They just don't come any closer
Exoplanet found right next door in Alpha Centauri
May be roughly Earth-sized, but blazingly hot.
by John Timmer - Oct 16 2012, 4:16pm USMST
Today, planet hunters announced evidence there's a planet orbiting one of our closest stellar neighbors. One of the three stars of the α Centauri star system shows the sort of periodic changes in brightness that are a hallmark of the presence of an orbiting planet. And, even though the new world would be far too hot to support liquid water, the astronomers who discovered it point out small planets tend to form in groups. Odds are good that there are additional planets lurking further out from the host star.
Rapid advances in planet-hunting have led to an ever-increasing catalog of exoplanets, but most of these orbit distant stars. In contrast, the a Centauri system "is a household name," as Greg Laughlin of UC Santa Cruz put it. Just over four light years from Earth, the system includes two bright stars, Centauri A and B orbiting each other with an 80 year period, along with a red dwarf called Proxima Centauri. Centauri B has a Sun-like mass, but is quite a bit dimmer.
The planet was detected using the radial velocity method. As a massive body orbits its host star, it exerts a gravitational pull on it, pulling the star in slightly different directions as its position shifts. These create a small acceleration in the star itself, usually on the order of a few meters per second. That, in turn, shows up in the light emitted by the star as Doppler shifts in the light it emits, which vary with the orbital period of the planet.
Detecting these, however, can be a challenge, as a long catalog of factors can also cause periodic changes in the star's output. The authors of the paper describing the find list them as, "instrumental noise, stellar oscillation modes, granulation at the surface of the star, rotational activity, long-term activity induced by a magnetic cycle, the orbital motion of the binary composed of a Centauri A and B, light contamination from a Centauri A, and imprecise stellar coordinates."
To get around these, the authors relied on a massive catalog of observations, made using the HARPS instrument on a 3.6 meter telescope at the European Southern Observatory. Over a span of nearly four years, the authors made multiple observations of Centauri B, often several observations a night, spaced out two hours apart. This let them average out short-term variability on the span of hours, and reconstruct that star's equivalent of the solar cycle, in which its activity increased over the course of their observations.
One by one, they factored out all of the periodicities they could account for. What was left was a hint of a signal with a periodicity of 3.3 days. This was incredibly weak—in fact, the smallest yet detected—at only 0.8 meters/second acceleration. But, even though this signal was much smaller than some of the noise they filtered out, the authors calculated there was a "false alarm probability" of less than one percent. In other words, it's probably a planet. As scientist told a press conference earlier today, if it was any place other than Alpha Centauri, there would be nothing extraordinary about the claims.
But don't start building the colony ship just yet. With a 3.3 day orbit, the planet is only 0.04 Astronomical Units (1 AU is the typical distance from the Earth to the Sun). That makes this planet blazingly hot, at about 1,500 Kelvin. One of its discoverers indicated this would ensure the surface is "not solid, more like lava." The radial velocity method lets you estimate the lower bound on the mass of the planet. Assuming it's orbiting roughly in a plane that faces edge-on to Earth, it has a mass roughly equivalent to our home planet.
Even though the new planet is likely well outside the habitable zone, we shouldn't give up on a Centauri. The plane of the two large stars of a Centauri is oriented nearly face on to Earth, and forces that govern star formation would make it likely that any planetary disks would form in this same plane. That means the planet is more likely to be on the low end of the mass estimates—in other words, close to Earth-sized. As the discoverers noted, about 70 percent of the small planets we've discovered have been in systems with multiple planets. So, the chances of finding something else further out are much higher than you might otherwise expect.
The HARPS team (which was represented by Stéphane Udry and Xavier Dumusque of the Geneva Observatory) estimate that, based on Centauri B's habitable zone (which is roughly centered on distance that's equivalent to Venus' orbit) they should be able to spot a Super-Earth (having five to 10 times Earth's mass) in the habitable zone. And, given the probability that the system's plane is oriented towards Earth, we could also use an orbiting observatory to watch for planets transiting in front of Centauri B.
We may have to wait a bit, though. The team told the press conference the orbit of the system's two large stars could be problematic; they were coming very close to each other over the next four years. This would make observations extremely challenging. It'll be eight years or more before we'll have good conditions for observations again. But, on the plus side, telescope tech is advancing dramatically these days, and a decade's worth of progress will put us in a much better position to learn something about our neighbors.
What about visiting? Laughlin estimates that, given our current technologies, any probe we sent wouldn't arrive for about 40,000 years. So that's probably a no-go, "given our propensity for instant gratification." But there are some unproven propulsion ideas that could get us there much more quickly, and Laughlin said that, should this find ignite enough interest, we may look into those more seriously.
While this putative exoplanet is far too hot to land safely on, let alone have any life on it . . . where there's one terrestrial planet, there are usually more. Assuming the observations are repeatable, the whole Alpha Centauri system suddenly just got a lot more desirable to visit.