My first Magazine Sky & Telescope - 04.2019

2022 2020 2018 2016 2014 2012 A 2010 cover the habitable zones for both stars. “Those distances around both Alpha Centauri A and B are safe for planets, so if they are there, they would survive,” says Debra Fischer (Yale University), who has led an intensive search for plan- ets around both A and B. As seen from Earth, Alpha Centauri A and B had a close approach in 2016 and are now slowly moving apart. This prevents effective planet searches using the radial velocity method. “The light from one star will bleed in as we try to take the spectrum of the other star, and that has led to con- tamination that ampliﬁed our errors and made it harder to get the precision we need,” Fischer says. While waiting for the stars to separate, Fischer and her graduate student, Lily Zhao, have focused their efforts on determining how big a planet could be hiding in the glare. Using observations from HARPS and two other instruments, they have shown that in their habitable zones, Alpha Cen B could harbor one or more unseen mini-Neptunes, and Alpha Cen A could hide a world three times Neptune’s mass. Any gas giants like Saturn would have been already detected with existing instruments. Alpha Centauri could also harbor circumbinary planets: There is a stable zone beyond 80 a.u. from the binary’s center of gravity. However, being so far from the stars, these planets would be cold, dark, and hard to ﬁnd. N E 5 arcseconds Are We There Yet? During 2019, Alpha Centauri A and B will move apart enough to resume radial velocity planet searches. When they do, the Echelle Spectrograph for Rocky Exoplanets and Stable Spectroscopic Observations (ESPRESSO), a new instrument 10 times more accurate than HARPS, will be ready at the European Southern Observatory’s Very Large Telescope in Chile. ESPRESSO can measure planet-induced wobbles in a star down to 10 centimeters per second, enough to detect Earth-mass planets in the habitable zone of Sun-mass stars. “I think in the next three to ﬁve years we will have an answer,” Fischer says. In anticipation of a discovery, a private consortium of researchers and institutions is working to design, build, and launch a small space telescope that could take images of plan- ets in the habitable zones of Alpha Centauri A and B. They call it Project Blue, after Carl Sagan’s famous “pale blue dot” moniker for Earth. According to Jon Morse, Project Blue’s Mission Executive, a small telescope specially designed and packed with all the p INCHING APART Alpha Cen A and B are moving apart again from our perspective, offering astronomers the opportunity to use the latest exoplanet-hunting instruments to scrutinize the system for alien worlds. necessary technology could be built and put in orbit for less than $50 million, a fraction of the cost of a mid-size space mission such as Kepler or Dawn. Such an endeavor would also go against the prevailing philosophy of planet-hunting missions, which now tend to rely on large surveys in order to maximize the chances of ﬁnding planets. “The possibility that there is a no result, that there are no planets that we are able to see, is real,” says Morse. “But we are saying, let’s look to Alpha Cen A and B, it’s the closest Sun-like star by a factor of 2.5, and the optical system you need is much smaller and less expensive.” Even before seeing blue dots around Alpha Centauri, another initiative is already making plans to send probes there. In April 2016, Silicon Valley billionaire and venture capitalist Yuri Milner created the Breakthrough Starshot B A sk yandtele scope.com • A PR I L 2 019 39

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