Focus on Alpha Centauri

At Palo Alto’s superb Amber India, I was thinking about Alpha Centauri. There are several Amber India locations in the Bay area, but the Palo Alto restaurant dishes up, among other delights, a spicy scallop appetizer that is searingly hot and brilliantly spiced. Greg and Jim Benford were at the table, Claudio Maccone and my son Miles. It was the night before Breakthrough Discuss convened. And while the topics roamed over many aspects of spaceflight, it was that star system right here in our solar neighborhood that preoccupied me.

How lucky could we be to have not one but two stars this close and so similar to our own? Centauri A is a G-class star, Centauri B a K, and if we hit the jackpot, we could conceivably find planets orbiting both. Then there is Proxima Centauri, an M-dwarf that is the closest star of all to the Solar System. The presence of so many astronomers on the Breakthrough Discuss roster made it clear we’d get the latest on the hunt for planets here, a vital factor as we assessed prospects for the Breakthrough Starshot mission. A nice blue target world would help.

The Binary Star Factor

Growing up, I would haunt the local library for books on astronomy, learning that despite its tantalizing proximity, Alpha Centauri was likely devoid of planets because Centauri A and B were so close to each other. After all, at times they close to within 11 AU — how could planets exist in such a gravitationally unsettled region? The question is still unanswered, as is the question of whether Proxima Centauri is truly part of a triple star system or simply shares a common motion with A and B. But our picture of Alpha Centauri has changed radically since my days in the local library, and the system is under scrutiny as never before.

Image: Gathering outside Stanford’s Arrillaga Alumni Center with coffee in hand as we waited for the first session of Breakthrough Discuss to begin.

Paul Wiegert and Matt Holman discussed stable orbits around Centauri A and B back in 1997, work that led to a generally accepted belief that out to a distance of perhaps 2.5 AU, small planets of Earth-like radius or a bit larger could exist. Giant planets are seemingly ruled out by radial velocity studies, although Jared Males (University of Arizona) would note that we might save James Cameron’s Polyphemus, a gas giant in the film Avatar, by postulating a large radius, low mass planet. But he was quick to add that the prospect was not likely.

The session, titled Exoplanet Detection Programs Focused on Alpha Centauri, was led by Olivier Guyon, but it was Michael Endl (University of Texas at Austin) who presented the overview of Alpha Centauri work so far. The issue of planet formation is far from settled, and as Centauri Dreams readers know, the question is not so much one of stable orbits but whether planet formation can deliver an intact planet in the first place. Key work here has been done by Philippe Thébault and Javiera Guedes, who have reached opposite conclusions, with Guedes arguing for small planet formation, and Thebault arguing against the proposition because of planetesimal encounters and the impact velocities at which they would occur.

Image: One of Michael Endl’s slides, this one discussing planet formation around Alpha Centauri.

The argument will be settled, as Endl pointed out, by our increasingly powerful ability to deploy new technologies. Radial velocity methods are pushing toward the region we’ll need to study, but even now we would need to work at a 10-12 centimeters per second level to find an Earth mass planet, a feat that Endl noted was orders of magnitude below what today’s best instruments can deliver. Remember, too, that the planet we thought we had found around Centauri B probably isn’t there, now considered to be a false positive probably caused by activity on the star itself. In fact, let me quote Xavier Dumusque on this, as he was at the conference and was on the team that performed the original Centauri B work:

We worked with 20,000 spectra on Centauri B taken over four years and found a small 50 cm per second signal that seemed to be a planet in a 3.2 day orbit. Subsequent papers have shown that the signal is in the data, but it is probably not due to a planet. All our techniques are at the limit of their capabilities, which means we should use all the techniques we have, so that if one tells us we have a planet, another can assure us it is real.

The problems Alpha Centauri presents, particularly right now, are manifest. Spectral contamination means that when you’re trying to tease a Doppler signal out of the light from a star like Centauri B, you get light mixing in from Centauri A, for at this point in their orbits, the two stars have closed to their closest point as viewed from Earth. The work Dumusque referred to, drawn from HARPS spectroscopic data at the European Southern Observatory’s La Silla Observatory, may well have been affected by magnetic effects on Centauri B’s surface. But right now we’re in that period when the primary Centauri stars are very hard to analyze.

I’ll remind readers that there has been one possible transit detected. Motivated by the HARPS work on the possible 3.2-day period planet, the search turned up what looked like the transit of an Earth-like planet with a period of less than 12 days, but if it was a transit, it did not recur. As to Proxima Centauri, we still have no planets there, but we can rule out larger worlds, while allowing the possibility of planets of two to three Earth masses in the habitable zone. We’ve followed the work of the Pale Red Dot project that has collected new spectra using the HARPS instrument in these pages and are awaiting the data analysis with great interest.

Image: Natalie Batalha (NASA Ames) raising a point during the panel discussion that followed the Alpha Centauri planet detection talks.

Bringing New Methods to Bear

The point that Michael Endl made, and it was echoed by other speakers, is that we need to throw everything we have at this intractable problem. One way forward is to keep improving radial velocity precision, but we also need to do x-ray monitoring of Centauri A and B to look for activity cycles, and consider the possibilities of astrometry and even direct imaging. Thomas Ayres (University of Colorado) noted the dramatic changes to Alpha Centauri A in x-ray imaging — the star goes dark at x-ray wavelengths in 2005 with an unprecedented darkening by a factor of 50, and is now showing a return to activity levels close to that of our own Sun.

The scientists at Breakthrough Discuss were generally upbeat about the prospects of finding planets in the Alpha Centauri system, though the feeling was not quite unanimous, with Peter Tuthill (University of Sydney) saying he found the likelihood of planets there in the range of 20 percent. Adding “I’ve just put myself out of a job with that comment,” he went on to explain JAM, the JWST Aperture Mask, which would use astrometric methods to look for the tiny stellar motion that a planet tugging either Centauri A or B would induce. A separate mission called TOLIMAN (a medieval name for Alpha Centauri) would use a diffractive pupil aperture mask, with the distortions optical systems produce becoming a ‘ruler’ that detects such motion.

Image: A view across the way from the alumni center during a break. After the intense sessions, it was a pleasure to walk outside for a few minutes to rest my eyes.

And what about observing Alpha Centauri planets from the ground? We’re moving into the era of enormous ground observatories with apertures from 20 meters up to 100 meters across in the works. These Extremely Large Telescopes (ELTs), like the European Extremely Large Telescope (Chile), the Thirty Meter Telescope (Mauna Kea, Hawaii) and the Giant Magellan Telescope (Chile) point toward future instruments as enormous as Colossus, a 100 meter telescope concept that could become the world’s largest optical and infrared instrument.

Needless to say, such instruments can become major tools for studying planetary systems around nearby stars. But as Markus Kasper (European Southern Observatory) explained, we can also perform upgrades on existing instruments — the Very Large Telescope (VLT), Magellan (Chile) and Gemini (sites in Hawaii and Chile) — to perform pathfinder work at thermal infrared wavelengths for future imaging with the giant instruments to come. Thus ground-based instruments become a complement to space telescopes for actual exoplanet imaging.

I was interested in Bruce Macintosh’s presentation on direct imaging from space, because years ago I talked to Webster Cash (University of Colorado) about the prospects of using a starshade, in which the optics for a mission are separated. Rather than using a coronagraph to block out the light of the central star, you create a starshade whose shape is precisely determined to block the same light, with the starshade operating some 25000 kilometers away from the telescope.

Macintosh (Stanford University) said the problem of seeing a planet next to the blazing star that it circles was akin to looking for bioluminescent algae next to a lighthouse, which is why we need a coronagraph or a starshade in the first place. The WFIRST mission (Wide-Field Infrared Survey Telescope), scheduled for launch late in the next decade, will carry an advanced coronagraph, but a starshade would also be compatible with this instrument.

Image: The starshade in position far from the space telescope observing the light, with the central star effectively masked. Credit: University of Colorado.

But maybe we don’t have to wait that long. During a break I had the chance to talk to Cash, who had been a huge help with my original Centauri Dreams book. Cash has been working with starshade concepts for a long time, but even he was surprised when his team began testing small starshades in the atmosphere. In a field of view that included the bright star Sirius, the star would simply disappear. While continuing work on a space telescope/starshade concept called the Aragoscope (after French optical scientist Francois Arago), Cash and team began testing an airborne starshade that could be observed by a telescope on the ground.

All of this could lead to serious results at Alpha Centauri. Cash hopes to use an airborne starshade no more than a meter across that will be observed by a balloon-lofted telescope several hundred kilometers away to probe the habitable zone of Alpha Centauri. “Anything you can do on ground, you should do on ground,” Cash explained. “If we can do it remotely with big telescopes, it’s not a key part of payload that actually goes to Alpha Centauri.”

I’m running out of time today, so I’ll start tomorrow with an Alpha Centauri observing platform called ACEsat, a dedicated space observatory, and move from there into some of the more speculative thoughts of the attendees on what we might find around these stars.