Stars like the sun typically form in clusters with other stars. Many clusters are spread out so that the stars drift apart, but others are denser, and gravity keeps their stars close together.
The sun now stands alone, so astronomers think our star—and its newborn solar system—was either ejected from its birth cluster or drifted away from its siblings about 4.5 billion years ago.
Messier 67, or M67, is a hundred-light-year-wide ball of stars that recently passed some crucial "paternity tests" for being the sun's birthplace.
The cluster not only harbors stellar bodies similar in temperature, age, and chemistry to our sun, but M67 also drifts a relatively close 2,900 light-years away.
A new study of M67, however, undermines the existing lines of evidence and leaves almost no chance that our star could hail from the region.
Computer simulations show that a rare chain of events—two or three massive stars lining up just right to make a gravitational slingshot—would have been needed to kick the sun out of M67 and get it where it is today.
Such a powerful event is a probabilistic Hail Mary and, even if it had occurred, the speed of the kick would have ripped our nascent solar system to shreds.
"When you have that kind of gravitational disruption, planetary disks evaporate, and existing planets acquire energy and can be expelled," said study leader Barbara Pichardo, an astrophysicist at the National Autonomous University of Mexico.
Sun and Its Siblings Born "Like Popcorn"
Practically all of our galaxy's 200 to 400 billion stars, including the sun, were born through the gravitational collapse of diffuse clouds of dust and gas sprinkled across the Milky Way by previous generations of long-dead stars.
(Related: "Big Bang Ripples Formed Universe's First Stars.")
These star-forming clouds mix a little bit, but stars of similar chemistry tend to appear within the same clouds around the same time.
"It's like popcorn," Pichardo said. "They heat up for a long time and then pop-pop-pop, they are born."
To look for solar siblings, astronomers can use the spectrum of light shining from a star of a similar age and tag its chemical makeup, which can then be compared to the sun's.
So far, only two probable sun siblings are know to exist anywhere close by—that is, within the best existing data set of stars, which includes about a hundred thousand stars just 325 light-years in any direction from Earth.
According to that data set, the closest existing cluster with sunlike stars is M67.
The star cluster is a bit too far away and possibly younger than the sun, but at first these didn't seem like insurmountable problems to astronomers. So Pichardo and her team performed their yearlong run of 3-D computer simulations, fully expecting to add another piece of evidence to the pile.
Star Cluster Ruled Out by Speed and Bob
The new simulations relied on a detailed model of the Milky Way, its spiral arms, and even its halo of mysterious dark matter.
The computations also accounted for the up-and-down bobbing motion of M67 and the sun through the galaxy's plane of stars—wavelike orbits that all celestial objects in the Milky Way exhibit, to some degree, thanks to complex gravitational interactions with other objects.
The goal was to rewind the clock and find all the moments in the past 4.5 billion years or so when the sun and M67 would have been lined up for a clean ejection.
"We thought we would find lots of approaching moments, but we didn't," Pichardo said.
The key factor is that the sun is currently traveling at roughly 67,000 miles (108,000 kilometers) an hour away from M67.
Pichardo and her team's simulations show that, in the unlikely event of the sun's ejection from M67 at the right place and the right time, the star's speed would have to have been closer to 130,000 miles (209,000 kilometers) an hour—much too fast to ensure the solar system's safety.
At such speeds, "if planets are not kicked out, they can at least be perturbed into noncircular orbits," Pichardo said. "A [roughly] circular orbit is a big reason the Earth is habitable and we're alive today."
If Earth's orbit were more elliptical, the planet would bake when it drifted too close to the sun and then go into a deep-freeze as it moved away, a scenario perhaps too hostile to support life.
Astrophysicist Julio Navarro of the University of Victoria in Canada, who wasn't involved in the study, said the work "casts quite a bit of doubt on M67" as the sun's birthplace.
Still, while he noted that many simulations are required to build up a good bed of robust data, he implied that much of the new work may in fact have been overkill.
Just examining the bobbing motion of the sun and the star cluster would have been enough, since stars ejected from clusters tend to continue moving with the same amplitude as their birthplaces.
"The vertical motion of M67 is five times larger than the [motion of the] sun. It's a more robust way to attack this problem, I think, because [M67's motion is] too energetic," Navarro said.
"The sun's so much lower, it makes you wonder how on Earth [M67] could ever be the progenitor of the sun."
Finding More "Suns" to Help the Search?
With M67 headed to the proverbial dustbin, astronomers interested in pinpointing the sun's origins have several alternative hypotheses to consider.
One theory suggests the sun's birth cluster has simply spread out into obscurity. Another posits that the sun drifted outward from close to the center of the galaxy, since many sunlike stars seem to lurk there.
To support any of these ideas, however, more detailed and far-reaching catalogs of stars and their chemistry are essential.
"You could start looking systematically for the sun's siblings. It's nice to know where your closest relatives are, and if you do, you might be able to pinpoint their origins," said stellar physicist Bengt Gustafsson of Uppsala University in Sweden.
The best current catalog of stars, their positions, and their chemistry was recorded in the 1990s by a European satellite called Hipparcos.
A similar but more capable European satellite called Gaia, scheduled to launch sometime next year, is ready to inventory a billion stars. About 40 million of these will be within a thousand light-years of Earth—close enough to get a good read on their chemistry.
"Unfortunately, we won't have results from Gaia for at least five years," the University of Victoria's Navarro said. "But in principle, we should see [more] stars like the sun and get much closer to an answer."
The study refuting M67 as the sun's birthplace has been accepted for publication in the Astronomical Journal.