Geology 50 light years away: Webb prepares to study rocky worlds

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NASA, ESA, CSA, Dani Player (STScI)

Illustration showing what exoplanet 55 Cancri might look like

With its mirror segments fully aligned and science instruments being calibrated, NASA’s James Webb Space Telescope is only weeks away from being fully operational.

Shortly after the first sightings come to light this summer, Webb’s in-depth science will begin.

Among the investigations planned for the first year are the studies of two hot exoplanets classified as “super-Earths” by their size and their rocky composition: 55 years and covered in lava, and LHS 3844b.

Researchers will train Webb’s high-precision spectrographs on these planets to understand the geological diversity of planets across the Galaxy and the evolution of rocky planets like Earth.

The super-hot super-Earth 55 Cancri and

55 Cancri e orbits within 2.4 million kilometers of its Sun-like star (1/25 of the distance between Mercury and the Sun), completing an orbit in less than 18 hours. With surface temperatures well above the melting point of typical rock-forming minerals, the dayside of the planet is believed to be covered in oceans of lava.

Planets orbiting this close to their star are assumed to be tidally locked, with one side always facing the star. Therefore, the hottest point on the planet must be the one face the star more directlyand the amount of heat coming from the day side should not change much over time.

But that doesn’t seem to be the case. Observations from 55 Cancri and NASA’s Spitzer Space Telescope suggest that the hottest region is not the part most directly facing the star, while the total amount of heat detected on the daytime side varies.

Does 55 Cancri have a thick atmosphere?

One explanation for these observations is that the planet has an atmosphere dynamic that displaces heat. “55 Cancri e could have a thick atmosphere dominated by oxygen or nitrogen,” said Renyu Hu of NASA’s JPL in southern California, who leads a team that will use NIRCam (Near-Infrared Camera) of the Webb and the MIRI (Mid-Infrared Camera). Infrared instrument) to capture the spectrum of thermal emissions from the day side of the planet.

“If you have an atmosphere, [o Webb] has the sensitivity and wavelength range to detect it and determine what it’s made of,” Hu added.

Or is it raining lava at night at 55 Cancri e?

Another intriguing possibility, however, is that 55 Cancri no tidal lock. Instead, it can be like Mercury, rotating three times for every two orbits (known as a 3:2 resonance). As a result, the planet would have a day-night cycle.

“That could explain why the hottest part is moved,” said Alexis Brandeker, a researcher at Stockholm University who leads another team studying the planet. “Just like on Earth, it would take time for the surface to warm up. The hottest time of the day it would be in the afternoon, not at noon.”

Brandeker’s team plans to test this hypothesis by using NIRCam to measure the heat emitted from the bright side of 55 Cancri and during four different orbits. If the planet has a 3:2 resonance, they will look at 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. At night, the steam cooled and condensed to form lava droplets that fell to the surface, making it solid again by nightfall.

The coldest super-Earth LHS 3844 b

While 55 Cancri e will provide insight into the exotic geology of a lava-covered world, LHS 3844 b offers a unique opportunity to analyze the solid rock on an exoplanetary surface.

Like 55 Cancri e, LHS 3844 b orbits very close to its star, completing one revolution in 11 hours. However, because its star is relatively small and cold, the planet is not hot enough for the surface to melt.

Additionally, Spitzer’s observations indicate that the planet is highly unlikely to have a substantial atmosphere.

What is the surface of the LHS 3844 b made of?

Although we cannot photograph the surface of LHS 3844 b directly with Webb, the lack of dark atmosphere makes it possible to study the surface by spectroscopy.

“It turns out that different rock types have different spectra,” explained Laura Kreidberg of 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 emitted by rocks.

Kreidberg’s team will use MIRI to capture the diurnal thermal emission spectrum of LHS 3844b, then compare it to spectra of known rocks such as basalt and granite to determine their composition. If the planet is volcanically activethe spectrum could also reveal the presence of traces of volcanic gases.

The significance of these sightings goes far beyond just two of the more than 5,000 confirmed exoplanets. “They will give us fantastic new perspectives on Earth-like planets in general, helping us understand what early Earth looked like when it was as hot as those planets are today,” Kreidberg said.

These observations of 55 Cancri ee LHS 3844 b will be made as part of the Webb Cycle 1 General Observer program. General Observer programs were competitively selected using an anonymous review system, the same system used to allocate time at Hubble.

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