April 1, 2016 at 1:15 PM
To paraphrase the old line about beetles, God sure must love red dwarf stars, because she made so many of them.
About three out of four stars in our galaxy are red dwarfs -- "the dim bulbs of the universe," in the words of Seth Shostak, senior astronomer at the SETI Institute in Mountain View, Calif.
But although these small, relatively gloomy stars are all over the place, they've historically been discounted as targets for SETI -- the search for extraterrestrial intelligence. Most SETI efforts have looked at larger stars like our own Sun.
The reason has to do with orbital dynamics, geometry and heat. Red dwarfs, with their relatively low mass, have narrow habitable zones as traditionally defined. That's the Goldilocks orbit in which a planet won't be too hot or too cold. The precise dimensions of HZs are debatable, but more massive stars have bigger ones. In our own solar system, Venus is too close to the Sun and Mars too far away, but Earth is just right.
A typical red dwarf has a narrow HZ -- and it's so close to the star that an orbiting planet would become tidally locked, meaning one side of the planet always faces the star and the other always is dark. The word on the street is that such a planet couldn't support life because the daylight side would be roasted and the dark side totally frozen.
But Shostak and his SETI colleagues have a sunnier view (dare one say!) of red dwarfs. Yes, the tidal lockage is a bummer, but oceans and atmospheres can do a lot of the heavy lifting in transporting heat from the hot side to the cold side and allowing the planet to retain at least some portion of habitability.
This week, the SETI Institute put out a press release saying it would use an array of telescopes in northern California to look at 20,000 red dwarf stars.
"Red dwarf stars are not as bad as they first seemed," Shostak told The Post.
Indeed they have obvious search advantages:
1. They're close, on average. That's because there's so many of them. It's a numbers game. And close does count in SETI, because a (very hypothetical) alien radio signal transmitted from twice as far away is going to be four times fainter when it reaches Earth.
2. They're old, on average. Our friend the Sun is built to last about 10 billion years, Shostak said. But a red dwarf star, cruising along at 55 mph with hands at 10 and 2 on the wheel and keeping an eye on the gas mileage indicator, can last 100 billion years. Now, mind you, the entire universe is only about 13.8 billion years old, but in general a red dwarf is more likely to be ancient than a yellow Sun-like star.
"This may be one case where being older may actually be better," Shostak told us. He reasons that any advanced life form would have had more time to develop technologies we might be able to detect.
In the news release, SETI Institute scientist Gerry Harp says the new search will use the Allen Telescope Array in the Cascade Mountains of northern California to look at a number of "magic frequencies" on the radio dial "that are directly related to basic mathematical constants. It's reasonable to speculate that extraterrestrials trying to attract attention might generate signals at such special frequencies."
We asked M.I.T. astrophysicist Sara Seager, an expert on extrasolar planets, if red dwarfs were good targets for SETI.
"Sure, why not?" she said by email. These stars "are on a pendulum -- sometimes favored for life, other times not."
By phone she elaborated Friday on why red dwarfs haven't been more of a focus. "They've definitely gotten a bad rap because they're so different from our own Sun," Seager said. "So we have difficulty understanding what the planets around them would be like."
At some point we need to add all the caveats about SETI: that it is innately speculative, that we don't know how often life percolates from non-living matter, that we can't say with any confidence that once life evolves it has a high likelihood of someday manifesting itself as a communicative technological civilization. Because the stars are all so far away, it would really be helpful if the aliens wanted to send us a signal.
We asked Shostak if he thinks this latest search for aliens will be any more successful than past attempts.
"Well, one never knows with SETI," he said. "I bet everybody a cup of Starbucks that we'll find ET in the next 20 years. But I haven't bet more than a cup of Starbucks."
Update: A reader in the comments raises the important issue of solar flares and other volatile behavior among red dwarf stars:
It's not the Tidal Locking that makes red dwarfs a problem in terms of habitability - it's the stellar life cycle. They've got an extended pre-main sequence phase where they'll roast any planets that happen to be in their main sequence habitable zones, and all but the largest one have really volatile output (i.e. they'll have giant sunspots that will drastically reduce the star's luminosity for stretches of time).
This is another blemish on red dwarf stars, but not a reason to shun them, according to Shostak and Seager. They say some red dwarfs are volatile and others aren't. Shostak said that even the active red dwarf stars can calm down after a couple of hundred million years.