A Catholic Citizen in America

This week, I’ll talk about Professor Ethan Siegel’s view that “the myth of the super-habitable super-Earth planet” is “a scientific catastrophe”, other non-catastrophes; and a problem with “super-Earths” as a label.

Along the way I’ll look at science, news, headlines and silliness. And finally, skip lightly over a 13th century academic debate that got out of hand.

• “…A Scientific Catastrophe”?
  • Earth ISN’T the Best of All Possible Worlds???
  • Bigger Isn’t (Always) Better: But Neither is Smaller
• Science News, Silliness, Headlines and “Catastrophe”
  • Proxima Chorizo, the Great Moon Hoax and Headlines
• Exoplanets: New Categories for Strange New Worlds
  • Sorting Exoplanets by — Radius?
  • Mass, Period and Discovery Method of Known Exoplanets (March 2022)
  • New Worlds Discovered by Kepler, TESS, and Everything Else
  • Still Seeking the Legendary Earth 2.0
• The Problem with “Super-Earths”
  • HD 219134 b: Data, Density and Uncertainty
  • “Super-Earths”: Not Necessarily Terrestrial
• Cosmic Pluralism, Aristotle, God, and Getting a Grip
  • Truth Matters

This post started more than two weeks ago, when a headline caught my eye.

Now, I’m more likely to click on links to articles about exoplanets and stars, than check on who’s dating who in Hollywood, or what a television sports analyst said about fans.

But words like “catastrophe” get my attention, even though I think they’re overused. Particularly when they’re describing something other than news media’s favorite obsessions.

And that is why the following item caught my eye:

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“…A Scientific Catastrophe”?

“Why ‘super-Earth’ exoplanets are a scientific catastrophe“
Ethan Siegel, Starts With A Bang, via Big Think (May 8, 2023)

“Key Takeaways

• “Of the more than 5,000 exoplanets known, the most common class of exoplanet is one that has no representation in our own Solar System: the Super-Earth.
• “Between 2 and 10 Earth masses — larger and more massive than Earth but smaller and less massive than Uranus or Neptune — it was the most common exoplanet class found by Kepler.
• “Many have speculated that super-Earths may be even more conducive to life, as well as more common, than Earth-like planets. That’s almost certainly untrue; here’s why.

“It’ time to expose a scientific catastrophe: the myth of the super-habitable super-Earth planet….”

I think Dr. Ethan Siegel (theoretical astrophysicist) has a point.

I also recommend his Big Think article. He pulled together a good sampling of what we know about exoplanets in general. With particular focus on those whose heft is between Earth’s and the Solar System’s ice giant worlds.

His text is heavy on facts and nearly devoid of filler or fluff. Just as commendable, he uses many graphics: including E. Pécontal’s animation, illustrating the radial velocity method of spotting exoplanets.

That said, I don’t think speculation about super-Earths is a “scientific catastrophe”.

Granted, I’m thinking of René Heller and John Armstrong’s “Superhabitable Worlds”, Astrobiology (2014).¹

Earth ISN’T the Best of All Possible Worlds???

“Superhabitable Worlds“
René Heller, John Armstrong; Astrobiology (January 2014) via arXiv

“…4. Conclusions….”

“…Terrestrial planets that are slightly more massive than Earth, that is, up to 2 or 3 M⊕, are preferably superhabitable due to the longer tectonic activity, a carbon-silicate cycle that is active on a longer timescale, enhanced magnetic shielding against cosmic and stellar high-energy radiation….”

“…Eventually, just as the Solar System turned out to be everything but typical for planetary systems, Earth could turn out everything but typical for a habitable or, ultimately, an inhabited world. Our argumentation can be understood as a refutation of the Rare Earth hypothesis. … While we agree that the occurrence of another truly Earth-like planet is trivially impossible, we hold that this argument does not constrain the emergence of other inhabited planets. We argue here in the opposite direction and claim that Earth could turn out to be a marginally habitable world. In our view, a variety of processes exists that can make environmental conditions on a planet or moon more benign to life than is the case on Earth….”
[emphasis mine]

If Siegel’s article identified the “many” folks who thought that maybe super-Earths were “more conducive to life” than our world, I missed it.

He did, however, say why many planets that are between two and 10 times as massive as ours are very likely not suitable for life. I’ll get back to that.

Heller and Armstrong — they wrote that “Superhabitable Worlds” paper — may turn out to be wrong about rocky planets that are more massive as Earth. But their 2014 paper gave reasons for their conclusions.

They also acknowledged that a planet just like Earth, orbiting a star just like ours, could be habitable; and that the odds of finding a world just like that were slim to none.

Heller and Armstrong started by discussing a “menagerie” of hypothetical exoplanets about 1.5 times as massive as Earth, with a radius 1.12 that of Earth’s, orbiting a star similar to Gl58: a red dwarf star in the general direction of Beta Librae.

Then they sketched out possible habitable zones for a rocky moon orbiting a Jupiter-size planet. They showed how the moon would be heated by sunlight from the star, sunlight that had reflected off the planet, and by tidal heating.

Tidal heating is the sort of thing that makes Io the most volcanically active body in the Solar System.² The most active that we know of, at any rate.

Bigger Isn’t (Always) Better: But Neither is Smaller

After that, they describe what they think would make a planet superhabitable.

That part of their paper goes on for five pages, so I’ll skip most of what they said.

One point they made was that bigger isn’t always better when it comes to habitability.

That’s because we’ve been learning that plate tectonics recycles chemicals and minerals living critters need. That process works on Earth, but apparently not for smaller worlds.

“…smaller planets have smaller diameters and thus higher surface-to-volume ratios than their larger cousins. Such bodies tend to lose the energy left over from their formation quickly and end up geologically dead, lacking the volcanoes, earthquakes and tectonic activity which supply the surface with life-sustaining material and the atmosphere with temperature moderators like carbon dioxide. Plate tectonics appear particularly crucial, at least on Earth: not only does the process recycle important chemicals and minerals….”
Planetary habitability, Wikipedia [emphasis mine]

A 2011 study presented at a meeting of the EPSC-DPS by L. Noack and D. Breuer (I put a link under footnote ³) said that plate tectonics works on Earth because our planet’s mantle is at the right temperature and pressure.

The good news is that there’s apparently a range of temperature and pressure that work: not some wildly-improbable exact balance.

The intriguing news is that, if Noack and Breuer are right, Earth is about as small as a planet can be and have enough heat and pressure inside to make plate tectonics go. Go and keep going long enough for life to get started and get interesting, at any rate.

But again, bigger isn’t always better. They figured that a rocky planet’s insides would be too hot for worlds more than five times as massive as ours.³

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Science News, Silliness, Headlines and “Catastrophe”

Bear in mind that I’m in my 70s. I remember McCarthyism’s dying gasps, Ehrlich’s “The Population Bomb”, and the Cold War.⁴

My notion of a “catastrophe” may be a tad more catastrophic than Professor Siegel’s.

Proxima Chorizo, the Great Moon Hoax and Headlines

I was annoyed when a high-profile scientist told his fans that a slice of chorizo was a Webb telescope image of Proxima Centauri.

But when he sobered up, he explained that he had a perfectly good reason for posting a picture of Proxima Chorizo.

So “annoyed” is about as far as I’ll go with that incident.

I see the Great Moon Hoax of 1835 as edging a bit closer to “catastrophe”.

Particularly since The New York Sun wouldn’t publish a retraction. Not even after folks realized that Sir John Herschel hadn’t discovered bat-people on the Moon.

That’s understandable. Their mini-bison and bipedal beavers sold papers. I can’t help wonder, though, how much their make-believe science series encouraged folks to write off science as flim-flam.

Then there are headlines like these:

• “There Is Only One Other Planet In Our Galaxy That Could Be Earth-Like, Say Scientists“
  Jamie Carter, Forbes (June 23, 2021)
• “Kepler-442b is more habitable than Earth“
  K. G. Orphanides, Science, Wired UK (July 10, 2015)

The Forbes article’s headline is accurate, partly.

And, although the scientists weren’t named in the article, there was a link to their paper. Turns out that there were four of them, and they did say Kepler-422 b had enough sunlight to support Earth-type photosynthesis.

But not that it was the only such planet in the whole galaxy, other than Earth.

“Efficiency of the oxygenic photosynthesis on Earth-like planets in the habitable zone“
Giovanni Covone, Riccardo M Ienco, Luca Cacciapuoti, Laura Inno; Monthly Notices of the Royal Astronomical Society (August 2021)

“…we also find that Kepler-442b receives a PAR photon flux slightly larger than the one necessary to sustain a large biosphere, similar to the Earth biosphere….”

In fairness, Forbes focuses on finance, industry, investing, and marketing. Not science.

The Wired UK article’s author was less imaginative.

Orphanides explained that a paper published in the Astrophysical Journal outlined a new method for assigning the ‘maybe habitable’ label. And that, by that method’s standard, Kepler-442 b might be just a bit more habitable than Earth.⁵

But the headline? Well, headlines are there to grab attention. And that one did its job.

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Exoplanets: New Categories for Strange New Worlds

If planetary scientists put the super-Earth label on every exoplanet with a mass between two and 10 times Earth’s, then that could be a problem. Because many are not Earth-like.

By 2017, researchers had found many planets that weren’t like anything in the Solar System, and didn’t fit into the old ‘terrestrial, gas giant, ice giant’ categories.

Hot Jupiters, for example, are at least as massive as the Solar System’s gas giants, but whip around their stars in tight orbits.

Ocean worlds are probably covered by profoundly deep oceans and/or have far more ice than Earth. From what I’ve read, the liquid on “ocean worlds” is water. Probably.

That’s assuming that the ‘big ocean’ model is accurate — as it almost certainly is for moons like Enceladus and Europa.

But an ice giant’s ice might be ammonia, methane or water. In this case, “ice” is any volatile chemical with a melting point above around 100 Kelvin. We’ve got two in our Solar System: Uranus and Neptune.

The Solar System’s planets are close enough for astronomical spectroscopy to show us what chemicals are in their atmospheres or on their surfaces. Scientists have used the same techniques for studying a few exoplanets.

But in many cases, the data we have to work with is an exoplanet’s orbital period, mass, and — for transiting planets — diameter.⁶

That information would tell hypothetical astronomers on a planet circling Tau Ceti that the Solar System’s Venus and Earth are almost certainly rocky planets with about the same mass. And that Venus should be warmer than Earth. But it wouldn’t tell them much else.

Sorting Exoplanets by — Radius?

Even so, just knowing a planet’s mass and diameter tells us quite a bit: particularly when researchers combine data from all known exoplanets.

That graph, used by Professor Siegel in his “Catastrophe” article, sorts 5,000 known (as of the start 0f 2022) exoplanets by radius.

I’m not sure how or where Open Exoplanet Catalogue got the radius of 5,000 exoplanets. When I checked their website, I didn’t find that graph.

We had catalogued about 5,500 exoplanets by early 2022. But as far as I know, we didn’t know the radius of each one.

Fast-forward to May, 2023. As of this month, scientists know of 5,300 and some odd exoplanets, a bit over 4,000 of which are transiting exoplanets.

Transiting exoplanets are worlds that pass between their star and us once every orbit. Observing and measuring these transits tells scientists how wide the worlds are.

I don’t know how Open Exoplanet Catalogue got radii for non-transiting worlds. Maybe it’s derived from their mass. Then again, maybe not.

Anyway, I gather that if Earth was on that graph, it’d be at -1.0 units. Neptune would be at -0.5 and Jupiter would be at 0.0.

And, again looking at that graph, it looks like the number of exoplanets peaks at a little shy of Neptune’s size, with another peak at Jupiter-size and larger. According to that graph.⁷

I spent more time than I maybe should have, trying to tack down where Open Exoplanet Catalogue got their “radius” data. Without success.

Maybe the graph’s “radius” label should have been “mass”.

There was, as of March 2022, a clustering of exoplanet masses around one Jupiter-mass and another around about 0.03 Jupiter-mass.⁷

Mass, Period and Discovery Method of Known Exoplanets (March 2022)

I’d enjoy geeking out over how scientists have been spotting exoplanets, and why they’ve spotted so many big planets with small orbits.

But you’re in luck.

If I’m going to get this thing finished in a reasonable time, my reasonable option is making a list of methods, plus a quick definition.

• Astrometry
  Precisely measuring the positions and movements of stars
• Imaging
  Getting a ‘photo’ of a planet, often in infrared
• Microlensing
  Observing the ‘flash’ when light from a planet gets focused by an intervening star’s gravity field
• Orbital brightness variations
  Just what it says: observing cyclic changes in a star’s brightness (CHECK THIS)
• Radial velocity
  Looking for changes in star’s spectrum caused by Doppler shift
• Transits
  Observing light from a star dimming as a planet moves across its face
• Timing variations
  Observing and timing transits

Maybe exoplanets really do come in two basic sizes, with two standard orbital periods: either Jupiter-size with a 1,000 day orbit, or between Uranus and Neptune-size with a 10-day orbit.

That’d make the Solar System, with its two gas giants, two ice giants and four terrestrial planets an oddball. And maybe that’s so.

On the other hand, each detection method we’ve got has its own selection bias.

It’s very possible that we’ve found a great many massive planets in either very tight orbits or in fairly big orbit — because that’s what our current methods are good at spotting.⁸

New Worlds Discovered by Kepler, TESS, and Everything Else

Until about 2012, scientists were mostly spotting new exoplanets by measuring the radial velocity of stars. Just to make things more complicated, radial velocity detection is often called Doppler spectroscopy, and that’s another topic.

Oddly enough, I’ve yet to see the flood of exoplanets discovered with the Kepler space telescope tied in with the 2012 ‘end of the world’ thing. Possibly because nerds like me focus more on the science side of the Mayan long count calendar, and less on pop prophecies.

The spike in confirmed exoplanets in 2016 probably came from NASA’s data dump in May of that year. Analysis dump, actually. Then, in July of 2018, TESS (Transiting Exoplanet Survey Satellite), started sending back data.

Kepler and TESS aren’t the only transit-detecting space observatories. And there’s the CNES/ESA COROT (Convection, Rotation and planetary Transits), and projects like SuperWASP and HATNet on Earth.⁹

That’s not a complete list. The point is that a whole lot of scientists have been gathering and analyzing data, using over a half-dozen methods: mostly transit, lately.

So how come, out of the five-thousand-plus new worlds they’ve spotted, they haven’t found ‘Earth 2.0’ — a planet pretty much like Earth, orbiting a star like ours?

Still Seeking the Legendary Earth 2.0

That scatter plot shows exoplanets know as of 2017, sorted by radius (vertical) and orbital period (horizontal).

The red oval is where we’d find a planet more-or-less like Mercury. Earth-size worlds in an orbit somewhat like ours would be in the green oval.

‘Earth 2.0’, a planet like ours orbiting a star like ours, would be right in the center of the green oval. We hadn’t spotted one like that in 2017, and still haven’t.

But we have found a few planets that are roughly Earth’s size: like Kepler-186f and TOI-700 d. They’re both in their star’s habitable zone. The stars are red dwarfs, though: so neither would be sure-fire ‘Earth 2.0’.

A few exoplanets have been called Earth analogs, and a few of those were temporarily dubbed ‘Earth 2.0’.

Some are too hot for life as we know it. Some are big enough to be called super-Earths.¹⁰ And that, finally, brings me to what bothers Professor E. Siegel about “super-Earths”.

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The Problem with “Super-Earths”

The earliest example I’ve found of the term “Super-Earths” is in a 2006 paper.

“…The first such planets were discovered during the past year, judging by their measured masses of less than 10 Earth-masses (M⊕) or Super-Earths. … Their composition can be either completely terrestrial or harbour an extensive ocean (water and ices) above a rocky core….”
(“Radius and Structure models for the First Super-Earth Planet“; Diana Valencia, Dimitar D. Sasselov, Richard J. O’Connell; The Astrophysical Journal (submitted October 4, 2006))

Having “a mass higher than Earth’s, but substantially below those of the Solar System’s ice giants” is a Wikipedia page’s definition for super-Earth. Apparently the label and mass-only description caught on.

But even in 2006 — or 2007, when the paper was published — it wasn’t the best moniker. Scientists knew that a fair number of “super-Earths” could be quite un-Earthlike.

Unlike Professor E. Siegel, I don’t think that’s a “catastrophe”.

Sloppy labeling, yes; but not a catastrophe.

Not unless scientists start forgetting distinctions and labels like ocean world, mini-Neptune, sub-Neptune, super-puff and Chthonian planet.

I’d be astounded if we’re still using all those labels a hundred years from now. We’ve been learning a lot, fast, about exoplanets.

A few decades back, we didn’t know that these worlds existed. A few decades from now, with new data and new analysis, many of today’s models for what’s inside exoplanets may turn out to be very wrong.

Diana Valencia et al.’s 2006 paper used GJ876d, Gliese 876 d, the first known “super-Earth”, as its model. I’d use it as an example, too, but not quite two decades later we still don’t know its radius.

And we won’t, until scientists come up with new observation and analysis methods. We do, however, have a pretty good handle on its mass: between six and a half and seven and a quarter times Earth’s.

So I’ll be looking at what we know about HD 219134 b, a super-Earth orbiting HD 219134, a star that’s a tad over 21 light-years out, in the general direction of Beta Cassiopeiae.¹¹

HD 219134 b: Data, Density and Uncertainty

The star HD 219134 is smaller and cooler than our star, with a K3V spectral class.

Its habitable zone, where a planet like Earth could have liquid water on the surface, is smaller than the Solar System’s.

And HD 219134 b is even closer to its star. It’s far too hot for life as we know it.

Astronomers have taken a close look at HD 219134 b: not directly, but by measuring shifts in its star’s radial velocity, and how much starlight it blocks when it transits HD 219134.

That’s given them HD 219134 b’s mass and radius, with a fair degree of accuracy: which in turn gives its density. Whatever its made of, on average the exoplanet is dense.

Density (grams per cubic centimeter) of:

• HD 219134 b
  6.36 (± 0.72)
• Earth
  5.5134
• Mercury
  5.427
• Neptune
  1.638

If those numbers are right, HD 219134 b is almost certainly not made of stuff similar to Neptune’s interior. And maybe not quite like Earth’s.

Slightly more recent data says that the exoplanet’s radius is smaller: about 1.5 times Earth’s. Which would make it even denser that Earth.

The last I checked, we haven’t detected an atmosphere around HD 219134 b. But scientists have worked out that it might have one: and if so, it probably isn’t mostly hydrogen.

Given how much data’s available, that’s pretty good work. And gives other scientists starting points for planning new observations of HD 219134’s planetary system.¹²

My guess is that HD 219134 b is a “super-Earth”: both in the sense of having more mass than Earth and less than Uranus or Neptune, and in the sense implied by “super-Earth“, being a planet that’s (probably) rocky.

“Super-Earths”: Not Necessarily Terrestrial

Many exoplanets with the super-Earth label are at least as dense as Earth, so they may be made of stuff like our home.

55 Cancri A e, for example is even denser than HD 219134 b: 6.66 grams per centimeter squared (+0.43 or -0.40).

But others, like Kepler-737b, with a density around three and a third grams per centimeter, are much less dense than Earth.

Since they’re also more massive than Earth, the odds are good that they’re not particularly Earth-like.

What is inside exoplanets — is something I’ll leave for another time.

I think having a label for exoplanets with masses between Earth’s and Uranus’ makes sense.

But “super-Earths” isn’t an ideal label. “Earth” can imply Earth-like. Some of them aren’t particularly Earthlike at all. Looking at their density, they’re probably not even terrestrial, like the Solar System’s inner worlds.¹³

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Cosmic Pluralism, Aristotle, God, and Getting a Grip

Aristotle was a very smart man.

Trust me, this relates to super-Earths, exoplanets and the search for extraterrestrial life. Like I said, Aristotle was very smart. But he wasn’t the only smart citizen of an ancient Greek city-state.

Take Anaximander, for example. He lived about two centuries before Aristotle, and said that we lived in a universe with many worlds. But he didn’t have fan base that kept Aristotle’s work front and center while the Roman Empire rose and crumbled.

About a thousand years back now, European scholars picked up where their ancient counterparts left off. They also put Aristotle in an exalted position.

I suspect that it didn’t hurt, either in Aristotle’s day or later, that Aristotle’s cosmology put Earth at the very center of the universe. Or, perhaps more accurately, at the bottom, and that’s yet another topic.

The point is that Earth was important: and the only earthly world. According to Aristotle.

Aristarchus and other ancient philosophers who said maybe we’re not standing on the only world weren’t entirely forgotten.

I figure we’d have realized that Earth wasn’t alone eventually, anyway. Truth has a way asserting itself.

The ‘one world or many’ debate heated up in the late 1200s.

Some European scholars said folks like Aristarchus were on the right track.

Others said there is only one Earth and we’re standing on it. Because Aristotle said so.

I’m oversimplifying developments in Western philosophy over a span of millennia something fearful, by the way.

The ‘because Aristotle said so’ thing got the Bishop of Paris involved. His Condemnation of 1277 said, at least by implication, that God’s God and Aristotle’s not.¹⁴

27A. That the first cause cannot make more than one world.
“Selections from the Condemnation of 1277“, Gyula Klima, Fordham University (November 23, 2006)

Truth Matters

Recapping, 27A of the Condemnation of 1277 implied that if we’re standing on the only earthly world, it’s because God (or ‘the first cause’ in Medieval academic-speak) wants it that way.

And that if the universe has many worlds like ours, that’s the way it is: whether Aristotle would have approved or not.

I suspect one reason the Condemnation of 1277 is so controversial these days is the ‘God’s God, Aristotle’s not’ thing.

Flat-out saying — even though Aristotelian cosmology fits nicely into the Mesopotamian cosmic poetry reflected in the Bible — that what’s true is true, even if it means we must readjust our assumptions?

That emphatically does not fit the ‘rigid, arbitrary and unthinking’ view of religion in general and Christianity in particular that’s been popular of late. I’ll grant that rabidly-righteous and frighteningly-faith-filled folks don’t help dispel that image.

A few more points, and I’m done.

I don’t “believe in” extraterrestrial life. I figure we’ll either find life that emerged on other worlds: or we won’t. I’m pretty sure we won’t stop looking.

I certainly wouldn’t mind if we learn that we have neighbors in this universe, and I’m drifting off-topic.

Trying to “not believe in” exoplanets would be silly, at best. Scientists have discovered thousands so far, and the odds are good that evidence of many more is in data that hasn’t been crunched yet.

The bottom line is that truth matters. A lot. That’s not just my opinion. Here’s what Saints, popes and a scientist have said about paying attention and accepting truth:

“Religion and natural science are fighting a joint battle in an incessant, never relaxing crusade against skepticism and against dogmatism, against disbelief and against superstition, and the rallying cry in this crusade has always been, and always will be: ‘On to God!’“
(“Religion and Natural Science”, Translated and published in “Max Planck: Scientific Autobiography and Other Papers” (1968); via Wikiquote [emphasis mine])

“…if methodical investigation within every branch of learning is carried out in a genuinely scientific manner and in accord with moral norms, it never truly conflicts with faith, for earthly matters and the concerns of faith derive from the same God. … we cannot but deplore certain habits of mind, which are sometimes found too among Christians, which do not sufficiently attend to the rightful independence of science and which, from the arguments and controversies they spark, lead many minds to conclude that faith and science are mutually opposed.…”
(“Gaudium et Spes“, Pope St. Paul VI (December 7, 1965) [emphasis mine])

“…God, the Creator and Ruler of all things, is also the Author of the Scriptures — and that therefore nothing can be proved either by physical science or archaeology which can really contradict the Scriptures. … Even if the difficulty is after all not cleared up and the discrepancy seems to remain, the contest must not be abandoned; truth cannot contradict truth….”
(“Providentissimus Deus“, Pope Leo XIII (November 18, 1893) [emphasis mine])

“Question the beauty of the earth, question the beauty of the sea, question the beauty of the air…. They all answer you, ‘Here we are, look; we’re beautiful.’…
“…So in this way they arrived at a knowledge of the god who made things, through the things which he made”.
(Sermon 241, St. Augustine of Hippo (ca. 411))

One more thing. God is large and in charge. And I’m okay with that.

“Our God is in heaven and does whatever he wills.”
(Psalms 115:3)

I was going to talk about our search for extraterrestrial life, hypothetical life chemistry, and why I hope we have neighbors. But this post is running long, so I’ll leave that for later.

Now, finally — really finally, this time — the usual links:

• “TRAPPIST-1 and the Mysterious Pea Pod Planets“
  (April 8, 2023)
• “Galaxies, Gravity and a Hot Terrestrial Planet“
  (February 25, 2023)
• “Two Nearby Habitable(?) Worlds; Elements for Life“
  (February 4, 2023)
• “Exoplanets, Air, and the Marshmallow Planet“
  (December 10, 2022)
• “Exoplanets, Iron, Evolution and Strange Geochemistry“
  (January 22, 2022)

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¹ Superhabitable, uninhabitable, and an opinion:

• Wikipedia
  • Ethan Siegel
  • Ice giant
  • Neptune
  • Super-Earth
  • Superhabitable planet
  • Uranus
• “Superhabitable Worlds“
  René Heller, John Armstrong; Astrobiology (January 2014) via arXiv
• “Professor Ethan Siegel, ‘Hyper Reality’“
  Featured Events, Pacific Northwest College (September 2010)

² Strange worlds, hypothetical and otherwise:

• Wikipedia
  • Beta Librae
  • Gliese 581
  • Superhabitable planet
  • Tidal heating
  • Tidal heating of Io
• “Superhabitable Worlds“
  René Heller, John Armstrong; Astrobiology (January 2014) via arXiv
• “Professor Ethan Siegel, ‘Hyper Reality’“
  Featured Events, Pacific Northwest College (September 2010)

³ Habitabable may not mean ‘just like Earth’:

• Wikipedia
  • Planetary habitability
  • Plate tectonics
  • Super-Earth
  • Superhabitable planet
• “The role of planetary interior in the long-term evolution of
  atmospheric CO2 on Earth-like exoplanets“
  M. Oosterloo, D. Höning, I. E. E. Kamp, F. F. S. van der Tak; Astronomy & Astrophysics, ESO (March 18, 2021) via arXiv
• “Superhabitable Worlds“
  René Heller, John Armstrong; Astrobiology (January 2014) via arXiv
• “Plate Tectonics on Earth-Like Planets“
  L. Noack, D. Breuer; EPSC-DPS (European Planetary Science Congress-Division for Planetary Sciences of the American Astronomical Society) Joint Meeting 2011, held 2-7 October 2011 in Nantes, France (October 2011)

⁴ Science and history, some of which I lived through:

• Wikipedia
  • Cold War
  • Great Moon Hoax
  • McCarthyism
  • The Population Bomb
  • Soviet Empire
• “Efficiency of the oxygenic photosynthesis on Earth-like planets in the habitable zone“
  Giovanni Covone, Riccardo M Ienco, Luca Cacciapuoti, Laura Inno; Monthly Notices of the Royal Astronomical Society, Volume 505, Issue 3, pages 3329–3335 (August 2021)
• A hoax, history, and not missing the ‘good old days’
  • “Green Sahara, Environmental and Climate News” (July 30, 2022)
    • Marketing and Menaces
  • “Where Have All the People Gone?” (November 23, 2018)
    • “The Population Bomb”
    • Caves of Soylent Dredd??
    • 50 Years After “The Population Bomb”
    • Fertility Rates
  • “‘Oumuamua: Data, Questions” (November 10, 2018)
    • Anaxagoras and the Great Moon Hoax

⁵ Publications and (pop?) science:

• Wikipedia
  • Forbes
  • Kepler-442b
• “Efficiency of the oxygenic photosynthesis on Earth-like planets in the habitable zone“
  Giovanni Covone, Riccardo M Ienco, Luca Cacciapuoti, Laura Inno; Monthly Notices of the Royal Astronomical Society, Volume 505, Issue 3, pages 3329–3335 (August 2021)
• “There Is Only One Other Planet In Our Galaxy That Could Be Earth-Like, Say Scientists“
  Jamie Carter, Forbes (June 23, 2021)
• “Kepler-442b is more habitable than Earth“
  K. G. Orphanides, Science, Wired UK (July 10, 2015)
• “Belief, Legend, and the Great Moon Hoax“
  Stephanie Hall, Folklife Today, American Folklife Center & Veterans History Project, Library of Congress Blogs (August 26, 2014)

⁶ Diverse and distant worlds:

• Wikipedia
  • 51 Pegasi b
  • Astronomical spectroscopy
  • Earth
  • Enceladus
  • Europa (moon)
  • Exoplanet
  • Gas giant
  • Hot Jupiter
  • Ice
    • Non-water
  • Ice giant
  • Gliese 1214 b
  • Neptune
  • Ocean world
  • Tau Ceti
  • Tau Ceti f
  • Terrestrial planet
  • Uranus
  • Venus
  • Volatility (chemistry)
• Studying light from distant worlds, my view
  • “TRAPPIST-1 and the Mysterious Pea Pod Planets” (April 8, 2023)
    • Possible Interiors: 2021
  • “TRAPPIST-1 b Measured by Webb: Hot, Airless” (April 1, 2023)
    • Taking TRAPPIST-1 b’s Temperature With Webb’s MIRI

⁷ Lists and statistics:

• Wikipedia
  • GitHub
  • Lists of exoplanets
• NASA Exoplanet archive (NASA Exoplanet Science Institute (NASA, ipac, NExScI/Caltech))
  • Exoplanet and Candidate Statistics
  • Exoplanet Plots (graphs and charts)
  • Transiting Planet Resources in the Exoplanet Archive
• Open Exoplanet Catalogue (“Fork me on GitHub”)

⁸ More lists, and how we study exoplanets:

• Wikipedia
  • Astrometry
  • Doppler spectroscopy
  • Gravitational microlensing
  • List of Solar System objects by size
  • Lists of exoplanets
  • Methods of detecting exoplanets
    • Astrometry
    • Direct imaging
    • Radial velocity
    • Transit photometry
• Exoplanets in Transit:Transits reveal planets orbiting other stars.
  Exoplanet Transit Poster, Professor Newby’s Educational Quanta, Temple University Sites. Temple Univrsity, Philadelphia, Pennsylvania
• NASA Exoplanet archive (NASA Exoplanet Science Institute (NASA, ipac, NExScI/Caltech))
  • Exoplanet and Candidate Statistics
  • Exoplanet Plots (graphs and charts)
  • Transiting Planet Resources in the Exoplanet Archive

⁹ Something silly, and a whole bunch of stuff that’s not:

• Wikipedia
  • 2012 phenomenon
  • CNES (Centre National d’Etudes Spatiales/National Centre for Space Studies)
  • CoRoT (Convection, Rotation et Transits planétaires/Convection, Rotation and planetary Transits)
  • Doppler spectroscopy
  • European Space Agency (ESA)
  • HATNet Project (Hungarian Automated Telescope Network)
  • Kepler space telescope
  • Maya civilization
  • Mesoamerican Long Count calendar
  • Methods of detecting exoplanets
  • Transiting Exoplanet Survey Satellite (TESS)
  • Wide Angle Search for Planets
• NASA
  • Exoplanet Plots graphs and charts)
  • “NASA’s Kepler Mission Announces Largest Collection of Planets Ever Discovered“; Felicia Chou, Michele Johnson; Karen Northon, Editor (May 10, 2016; last updated August 6, 2017)
  • Briefing Materials: Artificial Intelligence and NASA Data Used to Discover Eighth Planet Circling Distant Star (2017)

¹⁰ Searching for another place like Earth:

• Wikipedia
  • Earth analog
  • Kepler-42 (Three known planets, each with radius less than Earth’s)
  • Kepler-186
  • Kepler-186f (first planet with a radius similar to Earth’s discovered in another star’s habitable zone)
  • Kepler-452
  • Kepler-452b (radius about 1.5 times Earth’s, mass about five times Earth’s
  • Super-Earth
  • TOI-700
  • TOI-700 d (first Earth-sized exoplanet in its star’s habitable zone discovered by TESS (Transiting Exoplanet Survey Satellite))

¹¹ Stars and planets:

• Wikipedia
  • Beta Cassiopeiae
  • Chthonian planet
  • Gliese 876 d
  • HD 219134
  • HD 219134 b
  • Kepler-138
    • Planetary system
  • Mini-Neptune
  • Ocean world
  • Stellar classification
  • Sub-Neptune
  • Super-Earth
  • Super-puff
• “Radius and Structure models for the First Super-Earth Planet“
  Diana Valencia, Dimitar D. Sasselov, Richard J. O’Connell; The Astrophysical Journal (submitted October 4, 2006) via arXiv, Published 2007 via iop.org

¹² Searching for habitable worlds:

• Wikipedia
  • Circumstellar habitable zone
  • Earth
  • Exoplanet interiors
  • HD 219134
  • HD 219134 b
  • Mercury (planet)
  • Neptune
  • Super-Earth
• “From the stellar properties of HD 219134 to the internal compositions of its transiting exoplanets“
  R. Ligi, C. Dorn, A. Crida, Y. Lebreton, O. Creevey, F. Borsa1, D. Mourard, N. Nardetto, I. Tallon-Bosc, F. Morand, E. Poretti; Astronomy & Astrophysics, ESO (received July 5, 2019; accepted September 20, 2019) via Astronomy & Astrophysics and EDP Sciences
• “Secondary Atmospheres on HD 219134 b and c“
  Caroline Dorn, Kevin Heng; The Astrophysical Journal (January 23, 2018) via iop.org

¹³ Super-Earths and/or terrestrial planets:

• Wikipedia
  • 55 Cancri
  • 55 Cancri e
  • Kepler-737b
  • Terrestrial planet
• “Astronomers discover an unusually low-density super-Earth“
  Tomasz Nowakowski, Phys.org (May 16, 2023)
• “An unusually low-density super-Earth transiting the bright early-type M-dwarf GJ 1018 (TOI-244)“
  A. Castro-González, O. D. S. Demangeon, J. Lillo-Box, C. Lovis, B. Lavie, V. Adibekyan, L. Acuña, M. Deleuil6, A. Aguichine, M. R. Zapatero Osorio, H. M. Tabernero, J. Davoult, Y. Alibert, N. Santos, S. G. Sousa, A. Antoniadis-Karnavas, F. Borsa, J. N. Winn, C. Allende Prieto, P. Figueira, J. M. Jenkins, A. Sozzetti, M. Damasso, A. M. Silva, N. Astudillo-Defru1, S. C. C. Barros, X. Bonfils, S. Cristiani, P. Di Marcantonio, J. I. González Hernández, G. Lo Curto, C. J. A. P. Martins, N. J. Nunes, E. Palle, F. Pepe, S. Seager, A. Suárez Mascareño; Astronomy & Astrophysics (received March 31, 2023; acepted May 2, 2023; published May 9, 2023) via arXiv
• “Revisited mass-radius relations for exoplanets below 120 M⊕“
  J. F. Otegi, F. Bouchy, R. Helled1; Astronomy & Astrophysics (received 8 August 8, 2019; accepted November 10, 2019; published February 2020)
• “Mass-Radius Relationships for Exoplanets“
  D. C. Swift, J. H. Eggert, D. G. Hicks, S. Hamel, K. Caspersen, E. Schwegler, G. W. Collins, N. Nettelmann, G. J. Ackland; The Astrophysical Journal (December 13, 2011) via Office of Scientific and Technical Information, U.S. Department of Energy

¹⁴ Philosophers and history:

• Wikipedia
  • Anaximander
  • Aristotle
  • Aristotelian physics
  • Condemnations of 1210-1277
  • Cosmic pluralism
  • Geocentric model
    • Ptolemaic system
  • Ptolemy
• Condemnation of 1277
  Stanford Encyclopedia of Philosophy (January 30, 2003; revised November 18, 2018)
• “Selections from the Condemnation of 1277“
  Gyula Klima, Fordham University (November 23, 2006)