The James Webb Space Telescope explores hot exoplanets

After much apnea and anticipation, the James Webb Space Telescope (JWST) is only weeks away from being fully operational. The mirror segments are successfully aligned and the final step is to calibrate his scientific instruments. When ready, the JWST will spend part of its first year observing two exoplanets.

Hot exoplanets have been classified as “super-Earths” based on their sizes and rock compositions. Why do we say hot? The average temperature on Earth is 60 degrees Fahrenheit. Meanwhile, these two orbs, known as LHS 3844 b (“Hot Rocky”) and 55 Cancri e (“Super-Hot Rocky”) have average temperatures of 1,000 degrees and 3,100 degrees, respectively. Scientists plan to use JWST’s high-precision spectrographs (instruments that separate light by wavelength or frequency) to study planets, with the aim of better understanding the geological diversity and evolution of rocky planets, Earth included.

Related: 17 photos of the new James Webb Space Telescope

What is 55 Cancri e?

55 Cancri e, the hotter exoplanet of the two, orbits its sun-like star at a distance of less than 1.5 million miles, or about 1/25th the extent between Mercury and the Sun. Thus, one day in 55 Cancri e is 18 hours, and the average near and orbital surface temperatures are well above the melting point of the rock-forming minerals.

Supposedly, the exoplanet’s sun-facing surface is covered in seas of lava, however, further study by JWST will reveal exactly how it orbits its star and how that affects surface temperature. Interestingly, according to the Spitzer Space Telescope, the hottest area on the planet may not directly face the sun at 55 Cancri e, but instead is located out of the center.

“Planets that orbit this close to their star are assumed to be tidally locked, with one side facing the star at all times,” NASA explains. “As a result, the hottest spot on the planet should be the one facing the star most directly, and the amount of heat coming from the dayside should not change much over time.”

A comparison of Earth, 55 Canri e and LHS 3844 b. The latter two share compositional qualities with Earth, but are slightly larger. NASA, ESA, CSA, Dani Player (STScI)

JWST will study the atmosphere and composition of the exoplanet and consider the possibility that it is not, in fact, tidally locked, which is why the hotspot is shifted. Another proposal is that, like Mercury, 55 Cancri has a 3:2 resonance, meaning it will rotate three times for every two orbits, creating a day/night cycle that affects surface temperatures.

“That could explain why the hottest part of the planet is moving,” says 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 warm up. The hottest time of the day would be in the afternoon, not just at noon.

What is LHS 3844b?

Unlike 55 Cancri e, LHS 3844 b is significantly cooler and offers researchers the opportunity to study solid rock on an exoplanet. Although it is much closer to its “sun” (an orbit lasts only 11 hours), the star is quite small and cold, and therefore LHS 3844 b does not have a molten surface. Scientists plan to use spectroscopy because the exoplanet does not have an obscuring atmosphere.

“It turns out that different rock types have different spectra,” explains 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 emitted by rocks.

With a MIRI (Mid-Infrared Instrument), researchers will collect data from the dayside of the planet using the thermal emission spectrum. They will then compare this data to spectra of known rocks, such as basalt and granite, to determine their composition.

Why is it important?

These observations will be part of the Cycle 1 General Observer Program, which allocates observation hours on the JWST to various programs and surveys.

“They will give us fantastic new insights into Earth-like planets in general, helping us understand what early Earth was like when it was warm like these planets are today,” says Kreidberg.

Stewart C. Hartline