Sky & Telescope - May 2023 - 36

LEAH TISCIONE / S&T, SECTORS SOURCE: C. BEALS / MIT,
COVERAGE MAP SOURCE: D. DEMING / TESS TEAM
Alien Worlds
planets and stars are common. Planets smaller than 4 Earth
radii are the most common type of planet in our galaxy. Red
dwarf stars - typically at most 60% the size and temperature
of the Sun - are similarly the most abundant. Red dwarfs
(also known as M dwarf stars) make up most of the nearest
neighbors to our solar system (S&T: Jan. 2019, p. 34).
Second, it is generally easier to detect and study small
planets around small stars with the transit method than
around large stars, because what astronomers measure is the
ratio of the size of the planet (and its atmosphere) relative
to the size of the star. Correspondingly, it's more difficult to
detect a small planet around a Sun-like star than it is around
a red dwarf.
While Kepler has provided us with a bounty of small
planets, the mission focused on Sun-like stars, and they for
the most part lie far away from Earth. In fact, less than 1% of
Kepler planetary systems are located within 200 light-years, a
distance range well-suited for further follow-up.
Small Exoplanets Close to Home
TESS has brought our focus closer to home. We've discovered
285 confirmed exoplanets to date, with more than 4,000
additional candidate exoplanets awaiting confirmation (see
tipbox, page 35). At first glance, TESS's discoveries so far
match the expectations that we had before launch. TESS was
specifically designed to discover 50 planets with radii smaller
than 4 times that of Earth whose masses we could then measure
with ground-based observatories. We've far exceeded this
goal, with 174 small planets discovered to date, 116 of which
have measured masses.
Mass measurements are possible for so many TESS exoplanets
because the worlds orbit relatively bright stars, in
whose starlight we can more easily detect the tiny wobble
WHAT'S IN A NAME?
TESS exoplanets have a variety of names, simply
because there is no strict naming scheme employed
by the mission team. TESS exoplanets typically
receive a name that follows from an established star
catalog, particularly for bright stars, e.g., Pi Mensae
c. Otherwise, TESS targets are normally named as
TOI-, where TOI stands for
" TESS Object of Interest. " This indicates the mission
team flagged the object (star and planet) as a viable
planetary system and assigned it a TOI number, e.g.,
TOI-257.01. Once a planet candidate is confirmed as
a bona fide planet, we often change the number into a
letter, in this case TOI-257b.
caused by the planet's gravitational tug on the star. More
than 70% of all TESS exoplanets are found around stars
brighter than a visible magnitude of 12. In Kepler's case, just
8% of discoveries are around such bright stars. TESS planetary
systems are also located relatively close to home, with 40%
found within 200 light-years of Earth. And more than 20%
of TESS exoplanets orbit red dwarfs, compared to just a few
percent of Kepler exoplanets.
One of the first planets TESS discovered, Pi Mensae c,
orbits a star bright enough that it can be seen with the
naked eye - at least if you find yourself in a location with an
extremely dark sky and you have pretty good vision to boot.
While you can see the star Pi Mensae with your own eyes
on a clear night, it is not possible to see the planet with the
naked eye because it is only about twice the size of Earth and
it orbits very close to its star, with a period of just over 6 days.
OBSERVING SECTORS The combined field of view of TESS's four cameras spans a long, 24° × 96° strip of sky (A). Every 27 days or so, the
spacecraft will observe a different strip of sky (B), slicing the celestial sphere into 26 sectors (13 per hemisphere). These sectors overlap near the
ecliptic pole (C). The dashed circle around the pole shows the region that JWST can observe uninterrupted.
A
24°
Ecliptic
pole
96°
189 days
108 days
81 days
54 days
27 days
351 days
E
JWST
Ecliptic
latitude 6°
36 MAY 2023 * SKY & TELESCOPE
continuous
viewing zone
B
Ecliptic pole
C
CLIPT
IC

Sky & Telescope - May 2023

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