Sky & Telescope - April 2019 - 39

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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 planets 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 contamination 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.

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5 arcseconds

S&T ILLUSTR ATION, SOURCE: L. ZH AO E T A L. / ASTR ONOMICAL JOUR NAL 2018

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 planets 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

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Sky & Telescope - April 2019

Table of Contents for the Digital Edition of Sky & Telescope - April 2019