Funding has been approved to build a space telescope with the capacity to reveal if the Alpha Centauri system – the nearest planetary system to Earth – has Earth-like planets in its habitable zones. Although it may prove suitable for studying a handful of other stars, almost the entire value of the project rests in what we will learn about just two stars. Fortunately, the cost is about a thousand times less than other space telescopes.
Only a handful of stars are close enough that sending probes to them within a human lifetime is even a remote possibility. “These nearby planets are where humanity will take our first steps into interstellar space using high-speed, futuristic, robotic probes,” Breakthrough Watch's Professor Pete Klupar said in a statement. “If we consider the nearest few dozen stars, we expect a handful of rocky planets like Earth orbiting at the right distance for liquid surface water to be possible.”
Being both very close and similar to the Sun in brightness, Alpha Centauri A and B are particularly interesting, but so far we don't even know if they have planets to study. Existing methods for planet-finding are surprisingly ill-suited to discovering planets in the habitable zone of these vary sunlike stars so Professor Peter Tuthill of the University of Sydney has come up with something completely different.
Launching objects, particularly instruments as sensitive as telescopes, into space is so expensive the usual approach has been to seek multi-functionality. Tuthill, however, is building what he calls “a scalpel rather than a Swiss Army knife.” Using funding from the Breakthrough Institute and the Australian Government, he expects to launch in 2023. The TOLIMAN telescope will only be suited to looking for planets in very nearby bright binary star systems, particularly those where the stars are of similar brightness. So it's just as well we have a pair of stars almost perfectly suited to TOLIMAN's capabilities 4.3 light-years away.
TOLIMAN takes its name from the ancient Arabic word for Alpha Centauri. It uses a diffractive pupil lense to create a complex pattern that allows very precise measurements of a star's location (astrometry). It will detect tiny movements in the position of stars as a result of the tug of any orbiting planets. It's a similar idea to the radial velocity method, through which we discovered many of the first planets beyond our system, but is even more sensitive for short distances (astronomically speaking), enabling detections of lighter planets further from their stars.
Unfortunately astrometry not only loses capacity more quickly with distance than radial velocity, it also requires a frame of reference, Tuthill told IFLScience. The Gaia space telescope uses distant background stars for reference, but “for that you need at least a meter [3-foot] wide telescope," Tuthill explained to IFLScience. To put something that big and sensitive in orbit costs billions of dollars.
TOLIMAN instead is going much smaller, but using the two stars of Alpha Centuari as references for each other. Planets of even close to Earth mass at habitable distances from either star will make it wobble compared to its companion. The problem, Tuthill admitted, is we probably won't be able to tell which star is doing the wobbling and which is staying steady, that is, which one has the planet.
Nevertheless, the stars are similar enough in mass and brightness that the period of orbit for their habitable zones overlaps. The discovery of a planet around either will spur further projects both to study from Earth, and to visit.