In April 2018, NASA launched the Transiting Exoplanet Survey Satellite
(TESS). Its main goal is to locate Earth-sized planets and larger
“super-Earths” orbiting nearby stars for further study. One of the most
powerful tools that will examine the atmospheres of some planets that
TESS discovers will be NASA’s James Webb Space Telescope. Since
observing small exoplanets with thin atmospheres like Earth will be
challenging for Webb, astronomers will target easier, gas giant
exoplanets first.
Some of Webb’s first observations of gas giant exoplanets will be conducted through the Director’s Discretionary Early Release Science
program. The transiting exoplanet project team at Webb’s science
operations center is planning to conduct three different types of
observations that will provide both new scientific knowledge and a
better understanding of the performance of Webb’s science instruments.
“We have two main goals. The first is to get transiting exoplanet
datasets from Webb to the astronomical community as soon as possible.
The second is to do some great science so that astronomers and the
public can see how powerful this observatory is,” said Jacob Bean of the
University of Chicago, a co-principal investigator on the transiting
exoplanet project.
“Our team’s goal is to provide critical knowledge and insights to the
astronomical community that will help to catalyze exoplanet research
and make the best use of Webb in the limited time we have available,”
added Natalie Batalha of NASA Ames Research Center, the project’s
principal investigator.
Transit – An atmospheric spectrum
When a planet crosses in front of, or transits, its host star, the
star’s light is filtered through the planet’s atmosphere. Molecules
within the atmosphere absorb certain wavelengths, or colors, of light.
By splitting the star’s light into a rainbow spectrum, astronomers can
detect those sections of missing light and determine what molecules are
in the planet’s atmosphere.
For these observations, the project team selected WASP-79b, a
Jupiter-sized planet located about 780 light-years from Earth. The team
expects to detect and measure the abundances of water, carbon monoxide,
and carbon dioxide in WASP-79b. Webb also might detect new molecules not
yet seen in exoplanet atmospheres.
Phase curve – A weather map
Planets that orbit very close to their stars tend to become tidally
locked. One side of the planet permanently faces the star while the
other side faces away, just as one side of the Moon always faces the
Earth. When the planet is in front of the star, we see its cooler
backside. But as it orbits the star, more and more of the hot day-side
comes into view. By observing an entire orbit, astronomers can observe
those variations (called a phase curve) and use the data to map the
planet’s temperature, clouds, and chemistry as a function of longitude.
The team will observe a phase curve of the “hot Jupiter” known as
WASP-43b, which orbits its star in less than 20 hours. By looking at
different wavelengths of light, they can sample the atmosphere to
different depths and obtain a more complete picture of its structure.
“We have already seen dramatic and unexpected variations for this planet
with Hubble
and Spitzer. With Webb we will reveal these variations in significantly
greater detail to understand the physical processes that are
responsible,” said Bean.
Eclipse – A planet’s glow
The greatest challenge when observing an exoplanet is that the star’s
light is much brighter, swamping the faint light of the planet. To get
around this problem, one method is to observe a transiting planet when
it disappears behind the star, not when it crosses in front of the star.
By comparing the two measurements, one taken when both star and planet
are visible, and the other when only the star is in view, astronomers
can calculate how much light is coming from the planet alone.
This technique works best for very hot planets that glow brightly in
infrared light. The team plans to study WASP-18b, a planet that is baked
to a temperature of almost 4,800 degrees Fahrenheit (2,900 K). Among
other questions, they hope to determine whether the planet’s stratosphere exists due to the presence of titanium oxide, vanadium oxide, or some other molecule.
Habitable planets
Ultimately, astronomers want to use Webb to study potentially
habitable planets. In particular, Webb will target planets orbiting red
dwarf stars since those stars are smaller and dimmer, making it easier
to tease out the signal from an orbiting planet. Red dwarfs are also the
most common stars in our galaxy.
“TESS should locate more than a dozen planets orbiting in the
habitable zones of red dwarfs, a few of which might actually be
habitable. We want to learn whether those planets have atmospheres and
Webb will be the one to tell us,” said Kevin Stevenson of the Space
Telescope Science Institute, a co-principal investigator on the project.
“The results will go a long way towards answering the question of
whether conditions favorable to life are common in our galaxy.”
The James Webb Space Telescope is the world’s premier infrared
space observatory of the next decade. Webb will solve mysteries of our
solar system, look beyond to distant worlds around other stars, and
probe the mysterious structures and origins of our universe and our
place in it. Webb is an international program led by NASA with its partners, the European Space Agency (ESA) and the Canadian Space Agency (CSA).
Contact:
Christine Pulliam
Space Telescope Science Institute, Baltimore, Maryland
410-338-4366
cpulliam@stsci.edu
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