The problem with Alpha Centauri is that the system is too close. I don’t refer to its 4.3 light year distance from Sol, which makes these stars targets for future interstellar probes, but rather the distance of the two primary stars, Centauri A and B, from each other. The G-class Centauri A and K-class Centauri B orbit a common barycenter that takes them from a maximum of 35.6 AU to 11.2 AU during the roughly 80 year orbital period. That puts their average distance from each other at 23 AU.

So the average orbital distance here is a bit further than Uranus’ orbit of the Sun, while the closest approach takes the two stars almost as close as the Sun and Saturn. Habitable zone orbits are possible around both stars, making for interesting scenarios indeed, but finding out just how the system is populated with planets is not easy. We’ve learned a great deal about Proxima Centauri’s planets, but teasing out a planetary signature from our data on Centauri A and B has been frustrating despite many attempts. Alpha Centauri Bb, announced in 2012, is no longer considered a valid detection.

But the work continues. I was pleased to see just the other day that Peter Tuthill (University of Sydney) is continuing to advance a mission called TOLIMAN, which we’ve discussed in earlier articles (citations below). The acronym here stands for Telescope for Orbit Locus Interferometric Monitoring of our Astronomical Neighborhood, a mission designed around astrometry and a small 30cm narrow-field telescope. The project has signed a contract with Sofia-based satellite and space services company EnduroSat, whose MicroSat technology can downlink data at 125+ Mbps, and if the mission goes as planned, there will be data aplenty.

Image: Alpha Centauri is our nearest star system, best known in the Southern Hemisphere as the bottom of the two pointers to the Southern Cross. The stars are seen here in optical and x-ray spectra. Source: NASA.

The technology here is quite interesting, and a departure from other astrometry missions. Astrometry is all about tracking the minute changes in the position of stars as they are affected by the gravitational pull of planets orbiting them, a series of angular displacements that can result in calculations of the planet’s mass and orbit. Whereas both transit and radial velocity methods work best when dealing with planets close to their star, astrometry is the reverse, becoming more effective with separation.

Finding an Earth-class planet in the habitable zone around one of these two stars requires us to identify a signal in the range of 2.5 micro-arcseconds for Centauri A, an amount that is halved for a planet around Centauri B. Not an easy catch, but the ingenious TOLIMAN technology uses a ‘diffractive pupil’ to spread the starlight and increase the ability to spot and subtract systematic errors. I’ve quoted the team’s online description before but it usefully encapsulates the method, which has no need of field stars as references because it uses the binary companion to the star being examined as a reference, making a small aperture suitable for the work.

With the fortuitous presence of a bright phase reference only arcseconds away, measurements are immediately 2 – 3 orders of magnitude more precise than for a randomly chosen bright field star where many-arcminute fields (or larger) are required to find background stars for this task. Maintaining the instrument imaging distortions stable over a few arcseconds is considerably easier than requiring similar stability over arcminutes or degrees. Alpha Cen’s proximity to Earth means that the angular deviations on the sky are proportionately larger (typically a factor of ~10-100 compared to a population of comparably bright stars).

Image: Telescope design: The proposed TOLIMAN space telescope with a candidate telescope mirror pattern known as a diffractive pupil. Rather than concentrating the starlight into a tight focused beam as is usually done for optical systems, TOLIMAN has a strongly featured pattern, spreading starlight into a complex flower pattern that, paradoxically, makes it easier to register the fine detail required in the measurement to detect the small wobbles a planet would make in the star’s motion. Credit: Peter Tuthill / University of Sydney.

You can imagine the thermal and mechanical stability issues involved here. Doubtless Tuthill’s experience in the design of NIRISS (Near-Infrared Imager and Slitless Spectrograph ) and the aperture masking interferometry for the instrument on the James Webb Space Telescope will inform the evolution of the TOLIMAN hardware. As to EnduroSat, Raycho Raychev, founder and CEO, has this to say:

“We are exceptionally proud to partner in this mission. The challenges are enormous, and it will drive our engineering efforts to the extreme. The mission is a first-of-its-kind exploration science effort and will help open the doors for low-cost astronomy missions.”

A successful TOLIMAN mission could lead to what the team has referred to as TOLIMAN+, a larger instrument capable of detecting Earth-class worlds around both 61 Cygni and 70 Ophiuchi. But let’s get the Alpha Centauri results first, perhaps leading to detections around a target whose planetary signals would be much stronger than those of these other systems. We’ve seen how larger instruments like those aboard HIPPARCOS and Gaia have used astrometry to upgrade our view of vast numbers of stars, but it may be a small, dedicated mission with a unique technology that finally settles the question of planets around the two nearest Sun-like stars.

For more on TOLIMAN, see two previous posts: TOLIMAN Targets Centauri A/B Planets and TOLIMAN: Looking for Earth Mass Planets at Alpha Centauri. Also see this useful backgrounder.

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