Geology from 50 Light-Years: Webb Gets Ready to Study Rocky Worlds Imagine if
Earth were much, much closer to the Sun. So close that an entire year lasts only
a few hours. So close that gravity has locked one hemisphere in permanent
searing daylight and the other in endless darkness. So close that the oceans
boil away, rocks begin to melt, and the clouds rain lava. While nothing of the
sort exists in our own solar system, planets like this—rocky, roughly
Earth-sized, extremely hot and close to their stars—are not uncommon in the
Milky Way galaxy. What are the surfaces and atmospheres of these planets really
like? NASA’s James Webb Space Telescope is about to provide some answers. With
its mirror segments beautifully aligned and its scientific instruments
undergoing calibration, NASA’s James Webb Space Telescope is just weeks away
from full operation. Soon after the first observations are revealed this summer,
Webb’s in-depth science will begin. Among the investigations planned for the
first year are studies of two hot exoplanets classified as “super-Earths” for
their size and rocky composition: the lava-covered 55 Cancri e and the airless
LHS 3844 b. Researchers will train Webb’s high-precision spectrographs on these
planets with a view to understanding the geologic diversity of planets across
the galaxy, and the evolution of rocky planets like Earth. Super-Hot Super-Earth
55 Cancri e 55 Cancri e orbits less than 1.5 million miles from its Sun-like
star (one twenty-fifth of the distance between Mercury and the Sun), completing
one circuit in less than 18 hours. With surface temperatures far above the
melting point of typical rock-forming minerals, the day side of the planet is
thought to be covered in oceans of lava. Planets that orbit this close to their
star are assumed to be tidally locked, with one side facing the star at all
times. As a result, the hottest spot on the planet should be the one that faces
the star most directly, and the amount of heat coming from the day side should
not change much over time. But this doesn’t seem to be the case. Observations of
55 Cancri e from NASA’s Spitzer Space Telescope suggest that the hottest region
is offset from the part that faces the star most directly, while the total
amount of heat detected from the day side does vary. Does 55 Cancri e Have a
Thick Atmosphere? One explanation for these observations is that the planet has
a dynamic atmosphere that moves heat around. “55 Cancri e could have a thick
atmosphere dominated by oxygen or nitrogen,” explained Renyu Hu of NASA’s Jet
Propulsion Laboratory in Southern California, who leads a team that will use
Webb’s Near-Infrared Camera (NIRCam) and Mid-Infrared Instrument (MIRI) to
capture the thermal emission spectrum of the day side of the planet. “If it has
an atmosphere, [Webb] has the sensitivity and wavelength range to detect it and
determine what it is made of,” Hu added. Or Is It Raining Lava in the Evening on
55 Cancri e? Another intriguing possibility, however, is that 55 Cancri e is not
tidally locked. Instead, it may be like Mercury, rotating three times for every
two orbits (what’s known as a 3:2 resonance). As a result, the planet would have
a day-night cycle. “That could explain why the hottest part of the planet is
shifted,” explained Alexis Brandeker, a researcher from Stockholm University who
leads another team studying the planet. “Just like on Earth, it would take time
for the surface to heat up. The hottest time of the day would be in the
afternoon, not right at noon.” Brandeker’s team plans to test this hypothesis
using NIRCam to measure the heat emitted from the lit side of 55 Cancri e during
four different orbits. If the planet has a 3:2 resonance, they will observe each
hemisphere twice and should be able to detect any difference between the
hemispheres. In this scenario, the surface would heat up, melt, and even
vaporize during the day, forming a very thin atmosphere that Webb could detect.
In the evening, the vapor would cool and condense to form droplets of lava that
would rain back to the surface, turning solid again as night falls. Illustration
comparing rocky exoplanets LHS 3844 b and 55 Cancri e to Earth and Neptune. The
planets are arranged from left to right in order of increasing radius.
Illustration comparing rocky exoplanets LHS 3844 b and 55 Cancri e to Earth and
Neptune. Both 55 Cancri e and LHS 3844 b are between Earth and Neptune in terms
of size and mass, but they are more similar to Earth in terms of composition.
The planets are arranged from left to right in order of increasing radius.
Credits: ILLUSTRATION: NASA, ESA, CSA, Dani Player (STScI) Somewhat Cooler
Super-Earth LHS 3844 b While 55 Cancri e will provide insight into the exotic
geology of a world covered in lava, LHS 3844 b affords a unique opportunity to
analyze the solid rock on an exoplanet surface. Like 55 Cancri e, LHS 3844 b
orbits extremely close to its star, completing one revolution in 11 hours.
However, because its star is relatively small and cool, the planet is not hot
enough for the surface to be molten. Additionally, Spitzer observations indicate
that the planet is very unlikely to have a substantial atmosphere. What Is the
Surface of LHS 3844 b Made of? While we won’t be able to image the surface of
LHS 3844 b directly with Webb, the lack of an obscuring atmosphere makes it
possible to study the surface with spectroscopy. “It turns out that different
types of rock have different spectra,” explained Laura Kreidberg at the Max
Planck Institute for Astronomy. “You can see with your eyes that granite is
lighter in color than basalt. There are similar differences in the infrared
light that rocks give off.” Kreidberg’s team will use MIRI to capture the
thermal emission spectrum of the day side of LHS 3844 b, and then compare it to
spectra of known rocks, like basalt and granite, to determine its composition.
If the planet is volcanically active, the spectrum could also reveal the
presence of trace amounts of volcanic gases. The importance of these
observations goes far beyond just two of the more than 5,000 confirmed
exoplanets in the galaxy. “They will give us fantastic new perspectives on
Earth-like planets in general, helping us learn what the early Earth might have
been like when it was hot like these planets are today,” said Kreidberg. These
observations of 55 Cancri e and LHS 3844 b will be conducted as part of Webb’s
Cycle 1 General Observers program. General Observers programs were competitively
selected using a dual-anonymous review system, the same system used to allocate
time on Hubble. The James Webb Space Telescope is the world's premier space
science observatory. Webb will solve mysteries in 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, ESA (European Space Agency) and the Canadian
Space Agency. Banner Image: Illustration showing what exoplanet 55 Cancri e
could look like, based on current understanding of the planet. 55 Cancri e is a
rocky planet with a diameter almost twice that of Earth orbiting just 0.015
astronomical units from its Sun-like star. Because of its tight orbit, the
planet is extremely hot, with dayside temperatures reaching 4,400 degrees
Fahrenheit (about 2,400 degrees Celsius). Artwork Credit: NASA, ESA, CSA, Dani
Player (STScI) Margaret W. Carruthers Space Telescope Science Institute,
Baltimore, Maryland 410-338-4366 mcarruthers@stsci.edu Editor: Jamie Adkins
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