Astronomers will train the James Webb Telescope’s high-precision spectrographs on two intriguing rocky exoplanets

With its beautifully aligned mirror segments 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” for their size and their rocky composition: the 55 Cancri e covered with lava and the LHS 3844 b without air. 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.

Super-hot super-earth 55 Cancri e

55 Cancri e orbits less than 2.5 million kilometers from its sun-like star (one twenty-fifth of the distance between Mercury and the sun), completing one revolution in less than 18 hours. With surface temperatures well above the melting point of typical rock-forming minerals, the dayside 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 facing the star most directly, and 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 of 55 Cancri e from NASA’s Spitzer Space Telescope suggest that the hottest region is offset from the part that faces most directly 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 a dynamic atmosphere that moves heat. “55 Cancri e could have a thick atmosphere dominated by oxygen or nitrogen,” said Renyu Hu of NASA’s Jet Propulsion Laboratory in Southern California, who leads a team that will use the near-infrared camera (NIRCam) and Webb’s Mid Infrared Instrument (MIRI). ) to capture the thermal emission spectrum from the dayside of the planet. “If there is an atmosphere, [Webb] has the sensitivity and wavelength range to detect it and determine what it is made of,” Hu added.

Or does it rain lava at night on 55 Cancri e?

Another intriguing possibility, however, is that 55 Cancri e is not tidal locked. 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 of the planet is shifting,” said Alexis Brandeker, a Stockholm University researcher 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 day would be in the afternoon, not just at noon.”

Brandeker’s team plans to test this hypothesis by using NIRCam to measure the heat emitted from the illuminated 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 differences 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 steam would cool and condense to form droplets of lava that would fall to the surface, becoming solid again by nightfall.

Super-Earth LHS 3844 b a little colder

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 solid rock on the surface of an exoplanet.

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 image the surface of LHS 3844 b directly with Webb, the absence of an obscuring atmosphere allows the surface to be studied spectroscopically.

“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 dayside thermal emission spectrum of LHS 3844 b, then compare it to spectra of known rocks, such as basalt and granite, to determine its composition. If the planet is volcanically active, the 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 in the galaxy. “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,” Kreidberg said.

These observations of 55 Cancri e and LHS 3844 b will be conducted as part of Webb’s Cycle 1 General Observer Program. General Observer programs were competitively selected using a double-blind review system, the same system used to allocate time on Hubble.

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Provided by the Space Telescope Science Institute