With mirror segments beautifully aligned and its science instruments undergoing calibration, NASA’s James Webb Space Telescope is just weeks away from being fully operational. Shortly after the first observations are revealed, Webb will begin conducting in-depth scientific studies.
Among the investigations planned for the first year, are the studies of two exoplanets classified by their size and rocky composition as “super-Earths”: 55 Cancri e covered with lava and LHS 3844 b devoid of atmosphere. The researchers will use spectrographs Webb’s high-precision studies on these planets to understand the geological diversity of planets throughout the galaxy and how rocky planets like Earth evolve.
Imagine if the Earth were much closer to the Sun. So close that a year lasted a few hours. So close, that gravity has permanently blocked one hemisphere from scorching daylight and the other in endless darkness. So close that the oceans evaporate, the rocks begin to melt and the clouds precipitate lava.
While nothing of the sort exists in our solar system, planets like this—rocky, roughly the size of Earth, 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.
55 Cancri e: un planeta “Súper Tierra”, “Súper caliente”
55 Cancri e orbits at a distance from its Sun-like star of less than 2.5 million kilometers (one twenty-five times the distance between Mercury and the Sun), completing a full orbit in less than 18 hours. With surface temperatures well 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 so close to their star are assumed to be tidally locked, with one side “pinned” toward the star continuously. Therefore, the hottest point on the planet should be the one facing the star, and the amount of heat coming from the day side should not change much over time.
But this does not seem to be the case. Observations of 55 Cancri e from NASA’s Spitzer Space Telescope suggest that the hottest region is shifted away from the star-facing side and the total amount of heat detected on the dayside varies.
¿55 Cancri e tiene una atmósfera densa?
One possible explanation would be that the planet has a dynamic atmosphere that moves heat. “55 Cancri e could have a dense atmosphere dominated by oxygen or nitrogen,” explained Renyu Hu of NASA’s Jet Propulsion Laboratory in southern California, who is leading a team that will use the near infrared camera (NIRCam) of the Webb and the mid-infrared instrument (MIRI) to capture the thermal emission spectrum on 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’s made of,” Hu added.
¿O “llueve” lava en 55 Cancri e?
Another possibility would be that 55 Cancri e is not tidally locked. It could be like Mercury, spinning around three times for every two orbits (known as a 3:2 resonance). In that case, the planet would have a day-night cycle.
“That could explain why the hottest part of the planet is moving,” said Alexis Brandeker, a researcher at Stockholm University who leads another team studying the planet. “Just like on Earth, the surface would take time to warm up. The hottest time of day would be in the afternoon, not right at noon.”
Brandeker’s team plans to test this hypothesis by using NIRCam to measure the heat given off by the sunlit side of 55 Cancri e 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 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. At night, the steam would cool and condense to form lava droplets that would precipitate to the surface and re-solidify as night fell.
Credit: ILLUSTRATION: NASA, ESA, CSA, Dani Player (STScI).
El planeta Súper-Tierra LHS 3844 b: algo más fresco
While 55 Cancri e will provide insight into the characteristic geology of a lava-covered planet, LHS 3844 b will provide an opportunity to analyze the solid rock on the surface of an exoplanet.
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 melt. Furthermore, Spitzer’s observations indicate that the planet is highly unlikely to have a substantial atmosphere.
¿De qué está hecha la superficie de LHS 3844 b?
While we won’t be able to image the surface of LHS 3844 b directly with Webb, the lack of an atmosphere to darken the surface makes it possible to study it with spectroscopy.
“It turns out that different types of rock 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 that rocks emit.”
Kreidberg’s team will use MIRI to capture the dayside thermal emission spectrum of LHS 3844b and then compare it to spectra of known rocks, such as basalt and granite, to determine their 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 these two, as of today, there are more than 5,000 confirmed exoplanets in the galaxy. “They will give us fantastic new insights into Earth-like planets in general, helping us learn what early Earth might have been like when it was warmer, as these planets are today,” Kreidberg said.
These observations of 55 Cancri e and LHS 3844 b will be made as part of Webb’s Cycle 1 General Observers program. The programs General Observers they were 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. The Webb will solve mysteries of our solar system, observe distant planets around other stars, and investigate the mysterious structures and origins of our universe and our place in it. The Webb is an international program led by NASA with its partners, ESA (European Space Agency) and the Canadian Space Agency.
Editing: R. Castro.