10 SKYNEWS •NOVEMBER/DECEMBER 2019
is blocked by a planet. The amount of dim-
ming reveals the diameter of the planet.
The length of time between successive
passes gives the orbital period and the dis-
tance of the planet from its star.
The method was well tested by NASA’s
Kepler mission, launched in 2009, which
snared more than 2,600 planets by staring
continuously at a distant field of stars in
the Milky Way for over three years. Kepler’s
statistical snapshot has given exoplanet
experts a sense of the overall abundance of
these objects. The trade-off is that Kepler’s
discoveries are too distant to be studied in
great detail.
In contrast, TESS is built to ferret out
transits much closer to home. Its target
stars are fewer but are spread around thecelestial sphere. The mission consists of a
systematic search, during which the space-
craft’s four cameras scan a 24-by-96-degree
strip of sky for 27 days before moving on
to the next strip. In barely more than two
years, TESS will have covered most of the
celestial sphere.
“We’re finding everything we can,” says
Seager, as she describes the team’s efforts
to maximize its data. This includes the use
of artificial intelligence to help separate
real transits from normal variations in a
star’s brightness or to isolate glitches due
to space craft movement. The point, she
adds, is not simply to extend the list of
known exoplanets but to shed light on their
nature and origin.DIFFERENT WORLDS
TESS officially became a planet discoverer
in September 2018, when it found a little
companion around a Sun-like star in the
southern constellation Mensa. Located 59.
light-years away, Pi Mensae was already
known to researchers. An announcement
in 2001 declared that the star hosts a gas
giant roughly 10 times the mass of Jupiter,
with an orbital period of more than five
years. TESS’s object, Pi Mensae c, circles the
star in just 6¼ days.
Judging by the depth of its transits, Pi
Mensae c is perhaps twice the diameter of
Earth. Ground-based observatory mea -
surements of the gravitational interaction
between planet and star reveal that “c” is
4.8 times the Earth’s mass. Together, these
values imply that the planet’s density is
essentially equivalent to that of pure water.
It may contain mostly water or consist of a
rocky core surrounded by a vast envelope
of opaque gas. Either situation would be
unlike anything in our solar system.
“The fact is, a lot of these exoplanets are
very different from what we know,” says
Diana Dragomir, an astronomer at The
University of New Mexico, in Albuquerque.
It was Dragomir who, last April, reported
the first Earth-sized planet in the TESS
data. Known as HD 21749c, it hugs a star
somewhat smaller and dimmer than our
Sun. The planet is not quite as big as Earth,
so it’s almost certainly rocky. Scientists are
trying to estimate the mass of HD 21749c,
which would lead to further details, such
as whether or not it possesses a substantial
metal core.
TESS also netted a second, more distant
member of the HD 21749 system. Catego-READY
FOR ACTION
Seen here prior to
launch, in April 2018,
TESS measures about
one metre across,
minus its solar panels.
COURTESY NASA
CLEAN SWEEPFor the initial two-year mission, the TESS team has divided the sky into 26 strips,
13 per hemisphere. Each strip is made up of four “squares,” corresponding to TESS’s quartet of wide-
field cameras. During the extended mission, scientists plan to observe those areas that were missed
between the squares. COURTESY NASA/MIT/TESS
ECLIPTIC POLEecliptic
poleecliptic
latitude 6º96º24º