Our Sun Has a Sister

And it could have hundreds of thousands more of them.

The ancient Egyptians called it Ra. The ancient Greeks called it Helios. The ancient Mayans called it Kinich Ahau. The ancient Germans called it Sól.

Our longest-standing and most deeply held myths have so often revolved around the sun in large part because we humans have revolved around the sun. That distant sphere of glowing gas has been, to us fragile creatures, warmth and light and life itself. It has, we now know, been the center of everything we've known. No wonder we've assumed it was divine.

Which makes news just coming out of the University of Texas at Austin—soon to be reported in The Astrophysical Journal—particularly monumental. Our familiar star, it turns out, is not unique. Our sun has a sibling—a sister-star that almost certainly originated from the same cloud of gas and dust as our own shining orb. 

That sibling? A star with the deceptively dull name of HD 162826. Said star is 15 percent more massive than our sun, and located 110 light-years away from us (in the constellation Hercules, which is, appropriately, un-dully named). We can't see the sun's sister unaided, but even a set of low-power binoculars reveals HD 162826 to human eyes. It's situated near (well, relatively near) the bright star of Vega.

McDonald Observatory

The discovery was made by team of researchers led by the UT astronomer Ivan Ramirez, with help from several groups around the world. ​Using a combination of chemical analysis (high-resolution spectroscopy) and information about the stars' orbits (their "dynamics"), the team created a list of solar-sibling candidates that included 30 stars. Using information provided by telescopes at both the McDonald Observatory in Texas and the Las Campanas Observatory in Chile, they narrowed the field. In the end, there was one that matched our sun.

It was "lucky coincidence," Ramirez says, that HD 162826 emerged as our sun's sibling. As it turns out, HD 162826 has, for the past 15 years, been a subject of study by the McDonald Observatory Planet Search team. Studies conducted by that team, together with calculations provided by the University of New South Wales, mean that we already know a little bit about the our sibling-sun's solar system. It doesn't seem to have any "hot Jupiters," for example (massive planets that orbit close to the star itself). It also doesn't seem to have an analog to Jupiter itself.

It may, though, have other, terrestrial planets. And "there is a chance," Ramirez says—"small, but not zero"—that those planets could harbor life. In their earliest days within their birth cluster, he explains,

collisions could have knocked chunks off of planets, and these fragments could have travelled between solar systems, and perhaps even may have been responsible for bringing primitive life to Earth. Or, fragments from Earth could have transported life to planets orbiting solar siblings.

"So it could be argued," Ramirez continued, "that solar siblings are key candidates in the search for extraterrestrial life."

And if so: There are lots of candidates. HD 162826 may be the first solar sibling we know of, but it is, Ramirez thinks, not alone in that. That long-ago cloud of gas and dust kept busy. "The idea is that the Sun was born in a cluster with a thousand or a hundred thousand stars," Ramirez says. And that cluster, which formed more than 4.5 billion years ago, has long ago broken up, with member stars situating themselves into their own orbits around the galactic center—which in turn scattered them across the Milky Way as it exists today. "A few, like HD 162826, are still nearby," Ramirez says. "Others are much farther afield."