Satellite aims to discover thousands of nearby exoplanets, including at least 50 Earth-sized ones

Satellite aims to discover thousands of nearby exoplanets, including at least 50 Earth-sized ones
The Transiting Exoplanet Survey Satellite (TESS) will
discover thousands of exoplanets in orbit around the
brightest stars in the sky. In a two-year survey of the
solar neighborhood, TESS will monitor more than 200,000
stars for temporary drops in brightness caused by planetary
transits. This first-ever space borne all-sky transit
survey will identify planets ranging from Earth-sized to
gas giants, around a wide range of stellar types and
orbital distances. No ground-based survey can achieve this
feat. Credit: NASA’s Goddard Space Flight Center/CI Lab

There are potentially thousands of planets that lie just
outside our solar system—galactic neighbors that could be
rocky worlds or more tenuous collections of gas and dust.
Where are these closest exoplanets located? And which of them
might we be able to probe for clues to their composition and
even habitability? The Transiting Exoplanet Survey Satellite
(TESS) will be the first to seek out these nearby worlds.

The NASA-funded spacecraft, not much larger than a
refrigerator, carries four cameras that were conceived,
designed, and built at MIT, with one wide-eyed vision: to
survey the nearest, brightest in the sky for signs of passing .

Now, more than a decade since MIT scientists first proposed the
mission, TESS is about to get off the ground. The spacecraft is
scheduled to launch on a SpaceX Falcon 9 rocket from Cape
Canaveral Air Force Station in Florida, no earlier than April
16, at 6:32 p.m. EDT.

TESS will spend two years scanning nearly the entire sky—a
field of view that can encompass more than 20 million stars.
Scientists expect that thousands of these stars will host
transiting planets, which they hope to detect through images
taken with TESS’s cameras.

A set of flight camera electronics on one of the TESS
cameras, developed by the MIT Kavli Institute for
Astrophysics and Space Research (MKI), will transmit
exoplanet data from the camera to a computer aboard the
spacecraft that will process it before transmitting it back
to scientists on Earth. Credit: MIT Kavli Institute

Amid this extrasolar bounty, the TESS science team at MIT aims
to measure the masses of at least 50 small planets whose radii
are less than four times that of Earth. Many of TESS’s planets
should be close enough to our own that, once they are
identified by TESS, scientists can zoom in on them using other
telescopes, to detect atmospheres, characterize atmospheric
conditions, and even look for signs of habitability.

“TESS is kind of like a scout,” says Natalia Guerrero, deputy
manager of TESS Objects of Interest, an MIT-led effort that
will catalog objects captured in TESS data that may be
potential exoplanets.

“We’re on this scenic tour of the whole sky, and in some ways
we have no idea what we will see,” Guerrero says. “It’s like
we’re making a treasure map: Here are all these cool things.
Now, go after them.”

A seed, planted in space

TESS’s origins arose from an even smaller satellite that was
designed and built by MIT and launched into space by NASA on
Oct. 9, 2000. The High Energy Transient Explorer 2, or HETE-2,
orbited Earth for seven years, on a mission to detect and
localize gamma-ray bursts—high-energy explosions that emit
massive, fleeting bursts of gamma and X-rays.

NASA’s Transiting Exoplanet Survey Satellite (TESS), shown
here in a conceptual illustration, will identify exoplanets
orbiting the brightest stars just outside our solar system.
TESS will search for exoplanets orbiting stars within
hundreds of light-years of our solar system. Looking at these
close, bright stars will allow large ground-based telescopes
and the James Webb Space Telescope to do follow-up
observations on the exoplanets TESS finds to characterize
their atmospheres. Credit: NASA’s Goddard Space Flight Center

To detect such extreme, short-lived phenomena, scientists at
MIT, led by principal investigator George Ricker, integrated
into the satellite a suite of optical and X-ray cameras
outfitted with CCDs, or charge-coupled devices, designed to
record intensities and positions of light in an electronic
format.

“With the advent of CCDs in the 1970s, you had this fantastic
device … which made a lot of things easier for astronomers,”
says HETE-2 team member Joel Villasenor, who is now also
instrument scientist for TESS. “You just sum up all the pixels
on a CCD, which gives you the intensity, or magnitude, of
light. So CCDs really broke things open for astronomy.”

In 2004, Ricker and the HETE-2 team wondered whether the
satellite’s optical cameras could pick out other objects in the
sky that had begun to attract the astronomy community:
exoplanets. Around this time, fewer than 200 planets outside
our solar system had been discovered. A few of these were found
with a technique known as the transit method, which involves
looking for periodic dips in the light from certain stars,
which may signal a planet passing in front of the star.

“We were thinking, was the photometry of HETE-2’s cameras
sufficient so that we could point to a part of the sky and
detect one of these dips? Needless to say, it didn’t exactly
work,” Villasenor recalls. “But that was sort of the seed that
started us thinking, maybe we should try to fly CCDs with a
camera to try and detect these things.”

A path, cleared

In 2006, Ricker and his team at MIT proposed a small, low cost
satellite (HETE-S) to NASA as a Discovery class mission, and
later on as a privately funded mission for $20 million. But as
the cost of, and interest in, an all-sky exoplanet survey grew,
they decided instead to seek NASA funding, at a higher level of
$120 million. In 2008, they submitted a proposal for a NASA
Small Explorer (SMEX) Class Mission with the new name—TESS.

At this time, the satellite design included six CCD cameras,
and the team proposed that the spacecraft fly in a low-Earth
orbit, similar to that of HETE-2. Such an orbit, they reasoned,
should keep observing efficiency relatively high, as they
already had erected data-receiving ground stations for HETE-2
that could also be put to use for TESS.

But they soon realized that a low-Earth orbit would have a
negative impact on TESS’s much more sensitive cameras. The
spacecraft’s reaction to the Earth’s magnetic field, for
example, could lead to significant “spacecraft jitter,”
producing noise that hides an exoplanet’s telltale dip in
starlight.

NASA bypassed this first proposal, and the team went back to
the drawing board, this time emerging with a new plan that
hinged on a completely novel orbit. With the help of engineers
from Orbital ATK, the Aerospace Corporation, and NASA’s Goddard
Space Flight Center, the team identified a never-before-used
“lunar-resonant” orbit that would keep the spacecraft extremely
stable, while giving it a full-sky view.

NASA’s Transiting Exoplanet Survey Satellite (TESS), shown
here in a conceptual illustration, will identify exoplanets
orbiting the brightest stars just outside our solar system.
Credit: NASA’s Goddard Space Flight Center

Once TESS reaches this orbit, it will slingshot between the
Earth and the moon on a highly elliptical path that could keep
TESS orbiting for decades, shepherded by the moon’s
gravitational pull.

“The moon and the satellite are in a sort of dance,” Villasenor
says. “The moon pulls the satellite on one side, and by the
time TESS completes one orbit, the moon is on the other side
tugging in the opposite direction. The overall effect is the
moon’s pull is evened out, and it’s a very stable configuration
over many years. Nobody’s done this before, and I suspect other
programs will try to use this orbit later on.”

In its current planned trajectory, TESS will swing out toward
the moon for less than two weeks, gathering data, then swing
back toward the Earth where, on its closest approach, it will
transmit the data back to ground stations from 67,000 miles
above the surface before swinging back out. Ultimately, this
orbit will save TESS a huge amount of fuel, as it won’t need to
burn its thrusters on a regular basis to keep on its path.

With this revamped orbit, the TESS team submitted a second
proposal in 2010, this time as an Explorer class mission, which
NASA approved in 2013. It was around this time that the Kepler
Space Telescope ended its original survey for exoplanets. The
observatory, which was launched in 2009, stared at one specific
patch of the sky for four years, to monitor the light from
distant stars for signs of transiting planets.

By 2013, two of Kepler’s four reaction wheels had worn out,
preventing the spacecraft from continuing its original survey.
At this point, the telescope’s measurements had enabled the
discovery of nearly 1,000 confirmed exoplanets. Kepler,
designed to study far-off stars, paved the way for TESS, a
mission with a much wider view, to scan the nearest stars to
Earth.

“Kepler went up, and was this huge success, and researchers
said, ‘We can do this kind of science, and there are planets
everywhere,” says TESS member Jennifer Burt, an MIT-Kavli
postdoc. “And I think that was really the scientific check box
that we needed for NASA to say, ‘Okay, TESS makes a lot of
sense now.’ It’ll enable not just detecting planets, but
finding planets that we can thoroughly characterize after the
fact.”

Stripes in the sky

With the selection by NASA, the TESS team set up facilities on
campus and in MIT’s Lincoln Laboratory to build and test the
spacecraft’s cameras. The engineers designed “deep depletion”
CCDs specifically for TESS, meaning that the cameras can detect
light over a wide range of wavelengths up to the near infrared.
This is important, as many of the nearby stars TESS will
monitor are red-dwarfs—small, cool stars that emit less
brightly than the sun and in the infrared part of the
electromagnetic spectrum.

If scientists can detect periodic dips in the light from such
stars, this may signal the presence of planets with
significantly tighter orbits than that of Earth. Nevertheless,
there is a chance that some of these planets may be within the
“habitable zone,” as they would circle much cooler stars,
compared with the sun. Since these stars are relatively close
by, scientists can do follow-up observations with ground-based
telescopes to help identify whether conditions might indeed be
suitable for life.


Credit: Massachusetts Institute of Technology

TESS’s cameras are mounted on the top of the satellite and
surrounded by a protective cone to shield them from other forms
of electromagnetic radiation. Each has a 24 by 24 degree view of the sky,
large enough to encompass the Orion constellation. The
satellite will start its observations in the Southern
Hemisphere and will divide the sky into 13 stripes, monitoring
each segment for 27 days before pivoting to the next. TESS
should be able to observe nearly the entire sky in the Southern
Hemisphere in its first year, before moving on to the Northern
Hemisphere in its second year.

While TESS points at one stripe of the sky, its cameras will
take pictures of the stars in that portion. Ricker and his
colleagues have made a list of 200,000 nearby, bright stars
that they would particularly want to observe. The satellite’s
cameras will create “postage stamp” images that include pixels
around each of these stars. These images will be taken every
two minutes, in order to maximize the chance of catching the
moment that a planet crosses in front of its star. The cameras
will also take full-frame images of all the stars in a
particular stripe of the sky, every 30 minutes.

“With the two-minute pictures, you can get a movie-like image
of what the starlight is doing as the planet is crossing in
front of its host star,” Guerrero says. “For the 30-minute
images, people are excited about maybe seeing supernovae,
asteroids, or counterparts to gravitational waves. We have no
idea what we’re going to see at that timescale.”

Are we alone?

After TESS launches, the team expects that the satellite will
reestablish contact within the first week, during which it will
turn on all its instruments and cameras. Then, there will be a
60-day commissioning phase, as engineers and scientists at
Orbital ATK, NASA, and MIT calibrate the instruments and
monitor the satellite’s trajectory and performance. After that,
TESS will begin to collect and downlink images of the sky.
Scientists at MIT and NASA will take the raw data and convert
it into light curves that indicate the changing brightness of a
star over time.

From there, the TESS Science Team, including Sara Seager, the
Class of 1941 Professor of Earth, Atmospheric and Planetary
Sciences, and deputy director of science for TESS, will look
through thousands of light curves, for at least two similar
dips in starlight, indicating that a planet may have passed
twice in front of its star. Seager and her colleagues will then
employ a battery of methods to determine the mass of a
potential planet.

“Mass is a defining planetary characteristic,” Seager says. “If
you just know that a planet is twice the size of Earth, it
could be a lot of things: a rocky world with a thin atmosphere,
or what we call a “mini-Neptune”—a rocky world with a giant gas
envelope, where it would be a huge greenhouse blanket, and
there would be no life on the surface. So mass and size
together give us an average planet density, which tells us a
huge amount about what the planet is.”

During TESS’s two-year mission, Seager and her colleagues aim
to measure the masses of 50 planets with radii less than four
times that of Earth—dimensions that could signal further
observations for signs of habitability. Meanwhile, the whole
scientific community and public will get a chance to search
through TESS data for their own exoplanets. Once the data are
calibrated, the team will make them publicly available. Anyone
will be able to download the data and draw their own
interpretations, including high school students, armchair
astronomers, and other research institutions.

With so many eyes on TESS’S data, Seager says there’s a chance
that, some day, a nearby planet discovered by TESS might be
found to have signs of life.

“There’s no science that will tell us life is out there right
now, except that small rocky planets appear to be incredibly
common,” Seager says. “They appear to be everywhere we look. So
it’s got to be there somewhere.”

Explore further:
TESS
readies for takeoff

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