Sky and Telescope - March 2018 - 27

MINI-NEPT UNE: DAVID A . AGUIL A R / CFA; FL A RE: SOL A R DY N A MICS
OBSERVATORY / N ASA; DE TAIL WORK: ORBITA L ATK

reveal has water vapor and oxygen. But
there are several other questions TESS data
could help us answer.
One of the most perplexing questions
concerning exoplanets today, for example,
is the composition of the so-called miniNeptunes, exoplanets two to three times
the size of Earth. When combined with
the slightly smaller super-Earths, miniNeptunes appear to be the most common
planet in our galaxy, yet we have no counp DATA-DRIVEN ANSWERS TESS's observations may help astronomers determine the
terpart in the solar system (S&T: Mar. 2017,
compositions of mini-Neptunes (artist's concept of Kepler-19b at left) and how planets fare
p. 22). We think these planets are too large
around stars that release frequent, powerful flares.
and too ﬂuffy to be predominantly made of
rock, but they can't be Jupiter-like giants,
either. Since we don't know what they're made of, there is no
Transits Are the Future
consensus on how they might form.
TESS is by nature a "catalog machine": Its main goal is to creThe hope is that TESS will uncover dozens of mini-Nepate a big list of transiting exoplanets orbiting bright, nearby
tunes around a variety of nearby host stars. We will then be
stars. That might sound boring, but it's not. By creating this
able to combine the exoplanet radii measured from transit
catalog, TESS will help open the door to a breathtaking new
observations with masses from follow-up radial-velocity stud- era in exoplanet science. Exoplanet science's ﬁrst era, that of
ies to discover these worlds' average densities. Together with
1995-2009, saw the discovery and initial characterization of
atmospheric measurements, the densities might reveal patindividual objects; in the next decade (2010-2018), Kepler
terns that help us understand mini-Neptunes as an ensemble. ushered in the era of big data and exoplanet statistics. TESS
This data-fusion approach may yield satisfying results on a
will enable us to merge the two, fostering the study of indivariety of other topics, too.
vidual exoplanets' characteristics on a big-data scale.
Flares are another forefront topic in exoplanets (S&T: Nov.
Space-based exoplanet transit surveys are so popular an
2015, p. 22), because many M dwarf stars are active and ﬂare
endeavor, in fact, that astronomers have several more in the
frequently. Trappist-1, for example, ﬂared some 40 times in
pipeline on a range of scales. On the small end there's the
an 80-day observational period; Proxima Centauri has even
MIT and Jet Propulsion Laboratory's CubeSat ASTERIA, which
more frequent outbursts. We'll be able to search the entire
is a prototype for a ﬂeet of CubeSats that would search for
TESS data set for stars that show evidence of both planets and exoplanet transits around the very nearest, very brightest
ﬂares at optical wavelengths. This, together with the atmoSun-like stars. It launched in August 2017 and began working
spheric compositions of these planets and other stellar-activin November 2017. On the large end, there's the joint Swiss
ity indicators, may help shed light on whether active stars
and European Space Agency CHEOPS mission (launch 2018),
completely erode rocky exoplanets' atmospheres or not.
which will focus on one star with a transiting exoplanet at a
time, and ESA's PLATO (launch 2026), a compound telescope
with 26 cameras afﬁ xed to one platform that aims to ﬁnd
longer-period planets transiting nearby, Sun-like stars.
Despite the huge amount of activity, transits are the ﬁrst
part of a longer journey. Only planets fortuitously aligned
will show up as transits, but this alignment not only limits
us to systems tilted just right with respect to our perspective,
it also favors planets close to their stars (because geometrically they're more likely to cover the stellar disk). Thus the
technique leaves out worlds in larger or non-transiting orbits.
These systems will hopefully be caught up by projects such as
NASA's WFIRST microlensing survey (launch mid-2020s) and
both ground- and space-based direct imaging campaigns. But
transit surveys like TESS are a supreme stepping stone on our
way to understanding our galaxy's planets.

p DETAIL WORK Visible in this lab shot are TESS's four cameras (top),
which appear to sit like eggs inside the nest of the sunshield.

¢ SARA SEAGER is the TESS mission's deputy science director and a professor at MIT, which is one of eight partners on
the project. Learn more about the mission at tess.mit.edu.
sk yandtelescope.com * M A RCH 2 018

http://tess.mit.edu http://www.skyandtelescope.com

Sky and Telescope - March 2018

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