (The term “star system” is used instead of “stars” to describe new stellar discoveries, because they often turn out, on closer inspection, to be multiple stars.)

Henry, who has spent more than a decade studying nearby stars, says all of the new stars are red dwarfs, much smaller, cooler and dimmer than our own sun. Indeed, most of the stars closest to the sun – and possibly in our galaxy as a whole – are red dwarfs. Given the emerging view that some planets around red dwarfs may be hospitable to life, the discovery is good news for the search for life on other worlds.

To establish the stars’ precise locations, Henry and his collaborators studied visible-light images taken at the Cerro Tololo Inter-American Observatory in Chile. Over several years, they measured the change in the apparent position of each target star as Earth’s orbital motion caused it to shift back and forth relative to more distant stars. The effect, known as “parallax,” is similar to what happens if you hold a finger out in front of your face and close first one eye and then the other. Your finger appears to move back and forth relative to objects in the distance. With these parallax measurements and a bit of 10th-grade trigonometry, Henry and his colleagues were able to calculate the distance to the star.

The new data raise the census of known stars within a radius of 33 light years to 384 objects in 249 systems, Henry says. This near-Earth region includes the bright stars Sirius and Vega, but most local residents are red dwarfs, which astronomers term “M-dwarfs.” These small, dim stars have less than half the sun’s mass, and produce far less energy. Eventually, Henry expects a total of 500 stars to be found in the near-Earth region.

Of the 20 or so stars produced in the galaxy each year, 15 of them are dim M dwarfs.
Image Credit: NASA

One of the newly seen star systems is a binary star with an orbital period of several decades. A second system is just 12 light years away, Henry says. “It’s right on top of us, but nobody knew it was there.” This red dwarf is orbited by a brown dwarf, a sort of super-planet that blurs the distinction between “planet” and “star.” The motion of a third red dwarf hints at the existence of an orbital body that is probably a massive planet, smaller than a brown dwarf, with an orbital period of several years. “It may not pan out, we may be reading too much into the data,” Henry says, “but it’s the number-one priority in my head.”

Red dwarfs are the most common type of star in the nearby galaxy, and Henry suggests they could make up a significant part of the universe’s mass. “We keep finding more stars near the sun,” Henry says. If our neighborhood is representative of the galaxy, and the galaxy is representative of the universe, the new local density measurement may lead to a more precise calculation of the universe’s overall mass.

More nearby stars also translates into the possibility of more nearby planets, and perhaps life. The possibility of habitable worlds around red dwarfs is still being debated, though. Because red dwarfs have such low heat output, only planets in close orbit would be warm enough to have liquid water on the surface. But close-in planets could be subject to “tidal lock,” the same gravitational phenomenon that holds one face of our moon oriented toward Earth. If so, water would boil away on the side of the planet facing the star, and freeze on the opposite side, greatly reducing the likelihood of life.

This artist’s rendering shows a rocky iceball several times heavier than Earth orbiting a red dwarf star. Astronomers believe that some "super-Earths" form in a cosmic "snowstorm," collecting huge amounts of ices from the protoplanetary disk in which they formed.
Credit: David A. Aguilar (CfA)

But the odds of tidal lock “should be called into question,” says Alan Boss of the Carnegie Institution in Washington, D.C. The idea that close-orbiting planets would be tidally locked came from comparing these imagined planets to our moon, he observes, “but the moon had a very different origin than planets.”

The moon presumably formed after a giant impact released a chunk of Earth into a low orbit. Through gravitational “friction,” the Moon’s orbit gradually expanded, but it remained tidally locked from those early years in low orbit. Planets, on the other hand, emerge after dust coalesces from a whirling disk, Boss says, so they probably are rotating during formation. In our own solar system, the planet Mercury, “which is close enough to sun to be tidally locked, is spinning three times for every two orbital periods,” Boss notes.

Numbers alone give planets orbiting red dwarfs a major role in the search for life in the nearby galaxy, because the majority of nearby stars are red dwarfs. “M-dwarfs are all over the place,” Boss says. So the new finding is “absolutely good news” for the search for life.

Todd Henry agrees. “In the history of human exploration, you go to the closest places first. I think our future is out there, not in the black holes or distant galaxies. Local is good. In 500 years we may want to go visit these star systems.”


Related Web Sites

Red Dwarfs and Super-Earths
SETI Sets its Sights on M Dwarfs
Astrophysical Journal Letters
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