Sky and Telescope - March 2018 - 24

Scanning the Sky
Transits are hands down the current best way to ﬁnd small
exoplanets. The reason is that all of the starlight can be used
together. For other techniques, that is not always the case. The
radial velocity method, for example, measures the wiggle in the
star's position along our line of sight, but to do that we must
divide the light up into tiny bins of constituent colors (spectra). Inevitably, we lose some light when we do that. We also
use only a fraction of the starlight in the direct imaging technique, because the coronagraph device that blocks the starlight
to reveal the planet only works for small wavelength chunks.
But when a planet passes in front of a star, its bulk blocks
all of the wavelengths. Thus with transits, there's more bang
for our buck.
Because nearby stars are bright, TESS's aperture can
be small. The search for transiting planets orbiting bright
stars, therefore, does not rely on huge telescopes such as the
10-meter mirrors of the renowned Keck telescopes or even a

Time
p TRANSIT METHOD When a planet passes in front of its star from our
perspective, it blocks a small fraction of the star's light, which instruments such as TESS see as a dip in starlight. The graph of brightness
changes with time is called a light curve.

2.4-m mirror like Hubble's. The TESS cameras have an effective aperture of only 10 cm (4 inches).
But transits are rare, and so, like other transit surveys
before it, TESS must monitor a large number of stars. The
TESS cameras therefore each have custom f/1.4 lenses to
provide a wide ﬁeld of view that's 24°×24° - equivalent to
the area covered by about 50 full Moons, or roughly the
area of the constellation Orion. To get even more observing
real estate, TESS carries four of these cameras, all identical,
arranged in such a way as to cover a giant strip on the sky
(24°×96°). This setup of four small "telescopes" instead of
one large one makes TESS unique among space telescopes.
Each camera has a seven-element lens, plus a package of
detectors and electronics to enable transit detections. Lens
hoods shield the cameras from stray light from the Moon
and Earth, and a large sunshade encloses the plate that all
four cameras are attached to, all to improve TESS's chances of
ﬁnding other planetary systems.
Premier equipment isn't enough, though. To carry out a
two-year, all-sky survey of exoplanets, TESS also needs the

q TESS'S VIEW This sequence of simulated images illustrates the space telescope's field of view. Each camera contains four CCDs. A single CCD
covers a square of sky 12° wide, which is approximately the same size field as the Kepler space telescope observes. In all, each TESS camera images
a square 24° wide. This field of view would encompass the main stars of the constellation Orion (right).

1°

One CCD
12°

Camera Field
24°

24°

ORION

M A RCH 2 018 * SK Y & TELESCOPE

TR ANSIT ME THOD: LE AH TISCIONE / S&T, SOURCE: N ASA;
TESS'S VIE W: G REGG DINDER M A N / S&T, SIM UL ATED DATA: Z ACHORY BERTA-THO MPSON

blip as a small planet passes in front of the star and blocks
some of its light. Bright stars will also make detailed followup observations possible. Speciﬁcally, astronomers want to
be able to measure exoplanets' masses and to observe the
atmospheres of small, rocky planets - planetary properties
impossible to determine for the vast majority of the distant
(and therefore faint) stars observed during Kepler's original
mission. The TESS stars of prime interest, on the other hand,
will be only hundreds of light-years away and therefore typically a few hundred times brighter than Kepler's stars.
TESS's main raison d'être, or Level 1 Science Requirements
in formal NASA-speak, is to deliver to the community 50
planets with sizes less than 4 Earth radii and with measured
masses. Fifty might not sound like an impressive number,
but these 50 exoplanets will be the list of most promising
transiting planets for further study, a list of planets that with
follow-up observations will ultimately tell us a lot about the
small-exoplanet population.

Brightness

The Exoplanets Next Door

Sky and Telescope - March 2018

Table of Contents for the Digital Edition of Sky and Telescope - March 2018