Geology from 50 Light-Years: Webb Gets Ready to Study Rocky Worlds


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.

Imagine if Earth were much, much closer to the Sun. So close that an entire year lasts only a few hours. So close that gravity has locked one hemisphere in permanent searing daylight and the other in endless darkness. So close that the oceans boil away, rocks begin to melt, and the clouds rain lava. 

While nothing of the sort exists in our own solar system, planets like this—rocky, roughly Earth-sized, 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.

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. 

Illustration comparing rocky exoplanets LHS 3844 b and 55 Cancri e to Earth and Neptune. The planets are arranged from left to right in order of increasing radius.
Illustration comparing rocky exoplanets LHS 3844 b and 55 Cancri e to Earth and Neptune. Both 55 Cancri e and LHS 3844 b are between Earth and Neptune in terms of size and mass, but they are more similar to Earth in terms of composition. The planets are arranged from left to right in order of increasing radius.
Credits: ILLUSTRATION: NASA, ESA, CSA, Dani Player (STScI)

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. 

The James Webb Space Telescope is the world’s premier space science observatory. Webb will solve mysteries in our solar system, look beyond to distant worlds around other stars, and probe the mysterious structures and origins of our universe and our place in it. Webb is an international program led by NASA with its partners, ESA (European Space Agency) and the Canadian Space Agency.

Banner Image: Illustration showing what exoplanet 55 Cancri e could look like, based on current understanding of the planet. 55 Cancri e is a rocky planet with a diameter almost twice that of Earth orbiting just 0.015 astronomical units from its Sun-like star. Because of its tight orbit, the planet is extremely hot, with dayside temperatures reaching 4,400 degrees Fahrenheit (about 2,400 degrees Celsius). Artwork Credit: NASA, ESA, CSA, Dani Player (STScI)

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Ron Long
May 27, 2022 2:53 am

Difficult to imagine a planet that hot and that close to a star having an atmosphere. Coronal Mass Ejections that close to a star would blow any atmosphere away, probably molten surface rocks with it. Our planet Earth possesses a strong magnetic field which diverts incoming solar winds toward the poles, but can’t imagine a magnetic field strong enough to protect this 55 Cancri e planet.

Reply to  Ron Long
May 27, 2022 10:56 am

55 Cancri e, also known as Janssen, orbits a star called Copernicus only 41 light years away. The molten surface is completely uninhabitable, but above the burning horizon, Janssen’s sister planet, Galileo, hangs in a dark sky. Silicates in the atmosphere would condense into clouds on the tidally-locked planet’s darkside reflecting the lava below. So, the skies would sparkle.

55 Cancri e is a super-Earth exoplanet that orbits a G-type star similar to our Sun. Its mass is 8.08 Earths, it takes 0.7 days to complete one orbit of its star, and is 0.01544 AU from its star. Its discovery was announced in 2004.

It’s even more difficult to imagine an atmosphere enriched in silicates. I think the JWST will generate more questions than answers regarding exoplanet candidates.

May 27, 2022 3:55 am

Does the planet not also experience gravitation fluctuation in its orbit? Something like Io and Jupiter?

May 27, 2022 5:59 am

“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.”

could have“, Pure speculation causing the researchers to form their own confirmation biased dream research world.

Reply to  ATheoK
May 27, 2022 6:52 am

ATheoK ==> At least one more pragmatist commenting here….thank you. How much time and money and brain power is being wasted speculating about these ‘exo-planets’? Canceri e is 41 light-years distant.

John Campbell and his merry band of Sci-Fi writers of the 40s and 50s have already covered all this speculative ground — why spend more money on it — just go the the library.

Reply to  Kip Hansen
May 27, 2022 7:35 am

Agree. ‘Exo-planet’ science is very speculative and it makes one famous quote comes to mind:
“There is something fascinating about science. One gets such wholesale returns of conjecture out of such a trifling investment of fact.”

Reply to  Edim
May 27, 2022 7:39 am

Edim ==> One of those famous quotes of questionable authorship.. Often attributed to Mark Twain.

Reply to  Kip Hansen
May 27, 2022 8:07 am
Reply to  Edim
May 27, 2022 9:06 am

Edim ==> I would have to check –the internet is good at that sort of thing….

Smart Rock
Reply to  Kip Hansen
May 27, 2022 5:10 pm

It is from Life on the Missisipi. A very interesting, amusing and informative book, and a depiction of a slice of uniquely American history. Everyone should read it, at the very least to find out what Mark Twain’s name means.

Reply to  Kip Hansen
May 27, 2022 2:38 pm

I am a great fan of SF and have read much of it for entertainment. But it remains at best as inspiring entertainment, or morality sagas using SF as a platform.

We can continue to speculate or we can gather evidence.
As a scientist, I think it’s so much better to actually have data.
Let’s take a look and see what we find, or continue with fanciful ignorance.

Reply to  ATheoK
May 27, 2022 8:39 am

It seems they already have compiled the full story.
What remains is the confirmation of that biases via the “new toy”.

I think they believe that the “new toy” would not be able to show and brink enough data to threaten their story.


May 27, 2022 6:47 am

How far away is that ‘exo-planet’? What amount of unknown/unknowable stuff exists between the origin point and the observation point of the various electromagnetic signals our telescopes receive?

How many speculations are justified by the pragmatic reality of the above questions?

Personally, I do not believe that this is science….

Flash Chemtrail
Reply to  Kip Hansen
May 27, 2022 8:02 am

I can see the National Geographic headline now.

This Could Be Earth’s Future In 2100!

Curious George
Reply to  Kip Hansen
May 27, 2022 8:50 am

It is not about unknown/unknowable. It is about unobservable. Almost unobservable. The signal of the planet is very tiny compared to that of its sun. Untangling them is a highly speculative astrology.

Izaak Walton
Reply to  Kip Hansen
May 27, 2022 6:27 pm

just how far do you think telescopes can look before the observations stop being science? 50 light years is relatively close. Do you believe that other galaxies exist for example? The nearest is over 2 million light years away. Are we allowed to say anything about them?

Reply to  Izaak Walton
May 28, 2022 7:46 am

Izaak ==> The light we see with an optical telescope, and other radiation we detect with other types of telescopes, has traveled distances that we can name but not really comprehend.

50 light years is two hundred ninety-three trillion nine hundred thirty-one billion two hundred seventy million miles.

At the fastest speed of a Saturn V rocket, it would take us 100,000 years to travel there.

The light from 50 light years distance took 50 years to get from there to here, and was interrupted by traveling through an unknown quantity of interstellar matter, space dust clouds, energy fields, etc.

To be real science, what we say must be what we see (detect, etc). We can actually see the other planets around our Sun with our own unaided eyes — but can not distinguish them from stars unaided. With a good amateur telescope, we can see that they are visually different than other lights in the sky — we can see with a very good home telescope visual features.

What is detected about “exo-planets” is periodic differences in the amount of light arriving from far distant stars. The rest is computation and speculation.

Izaak Walton
Reply to  Kip Hansen
May 28, 2022 5:02 pm

That is just pure nonsense. Do you think that neutrinos exist? We cannot see them and no detector can for example actually detect neutrinos and certainly can’t distinguish between the various flavours of them. However the fact that posulating their existence allows us to make sense of a wide range of different experiments allows us to be confident that neutrinos exist. And the same is true with particles electrons despite the fact that nobody has seen one.

And in fact we do know how much interstellar matter is out there. Space dust for example has strong and well known absorption bands and can be detected easily through spectroscopy. And what exactly is an “energy field”? That is just pure nonsense with no physical meaning. So unless you want to claim that the normal laws of physics stop at the boundaries of what can be seen with a “good home telescope” then you have all the evidence you need for the existence of exo-planets.

Reply to  Izaak Walton
May 29, 2022 12:32 pm

Izaak ==> Perhaps you are intentionally misunderstanding my point in your enthusiasm.

My point is only that we only have a tiny itty bitty bit of observational evidence about the existence and properties of any given claimed exo-planet.

From that very thin evidence, NASA makes claims of physical properties etc etc of as yet unproven particular exo-planets.

Personally, my opinion is not only that planets exist circling other stars but that many of those stars are inhabited not only with life but with sentient life much like ours. What evidence do I have to support this opinion? — almost none.

But NASA does not have adequate evidence for their claims of exo-planets — not much more than a strong desire and a well-read-in-SciFi workforce.

If there is more evidence, they have not made it available to the general public.

If you know of more evidence than periodic dimming of distant starts — feel free to link to it.

Jeff Alberts
Reply to  Izaak Walton
June 2, 2022 9:35 am

However the fact that posulating their existence allows us to make sense of a wide range of different experiments allows us to be confident that neutrinos exist.”

So, something we don’t really know exists, helps speculation about other things?

David Anderson
May 27, 2022 9:37 am

From NASA:

I estimate 5% of all science based articles aimed at a popular audience contains no actual discoveries. It’s all speculation and predictions about future possible discoveries.

Tom Abbott
May 27, 2022 9:49 am

Love it! I look forward to the new knowledge Webb will give us.

May 27, 2022 12:06 pm

If NASA’s James Webb Space Telescope wants to verify it can study these exoplanets dozens of light years from Earth, why not start with Pluto and show how well it might work for a small rocky ‘planet’ in our own solar system that is about 4.5 light hours away. Not 47 light years away. Since we know a lot more now about Pluto from New Horizons flyby a few years back, JWST should be able to accurately verify what we already know. And show their work as to how they arrive at any conclusions. That should be a walk in the park. 

I fear a lot conjecture may be going on, based upon a small fraction of a pixel. I am all for JWSP and finding out more about the universe, but Hubble couldn’t even take a clear optical picture of Pluto, and now we have all these conjectures about planets multiple orders of magnitude further away than Pluto in further away solar systems. It matters, because as we see from the conjecturing about CO2 being the control knob for climate, these wild eyed guesses and may not be what the data presents. In this case, it doesn’t matter a whole lot, except gives new types of powers to scientists making pronouncements on how things are. And before you know it, some policy wanker will outlaw the perfected internal combustion engine based upon some scientific hypothesis about CO2 being the control knob for the climate. 

Gord in Calgary
Reply to  Earthling2
May 27, 2022 2:40 pm

Pretend to study remote planets that we can never hope to go to with made up visuals that cannot be resolved into anything other than a fuzzy computer generated blob. It’s a PR stunt by NASA. Click bait. Perfectly good planets of great variety to study in our own solar system if that was the actual goal.

Reply to  Gord in Calgary
May 27, 2022 7:50 pm

I think you are correct about it being a PR stunt. It seems like a political ploy to say they can look into another solar system’s planets before our nearby ones are more than perfunctorily explored.

I feel we should concentrate on developing moon bases and use them as stepping stones to other planets in our own system. The moon could also be handy to observe from. I’m more interested in detecting an incoming asteroid than the exotic atmosphere on a planet light years away.

Thomas Burk
Reply to  Earthling2
May 27, 2022 2:57 pm

Right you are. Exoplanets are one of the new things in observational astronomy and an absolutely massive amount of money is being requested to study them. Witness the WFIRST spacecraft (now called the Roman Space Telescope — in fact, since christened the Nancy Grace Roman Space Telescope). It has a new coronagraph onboard designed to block out the light of the star and allow better viewing of the exoplanet. Its all the name of finding “Earth-like” worlds out there. So far, they’ve found all sorts of weird and utterly nasty exoplanets and nothing even close to Earth. But they will never give up trying, despite the huge cost. They are desperately hoping Earth is not unique, because if it is, their whole philosophy of the universe will have its edifice shaken to the core. But the more we find out, the more unique our planet seems to be. That will not stand in the science community. They hate that. I’m in the business, so I’m fine with spending the money and learning more. And you never know what they will find. But it is awfully expensive.

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