Our corner of the universe just got a little bit lonelier.
Take a look at the picture at the top of this page. See that star in the lower right hand corner, the big, bright one with the arrow pointing at it? That's Barnard's star, a 10 billion year old red dwarf that sits just 6 light years from Earth -- a stone's throw in cosmic terms. Only the Centauri system's three stars are closer to us than Barnard's Star, and the next closest is a full two light years more than that.
Due to its proximity, Barnard's star is often short-listed as a target for humanity's first interstellar probe. Astronomers have long hoped to find a habitable planet around it, an alien Earth that might someday bear the boot prints of a future Neil Armstrong, or the tire tracks of a souped-up 25th-century Curiosity rover. Which is a bummer, because new observations indicate that Barnard's Star is likely barren, its habitable zone empty of planets Earth's size or larger.
It wasn't always this way. As Paul Gilster of Centauri Dreams reports, in 1938, an astronomer named Peter van de Kamp looked for and found -- or so he thought -- evidence of planets around Barnard's Star. Van de Kamp's measurements indicated that at least one, and possibly two, Jupiter-class planets orbited the star. But over the years, astronomers looking to confirm his observations consistently came up empty. And last week, a group of researchers led by UC Berkeley's Jieun Choi delivered the fatal blow to his data, when they revealed the results of 248 precise Doppler measurements that were designed to examine the star for wobbles indicative of planets around it. The measurements, taken over a period of 25 years, led to a depressing conclusion: "the habitable zone around Barnard's star appears to be devoid of roughly Earth-mass planets or larger . . . [p]revious claims of planets around the star by van de Kamp are strongly refuted."
Just like that, Barnard's Star's Jupiters went the way of Pluto -- de-planeted.
Part of me was surprised to hear this news. Not because of any special attachment to the van de Kamp data, but because lately it seems like everywhere astronomers look they find planets. NASA's Kepler space telescope, which studies a group of distant Milky Way stars, has found more than 2,000 exoplanet candidates in just the past two years, leading many to suspect that our galaxy is home to billions of planets, a sizable portion of which could be habitable. And that doesn't even include "orphan planets," billions of which are said to float freely around our galaxy. Perhaps nothing communicates this profusion of new planets better than this new video by Harvard Planetary Scientist Alex Parker, which depicts all of Kepler's exoplanets orbiting a single star:
This sense that planets are commonplace, that they lurk in every unexplored corner of the universe, may explain why not everyone is writing off Barnard's Star. Some are holding out hope that the star could surprise us, yet. I reached out to Project Icarus' Andreas Tziolas, who I previously interviewed about interstellar travel, to see if he was disappointed to hear that Barnard's Star was barren. He told me that "knowing where planets do not exist is a good result in and of itself," but he was also quick to note that radial velocity methods (of the sort used in the study) aren't well suited to looking for terrestrial planets. "Any exoplanet hunter you ask [will agree] that there's still room for something unexpected," he said.
And he was right. When I asked Sara Seager, exoplanet hunter extraordinaire, about Barnard's Star, she expressed confidence that astronomers will keep searching for smaller and smaller planets there, because "the nearest stars are such prime real estate." How prime? Only 4 stars, including Barnard's Star, are within 6 light years of the Sun, and only 11 are within 10 light years. If the Milky Way is a vast city, our local stellar group is a tiny, exurban cul de sac in a neighborhood where the houses sit on enormous lots. With so few houses in visiting range, we'll need to knock on every door twice. That was the lesson I took from my exchanges with Tziolas and Seager.
The other interesting thing about this story is how little attention it's gotten. If it weren't for Paul Gilster's (characteristically) superb post on the subject, I think I would have missed it altogether. Run a Twitter search for "Barnard's Star," and only a handful of results pop up, most of which are tweets from professional astronomers. Google is even worse, turning up the original paper, a short item from Universe Today, and not much else. That says something about how seriously we take idea of interstellar travel. If a mission to the stars were considered a viable near-term possibility, the Barnard's Star observations would be front-page news. But in our time, a time when star travel is viewed as something inconceivable, the study merited no more than a single line on science news aggregator sites.
Back in July, Adam Frank, professor of physics and astronomy at the University of Rochester argued, in an op-ed for The New York Times, that interstellar travel would likely be beyond the reach of humanity for hundreds, if not thousands, of years. Frank pointed out that the Voyager space probe, the most distant manmade object from Earth, would, at its current velocity, take more than 700 centuries to reach another star. He noted that the technologies that power star travel in science fiction, futuristic-sounding techniques like "warp drive" and "hyper drive," were just that: fictional. Like it or not, he wrote, "we are probably trapped in our solar system for a long, long time." Thinking back on Frank's piece it occurred to me that whether he's right or wrong, our culture surely agrees with him, and the muted reception to Barnard's Star story bears that out. Unless you're an interstellar engineer or an exoplanet hunter, the happenings around a star 35 trillion miles away are pretty remote from your every day life.
They just don't register as local news.
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