Sky & Telescope - May 2023 - 35
LEAH TISCIONE / S&T, SOURCE: NASA
Brightness
The TESS space telescope is expanding our
knowledge of exoplanets, helping us focus on
those orbiting stars that are the smallest and
closest to Earth.
O
ver the past few decades, there has been an exoplanet
revolution. Thirty years ago, we only knew of
a handful of planets outside our solar system. Today,
we know of more than 5,000 of these distant worlds, with
some similar in size to Earth and some orbiting stars like the
Sun. A major player in this exoplanet revolution was NASA's
Kepler space telescope, which was retired in 2018. More than
50% of all exoplanets discovered to date have come from
searching through Kepler data.
But our planet-hunting days are far from over. NASA's
Transiting Exoplanet Survey Satellite (TESS) launched in
2018 and quickly picked up where Kepler left off (S&T: Mar.
2018, p. 22). Its mission: to search the entire sky for the
nearest exoplanets.
One of Kepler's main goals was to determine how common
exoplanets are in our galaxy, the Milky Way. Kepler
surveyed a single part of the sky for several years to search
for exoplanets via the transit method, in which periodic dips
in a star's brightness reveal a planet passing in front of that
star. The exquisite data Kepler collected during this initial
mission, which lasted from 2009 to 2013, showed us that
exoplanets are indeed abundant in our galaxy. We can now
say with confidence that there are more planets than stars in
the galaxy and so, on average, every single star in the galaxy
hosts at least one planet. Of course, this does not mean every
star actually hosts a planet. Rather, some stars host no planets
and some stars host multiple planets.
However, there is more to the story than simply counting
how many exoplanets there are. We want to know what
exoplanets are made of and what their atmospheres are like.
To find that out, we need to take a close look at these worlds.
But the systems Kepler surveyed predominantly orbit faint,
distant stars - poor targets for detailed study.
This is where TESS comes in. TESS is the only space observatory
capable of surveying the entire sky to discover many
of the closest, most accessible exoplanets in the Milky Way.
Because TESS discoveries are predominantly around small,
bright, nearby stars, astronomers can readily use other observatories
to follow up and determine the exoplanets' masses
and the composition of their atmospheres. With its discoveries,
TESS is thus moving us further along in our journey to
determine what exoplanets are made of and whether there is
any other planet out there like Earth.
The Story Behind TESS
While TESS and Kepler both use the transit method to
discover exoplanets in our galaxy, they differ in a few key
ways. Kepler was a 0.95-meter telescope with a field of view
about 12° wide, capable of detecting the change in brightness
caused by an Earth-sized planet passing in front of a faint,
distant, Sun-like star. In its primary mission, Kepler surveyed
just one region of the sky for about four years. For another
four and a half years after that, it stared at a series of patches
of sky along the ecliptic plane as part of its " K2 " mission.
TESS, on the other hand, has four CCD cameras each with
10.5-cm apertures that together cover an area of 24° by 96°
on the sky - more than 20 times greater than Kepler's field
of view. TESS's observations step around the sky every 27 days
or so to produce a nearly all-sky survey over the course of two
years. With these characteristics and approach, TESS is capable
of detecting the change in brightness caused by Earth-size
planets passing in front of bright, nearby stars similar in size
to the Sun or smaller.
While TESS has relatively small camera apertures compared
to Kepler's large mirror, it is very efficient. It observes
millions of stars in order to find the kinds of systems we're
looking for. TESS focuses on relatively small planets and
small, bright, nearby stars for a few reasons. First, such
CANDIDATE VS. CONFIRMED
The process of discovering and confirming bona fide
exoplanets is quite involved, which is why there are so
many more candidate exoplanets compared with confirmed
exoplanets. The confirmation of an exoplanet requires
astronomers to determine with extremely high confidence
that the only object that could be causing the tiny periodic
dip in starlight is a planet. The alternatives include a
binary star system, where one star eclipses another star
periodically, or a false alarm, in which the " transit signal "
is actually due to something like starspots or instrumental
artifacts mucking up the data.
Time
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 appears as a dip in starlight. The graph of how the star's
brightness changes with time is called a light curve.
sk yand tele scope .o r g * MAY 2023 35
https://skyandtelescope.org/
Sky & Telescope - May 2023
Table of Contents for the Digital Edition of Sky & Telescope - May 2023
Contents
Sky & Telescope - May 2023 - Cover1
Sky & Telescope - May 2023 - Cover2
Sky & Telescope - May 2023 - 1
Sky & Telescope - May 2023 - Contents
Sky & Telescope - May 2023 - 3
Sky & Telescope - May 2023 - 4
Sky & Telescope - May 2023 - 5
Sky & Telescope - May 2023 - 6
Sky & Telescope - May 2023 - 7
Sky & Telescope - May 2023 - 8
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