22 AUSTRALIAN SKY & TELESCOPE April 2018
THE EXOPLANETS NEXT DOORTRANSIT METHOD: LEAH TISCIONE /S&T, SOURCE: NASA; TESS’S VIEW: GREGG DINDERMAN /S&T, SIMULATED DATA: ZACHORY BERTA-THOMPSONas a small planet passes in front of the star and blocks some
of its light. Bright stars will also make detailed follow-up
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 find for the scientific
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.Scanning the sky
Transits are 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 colours (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 the10-metre mirrors of the renowned Keck telescopes or even a
2.4-m mirror like Hubble’s. The TESS cameras have effective
apertures of only 10 cm.
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 theBrightnessTimeORION1° 12°One CCD Camera Field
24° 24°STRANSIT 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.TTESS’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).