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.
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.
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