Eyeball Planets, Lobster Oceans? Studying Exoplanet Climates

Posted on August 10, 2024 by Brian H. Gill
NASA Exoplanet Catalog: 'LHS 1140 b is a super Earth exoplanet that orbits an M-type star. Its mass is 6.38 Earths, it takes 24.7 days to complete one orbit of its star, and is 0.0957 AU from its star. Its discovery was announced in 2017.'
LHS 1140 planetary system.

Headlines about an “eyeball planet” got my attention last month.

Then I got distracted by what I thought were more time-sensitive topics — and remembered what two scientists learned when they simulated ocean currents and winds on a tidally-locked exoplanet.

That last item was from 2013. It’s still the best discussion I’ve seen of what an “eyeball planet” might actually look like. Turns out that a patch of open ocean on a tidally locked exoplanet’s ocean wouldn’t necessarily be circular.

But I’ll admit that “eyeball planet” is a cool description. And may be easier to remember than terms like “lobster-like spatial pattern”.

So this week I’ll be talking about LHS 1140 b, which may not be an “eyeball planet” after all, ocean planet simulations; and — briefly, for me — how I see extraterrestrial life.

LHS 1140 b: Water, With Nitrogen in the Atmosphere — Maybe

B. Gougeon/University of Montreal's illustration: 'Temperate exoplanet LHS 1140 b may be a world completely covered in ice (left) similar to Jupiter’s moon Europa or be an ice world with a liquid substellar ocean and a cloudy atmosphere (centre). LHS 1140 b is 1.7 times the size of our planet Earth (right) and is the most promising habitable zone exoplanet yet in our search for liquid water beyond the Solar System.' B. Gougeon/Université de Montréal via Sky and Telescope (July 24, 2024) used w/o permission and see https://news.umich.edu/astronomers-find-surprising-ice-world-in-the-habitable-zone-with-jwst-data/
B. Gougeon’s illustration, comparing an ice-covered LHS 1140 b (with and without open water) and Earth.

“A Closer Look at a Potential ‘Eyeball Planet’“
Arielle Frommer, Sky & Telescope (July 24, 2024)
“New James Webb Space Telescope observations of LHS 1140b hint at a temperate water world with a nitrogen-rich atmosphere.”

“Imagine a world hospitable to life, with a single temperate ocean surrounded on all sides by ice. This ‘eyeball planet’ might sound straight out of science fiction, but it is entirely possible — and astronomers think they might have found such a world in LHS 1140b.

“Located only 49 light-years away in the constellation Cetus, LHS 1140b is one of the closest discovered planets that lies within its star’s habitable zone — the region where a planet could retain liquid water….”

Roger Sinnott, Rick Fienberg/ Sky and Telescope/IAU's sky chart: constellation Cetus, LHS 1140's position marked by a red circle.We’ve known about LHS 1140 b since 2017.

It’s a transiting exoplanet, passing between us and its sun every 24 and three-quarters days, which gives scientists a chance to study light passing through its atmosphere.

The Sky & Telescope article talks about what scientists found in data from the James Webb Space Telescope’s Near Infrared Imager and Slitless Spectrograph.

Seems that the planet could have a nitrogen-rich atmosphere.

“…However, the team only has hints about the atmosphere’s composition so far — the researchers couldn’t rule out that the planet might have no atmosphere at all, making it a world of barren rock or ice.

“‘What I find to be the most significant about this result is that there’s an indication that LHS 1140b might have an atmosphere at all,’ says Jason Dittmann (University of Florida), a co-discoverer of LHS 1140b who was not involved in this study.

“LHS 1140b is the first rocky exoplanet to have shown hints of an atmosphere, and while the study’s atmospheric results are tentative, LHS 1140b is certainly a promising habitable candidate to keep an eye on….”
(“A Closer Look at a Potential ‘Eyeball Planet’“, Arielle Frommer, Sky & Telescope (July 24, 2024))

I’m not sure about “first rocky exoplanet to have shown hints of an atmosphere” in that last paragraph.

The last I checked, nobody had confirmed that planets in the TRAPPIST-1 system had atmospheres. But 55 Cancri e is a “rocky” planet, in the sense of being mostly silicate, rock, or metal. And it’s got an atmosphere.

On the other hand, there’s a chance that 55 Cancri e is a carbon planet.

I suppose “first rocky exoplanet” depends on what “rocky” means in context.

I’ve talked about some of this stuff before. As usual you’ll find links in the footnotes.1

Next, a quick look at “eyeball planets” and — maybe — lobster-shaped oceans.

Tidally Locked Ocean Planets: Simple, and Not-So-Simple, Models

Yongyun Hu, Jun Yang's Figure S1: 'Sea-ice coverage and surface air temperature in AGCM simulations with a slab ocean. (Left) Sea-ice fraction (unit, %); (Right) surface air temperature (unit, °C); (Upper) 355 ppmv of CO2; and (Lower) 200,000 ppmv of CO2. For A and B, blue indicates open ocean, and white indicates sea ice. Note that the color scale is not linear in C and D.' (2013)
Hu & Yang’s Figure S1: hypothetical eyeball planet, with a slab ocean. (2013)
(left) sea-ice fraction
(right) surface air temp
(upper) low CO2
(lower) high CO2

“Role of ocean heat transport in climates of tidally locked exoplanets around M dwarf stars”
Yongyun Hu, Jun Yang; PNAS (2013))

“…Simulation with a comprehensive Earth atmospheric general circulation model (AGCM) coupled to a slab ocean, without dynamic ocean heat transport, revealed an ‘eyeball’ climate state, with a round area of open ocean centered at the substellar point and complete ice coverage on the nightside, even for very high CO2 concentrations….”

Yongyun Hu and Jun Yang’s “slab ocean, without dynamic ocean heat transport”, may be among the simplest climate models for an ocean planet.

With no ocean currents, a tidally locked planet’s ocean might be covered with ice, with a single round spot of clear blue water gazing at its sun.

I haven’t tracked the term “eyeball planet” or “eyeball Earth” any further back than 2013. Maybe I’ll go deeper down that rabbit hole someday, but not this week.

Anyway, an “eyeball planet” is, I gather, an ocean world that’s tidally locked with its star, like the Moon is with Earth.

The Moon has a day-night cycle, since it turns around relative to the sun once a month.

NASA/JPL-Caltech picture: artist's impression of TRAPPIST-1 f, as the planet would look if covered with water and tidally locked: liquid on the day side, ice on the night side. (2017) see https://photojournal.jpl.nasa.gov/catalog/PIA21468 / via Wikipedia, used w/o permissionBut a planet that’s tidally locked with its star would have one side where it’s always day, with nothing but night on the other.

For a planet with no air to speak of, like Mercury, being tidally locked would leave it baking on one side, freezing on the other.

Scientists thought those were the conditions on Mercury. Until the 1960s, when we learned that the planet rotates three times for every two orbits.

Mercury is tidally locked, by the way, but with a 3:2 ratio instead of 1:1. That’s almost another topic.

At any rate, LHS 1140 b — I’m back to that planet for a moment — is around 1 and three-quarters times Earth’s diameter, but less dense. It might be a sub-Neptune.

But since nobody’s detected hydrogen in its atmosphere, the odds are that it’s an ocean world, a rocky planet covered with an ocean hundreds of miles deep.2

Exoplanet Climate Simulations and a “Lobster-Like Spatial Pattern”

Yongyun Hu, Jun Yang's's Fig. 1: 'Spatial distributions of sea-ice fraction and surface air temperature. (Left) Sea-ice fraction (unit, %); (Right) surface air temperature (unit, °C); (Upper) 355 ppmv CO2; and (Lower) 200,000 ppmv CO2. In A and B, arrows indicate wind velocity at the lowest level of the atmospheric model (990 hPa), with a length scale of 15 m s−1. In C and D, arrows indicate ocean surface current velocity, with a length scale of 3 m s−1. Note that the color scale for surface air temperature is not linear. The substellar point is at the equator and 180° in longtitude.' (2013)
Hu & Yang’s Fig. 1: hypothetical ocean world. (2013)
(left) sea-ice percent
(right) surface air temperature °C
(upper) atmosphere with 355 ppmv CO2
(bottom) 200,000 ppmv CO2

Sooner or later, “ocean world” may mean a planet with closely-defined characteristics.

That hasn’t happened yet, so sometimes Earth is called an “ocean world”.

Along, again sometimes, with Saturn’s moon Titan; exoplanets like CoRoT-7b, Kepler-10b, and Kepler-78b, that may be covered in lava; and LHS 1140 b — assuming that recent observations and analysis are right.

If LHS 1140 b is an ocean world, then it’s probably not an “eyeball planet”. At least not one with a round ice-free patch facing its sun.

For one thing, I’ve seen research that says the sort of tidal locking needed for eyeball planets won’t work.

I haven’t read it, though. It’s been one of those weeks, which seem to be coming more often these days, and that’s yet another topic.

Assuming that 1:1 tidal locking for worlds like LHS 1140 b is possible, and their surface conditions are right, looks like their ice-free patches won’t be round.

That’s because ocean currents happen: certainly on Earth, and near-certainly elsewhere.

It’s hard to imagine an ocean where the water doesn’t move around. Maybe in a post-plastic-apocalypse scenario, where the ocean’s water is entirely contained in discarded plastic bags. Oh, boy. Never mind. Moving along.

Folks developed the first weather and climate simulation programs around 1950. By the 1970s, forecasting where tropical cyclones would go — is yet again another topic.

The point is that we’ve got moderately-accurate simulations for how Earth’s ocean and atmosphere work. Plugging in different values lets scientists make informed guesses about the climate on other worlds: even still-hypothetical ones we haven’t spotted yet.

Like the ‘lobster-shaped’ ice-free area described in this 2013 paper:3

“…Fig. 1A shows AOGCM [Atmospheric-Oceanic General Circulation Model] simulation results of sea-ice fraction and wind velocity at the lowest model layer for 355 ppmv of CO2. This level of CO2 roughly equals the present-day CO2 concentration in the Earth atmosphere. In the presence of a dynamic ocean, the open-ocean area (blue) is not like the round iris of an ‘eye’ such as that in AGCM simulations coupled with a slab ocean (Fig. S1A; also figure 3 in ref. 4). Instead, the spatial pattern of the open-ocean region is more like a ‘lobster,’ showing two ‘claws’ symmetric to the equator and a long tail along the equator. The tail of open water extends eastward to the nightside. At the western side of the substellar point, sea ice is drifted eastward from the nightside toward the substellar point. The open-ocean region remains even for 3.6 ppmv of CO2 and shows the similar lobster-like spatial pattern. For very high-level CO2 (200,000 ppmv), sea ice is completely melted (Fig. 1B). By contrast, the nightside and a large part of the dayside remain frozen for the same level of CO2 in the AGCM simulation (Fig. S1B), and the open-ocean region is only slightly expanded compared with that in Fig. S1A….”
(“Role of ocean heat transport in climates of tidally locked exoplanets around M dwarf stars“; Yongyun Hu, Jun Yang; PNAS (2013)) [emphasis mine]

That’s No Lobster, That’s a Spaceship!

Yongyun Hu, Jun Yang's Fig. S3. 'Sea-ice thickness for 355 ppmv of CO2. Color interval is 0.5 m.' (2013)I haven’t talked with my oldest daughter for some time, mainly because we spend a couple hours each day on a text/media messaging service.

It’s not the same being in the same room, but suits us pretty well, and that’s still another topic.

The point of that digital detour is that I showed her the ‘lobster-like spatial pattern’ those scientists described.

Promotional art for George Pal Productions / Walter Lantz Productions' 'Destination Moon' (1950) 50th Anniversary EditionShe saw the pattern just fine, but didn’t see the “lobster”:

“Image A looks like a sleek spaceship.” …
“Yeah, really not getting ‘lobster’ out of any of these.”
(excerpt from my oldest daughter’s remarks, earlier this week: ca. August 7, 2024)

I think she’s right.

My first take on that hypothetical patch of open water was that it looked like “a sleek spaceship”: the sort writers and artists imagined, before we learned that real spaceships look like piles of storage tanks in a hurry.

Somewhat Simple Simulations, Dynamic Oceans

Yongyun Hu, Jun Yang's Fig. 2: 'Depth–latitude cross-sections of zonal-mean ocean potential temperatures and zonal-mean zonal velocity. (Left) Ocean potential temperature (unit, °C); (Right) ocean zonal velocity (unit, m s−1); (Upper) 355 ppmv CO2; and (Lower) 200,000 ppmv CO2. In C and D, yellow-red colors indicate westerly flows, blue colors indicate easterly flows, and contours are the mean meridional mass streamfunction. Solid contours indicate clockwise streamlines, and dashed contours are anticlockwise streamlines. Contour interval is 100 Sv.' (2013)
Hu & Yang’s Fig. 2:
“Depth–latitude cross-sections of zonal-mean ocean potential temperatures and zonal-mean zonal velocity.
(Left) Ocean potential temperature (unit, °C);
(Right) ocean zonal velocity (unit, m s−1);
(Upper) 355 ppmv CO2; and
(Lower) 200,000 ppmv CO2.
“In C and D, yellow-red colors indicate westerly flows, blue colors indicate easterly flows, and contours are the mean meridional mass streamfunction. Solid contours indicate clockwise streamlines, and dashed contours are anticlockwise streamlines. Contour interval is 100 Sv.” Hu & Yang (2013)

My hat’s off to Hu and Yang’s 2013 study, mainly for showing how ocean currents might take heat from a tidally locked planet’s sunlit side to its dark half.

Their simulation is, I strongly suspect, much simpler than any real planet’s ocean.

For one thing, their hypothetical ocean is four kilometers, two and a half miles, deep: the same as most of Earth’s ocean, once you get past the continental shelves.

That’s four kilometers deep everywhere on the planet.

Maybe there are worlds with perfectly flat ocean beds. My guess is that at least some would have features like Earth’s continents, complicating what might otherwise be elegantly simple looping currents.4

But I figure Hu and Yang helped others see water and air on ocean worlds as “dynamic”.

That’s not, I think, wishful thinking on my part. I found catchy(??) titles like these in my notes, when I got started writing about ‘lobster oceans’ this week —

  • “The middle atmospheric circulation of a tidally locked Earth-like planet and the role of the sea surface temperature”
    Elisavet Proedrou, Klemens Hocke, Peter Wurz; Progress in Earth and Planetary Sciences (2016)
  • “Connecting the dots — II. Phase changes in the climate dynamics of tidally locked terrestrial exoplanets”
    L. Carone, R. Keppens, L. Decin; MNRAS (Monthly Notices of the Royal Astronomical Society) (2015)

Extraterrestrial Life, the Universe, and Me

NASA/JPL-Caltech/R. Hurt's artist's concept: how rocky, potentially habitable planets might appear. (April 13, 2022)
R. Hurt’s illustration: what potentially habitable planets might look like. (2022)

By now, I suspect that someone’s tried simulating an ocean that’s hundreds of miles deep, like the one that’s (maybe) covering LHS 1140 b. But I haven’t run across it.

This item, however, popped up Monday:

As usual, it started with an Abstract, which told me that it’ll be fascinating reading — and not something I’d try digging into this week:

“…Most studies assessing the effect of flares on planetary habitability assume a 9000 K blackbody spectral energy distribution that produces more NUV flux than FUV flux (R=FFUV/FNUV ~ 1/6)…”
(“Stellar flares are far-ultraviolet luminous“, MNRAS (August 5, 2024))

These scientists looked at stars in a Gaia catalog, including red dwarfs like LHS 1140, and learned that their stellar flares make more high-energy ultraviolet light than expected.

That probably affects the odds for an exoplanet supporting life. But whether it means we’re more, or less, like to find extraterrestrial life: it’s probably to early to tell.5

Still Looking For Life, Still Learning

Detail, Hubble Space Telescope's ACS' view of NGC 602 and N90. (July 14/18, 2004) from NASA/Hubble, used w/o permission. (NGC 602 is an open cluster of stars in the Small Magellanic Cloud.)We know that there’s life in the universe. Earth is swarming with it.

The question is whether we’ll find life that started on — or in — other worlds.

So far, we’ve found a profusion of chemicals used by our sort of life in this part of the Milky Way galaxy.

But we’ve found no solid evidence that life exists elsewhere. And there’s nothing even close to a scientific consensus on the odds that we are — or are not — alone.

I’ll admit to a bias. I’d prefer that we find life that didn’t come from Earth — even better, that we find people like us; free-willed creatures with souls and bodies, but not human.

If that happens, studying similarities and differences would give us marvelous opportunities to learn more about life — and ourselves.

But I don’t think that there must be life elsewhere in the cosmos, or that there must not be. It’s God’s universe and God’s decision. Our job, part of it, is studying God’s work; and learning what we can.

I’ve talked about this, and related ideas, before:

1 Stars, planets, astronomy, and rocks:

2 Planets, old and newly-discovered:

3 Weather forecasting and simulating exoplanet climates, oceans and lava worlds:

4 Wind, water, and weird worlds:

5 habitability and UV:

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About Brian H. Gill

I was born in 1951. I'm a husband, father and grandfather. One of the kids graduated from college in December, 2008, and is helping her husband run businesses and raise my granddaughter; another is a cartoonist and artist; #3 daughter is a writer; my son is developing a digital game with #3 and #1 daughters. I'm also a writer and artist.
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