TRAPPIST-1 and the Mysterious Pea Pod Planets

NASA/JPL-Caltech's illustration: TRAPPIST-1 planetary system, orbits of known planets in 2016 and then in 2017.
TRAPPIST-1 planetary system: known planets in 2016 and 2017.

There may have been times when one generation’s world was much like another’s.

This is not one of those times.

Science textbooks of my youth included speculation that Earth’s mountains exist because our planet has been cooling and shrinking. One of my geology professors didn’t “believe in” continental drift, and that’s another topic.1

Back then, we knew that planets orbit our star, but weren’t sure how the star we call the Sun and the Solar System formed.

We still don’t, for that matter. Not for sure. But the nebular hypothesis, or something very much like it, is a pretty good fit with observations.

I’ll get back to that, and some of what we’ve been learning about planetary systems: including TRAPPIST-1 and its seven worlds.

Working Out How Suns and Planets Form

Rene Descartes' illustration from 'The World, or Treatise on Light', Chapter eight On the Formation of the Sun and the Stars of the New World, Rene Descartes, (ca. ca. 1633-1664). Translated by Michael S. Mahoney (1979). via Princeton University, used w/o permission.Oversimplifying something fierce, around 1630 René Descartes said that the Solar System got started as a big swirling vortex. These days, we see Descartes’ vortex as an early version of the nebular hypothesis.

He figured the swirling vortex would explain the Sun’s rotation and the planets’ circular orbits. Then Newton wrote about gravity, which Descartes hadn’t covered. Not the way Newton had, at any rate.

Emmanuel Swedenborg, Immanuel Kant, Pierre-Simon Laplace and others either formed or tweaked nebular hypotheses for how the Solar System started.

Fast-forward to the 20th century.

In 1904 or maybe 1905, Forest Moulton and Thomas Chamberlin came up with the planetesimal hypothesis as a model for the Solar System’s formation.

They said that maybe, early on, another star had passed very close to the Sun. This would have raised tidal bulges on both stars. And that, along with supercharged solar prominences, would have left enough material orbiting the sun to form planets.

About three decades later, Raymond Lyttleton said pretty much the same thing: except that the other star collided with the Sun’s companion star, and that debris from the collision formed the planets.

Then in 1951, 1961 and 1981 three Swiss astronomers — never mind the names and don’t bother memorizing this, there won’t be a test — said maybe the Sun was spinning really fast at first, and blorped out material for the planets.2

Clashing Stars and Pulsar Planets

ALMA, C Brogan, B Saxton's image: 'A protoplanetary disc has formed around the young star HL Tau'. Jonathan Webb, BBC News (November 6, 2014) used w/o permission.One more ‘stellar collision’ idea, and I’ll move on.

In 1917, James Jeans said the Solar System happened when another star and the Sun almost ran into each other. Tidal effects would pull massive amounts of stuff out of both stars, which would then condense into planets. That was the idea, anyway.

Harold Jeffreys said that a near-miss like that was wildly unlikely, and Henry Norris Russel said the tidal model had problems with angular momentum. They were both right. But nebular hypotheses have angular momentum issues, too. Problem is, the Sun has 99%-plus of the Solar System’s mass, but only 2% of its angular momentum.

Starting in the 1930s, some scientists started sorting ‘how the Solar System formed’ ideas into categories. There were loads of ideas — I’ve been listing a selection — so they had no shortage of material.

Here’s an over-simplified version of the categories:

  1. The Sun formed first, then planets formed
    • Planets formed from outside dust and gas that just happened to come along
    • Another star hit the Sun and that’s where planet-stuff came from
      • And the Sun spit out planet-forming stuff
  2. The Sun and planets formed at pretty much the same time, from the same stuff

It wasn’t until the 1970s that option number 2 got traction again.

A version of the nebular hypothesis had been generally accepted by the end of the 1980s. But it didn’t explain why the Solar System’s planets don’t orbit in exactly the same plane.

And we’re still not sure how the Solar System’s double planet formed. Or whether Earth-Moon should be called a double planet.

I’m okay with the idea, but I’m not a scientist: and don’t have a career built partly on saying that the Moon is a satellite, not the other unit of a binary world.

Maybe I’m being unfair.

Anyway, in 1992 Aleksander Wolszczan and Dale Frail found planets orbiting the pulsar PSR B1257+12.3

Not Quite Like the Solar System: 55 Cancri’s Planets

NASA's diagram, comparing Cancri 55 planetary system and the Solar System's Earth and Jupiter. (2006)
Comparing Solar System’s Earth and Jupiter, 55 Cancri system. NASA (2006)

A little over three decades later, we’ve cataloged upwards of 5,000 exoplanets in nearly 4,000 planetary systems.

Of those, 855 systems have more than one known planet.

That’s the count as of April 1, 2023, at any rate: with thousands of unconfirmed worlds still in Kepler and TESS archives, waiting to be processed.

A few planetary systems look a bit like ours; like 55 Cancri, with its planets e (Janssen), b (Galileo), c (Brahe), f (Harriot) and d (Lipperhey).

Lipperhey is a gas giant, a bit more massive than Jupiter but orbiting about as far from 55 Cancri as Jupiter is from Sol. The others are closer to their sun, spaced very roughly like Mercury, Venus and Earth.

But Galileo, the second planet out from 55 Cancri, is nearly as massive as Jupiter.4

Kepler-90: Solar-Style Planetary Size Selection, With Compact Orbits

NASA/Ames Research Center/Wendy Stenzel's illustration, comparing sizes of Kepler-90 and Solar System planets. (December 14, 2017)Then there’s the Kepler-90 system. I talked about it last week.

Its eight planets are nicely arranged, with rocky worlds near the sun and gas giants farther out.

So far, this sounds like the planetary systems some of us were expecting. Or hoping for.

NASA/Ames Research Center/Wendy Stenzel's illustration: 'Kepler-90 Planets Orbit Close to Their Star'. Kepler=90 and Solar System orbits compared. (December 14, 2017)The orbit of Kepler-90’s most distant world is just barely bigger than Earth’s.

And, like I said, it’s about as big and massive as Jupiter.

That was a surprise.

But it wasn’t the first one in our study of exoplanets. I’m pretty sure it won’t be the last.

Before we started charting other planetary systems, assuming that they’d be pretty much like our Solar System made sense.

If there was more than maybe one other planetary system in this galaxy.

Thinking that the Solar System began with an extremely unlikely near-miss or collision with another star had two advantages, of a sort. It kept the study of other worlds close to home, and made our star one of the highly-exclusive ‘I’ve got planets’ club.

After 1992, that obviously wasn’t the case.

Then, after discovering planets around a pulsar, Astronomers figured they might find planets around other pulsars. They did, but not many.

After that first three-planet pulsar system, and after checking at least 800 pulsars, scientists found only four pulsars with a planet. One planet around each.5

I think a take-away here is that what’s discovered early-on isn’t necessarily typical.

A Profusion of Pea Pod Planets

Lauren Weiss/The Astronomical Journal's table from a California-Kepler study multi-planet systems with four planets or more. Architectures planetary systems. Each row corresponds to the planets around one and the circles represent the radii of planets in the system. Note how many have lines of planets that are roughly the same size. Inner Solar System is included for comparison. (January 3, 2018)By 2018, we’d been studying a fair number of multi-planet systems.

Many of the ones spotted by the Kepler space telescope followed a pattern. But it wasn’t the Solar System’s.

[text describing chart at right]
“The architectures of California-Kepler study multi-planet systems with four planets or more. Each row corresponds to the planets around one and the circles represent the radii of planets in the system. Note how many have lines of planets that are roughly the same size.
(Image credit:Lauren Weiss, The Astronomical Journal (January 3, 2018) via NASA News (July 13, 2018))

That chart is from a NASA News article:

The Architecture of Solar Systems
Marc Kaufman, NASA News (July 13, 2018)

In many systems, the planets were all roughly the same size as the planet in orbit next to them. (No tiny-Mars-to-gigantic-Jupiter transitions.) This kind of planetary architecture was not found everywhere but it was quite common — more common than random planet sizing would predict….

“…What’s more, [University of Montreal astronomer Lauren] Weiss and her colleagues found that the orbits of these ‘planets in a pod’ were generally an equal distance apart in ‘multi’ of three planets or more. In other words, the distance between the orbits of planet A and planet B was often the same distance as between the orbits of planet B and planet C….”

The NASA article talked about a “peas in a pod” paper in The Astronomical Journal, which is where they got that chart: a screen capture. I’ve put a link in the footnotes.

Kepler space telescope's viewing area. Each rectangle rectangles shows where one of Kepler's 95-megapixel charge-coupled device, or CCD cameras is pointed. Scientists selected these areas to avoid the region's brightest stars (the largest black dots).Since the “peas in a pod” study involved Kepler data, it didn’t include the TRAPPIST-1 planetary system. Kepler hadn’t been looking in that direction.

The ‘pea pod paper’ included a close look at 355 planetary systems.

If I’m counting them right, that chart shows the 51 systems where the star has a mass from 0.67 to 1.26 times the Sun’s and four or more planets. Make that 52, since it includes Sol, our star, a little shy of halfway down the list.

Compared to other multi-planet systems described in that 2018 paper, the Solar System looks like an oddball.

That’s one reason I’ll probably take a closer look at the ‘pea pod paper’ later on.

The Solar System may be well off the 50th percentile, but the TRAPPIST-1 planets follow the similar-size and close-orbits pattern. And that, finally, brings me to TRAPPIST-1 and its planets.6

TRAPPIST-1’s Pea Pod Planets

NASA/JPL-Caltech's illustration: Comparison of TRAPPIST-1 system and inner Solar System planets. 'A planet’s density is determined by its composition as well as its size: Gravity compresses the material a planet is made of, increasing the planet’s density. Uncompressed density adjusts for the effect of gravity and can reveal how the composition of various planets compare.' (January 22, 2021)
Comparing TRAPPIST-1’s worlds and Solar System’s inner planets. (2021)

ESO/IAU, Sky and Telescope's sky chart: the constellation Aquarius, with TRAPPIST-1's position marked with a red circle.The TRAPPIST-1 planetary system fits the ‘pea pod planets’ pattern.

I doubt that’ll be the pattern’s generally-accepted label, although scientists have gotten less stuffy about names and descriptions. And that’s yet another topic.

TRAPPIST-1 is in the constellation Aquarius, in the general direction of Phi Aquarii; but a whole lot closer: about 40 light-years out.

The star is an ultra-cool red dwarf, with a surface temperature of 2,566 kelvins, give or take 26.

That’s cool for a star, but still hot: around 2,840° centigrade or 4,650° Fahrenheit.

Bhutajata's illustration: 'Color emitted by a black body on a linear scale from 800 kelvins to 12200 kelvins, given by Planck's Law, assuming a monitor properly calibrated to sRGB color space - D65 white point - 2.2 gamma. Colors are out of gamut below 1934 K, so those have been desaturated to fit in sRGB color space (relative mapping)....' (October 11, 2015)By comparison, a candle flame’s luminous zone is around 1,200° to 1,500° centigrade. I talked about color temperature, stars, traffic lights and space art last week.

I also mentioned a 2017 paper that talked about TRAPPIST-1’s magnetic field, and how it might set up induction heating in the system’s three innermost worlds.

NASA/JPL-Caltech's illustration: TRAPPIST-1 and Solar planetary systems, TRAPPIST-1 system enlarged 25x. The green areas are the two stars' habitable zones, where liquid water could exist on an Earth-like planet. (2017)That’d be TRAPPIST-1 b, c, d, e, f, g and h.

Oddly enough, none of the TRAPPIST-1 planets have names yet. I don’t know whether that’s because or in spite of it being such a high-profile planetary system.

Getting myself back on-topic, the TRAPPIST-1 planetary system is arguably more orderly than ours.

The planets’ orbits are very nearly circular, and stay within 0.1° of the system’s ecliptic. The Solar System’s planets aren’t nearly as well-aligned, with orbits inclined up to seven degrees from Earth’s:7

  • Mercury 7.01°
  • Venus 3.39°
  • Earth 0°
  • Mars 1.85°
  • Jupiter 1.31°
  • Saturn 2.49°
  • Uranus 0.77°
  • Neptune 1.77°

Possible Interiors: 2021

NASA/JPL-Caltech's illustration: TRAPPIST-1 planets; possible interiors, based on precise measurement of their masses, radii and dynamics. 'Three possible interiors of the TRAPPIST-1 exoplanets. The more precisely scientists know the density of a planet, the more they can narrow down the range of possible interiors for that planet. All seven planets have very similar densities, so they likely have a similar compositions.' (January 22, 2021)
Three possible interiors for TRAPPIST-1 planets (2021).

Recapping, the TRAPPIST-1 system’s orbits are all basically in the same plane. That plane passes through the Solar System, so the planets regularly pass across the face of their sun from our viewpoint.

This lets astronomers measure the TRAPPIST-1 planets’ size and orbits. And that lets them work out their masses and densities.

Or, rather, their approximate masses and densities. Margins of error get smaller as astronomers get more and more precise data. So I’m hoping we’ll see an update to topics covered by a 2021 paper.

The 7 Rocky TRAPPIST-1 Planets May Be Made of Similar Stuff
Calla Cofield, Jet Propulsion Laboratory, Pasadena, California; Exoplanets, Feature, NASA (January 22, 2021; updated January 25, 2021) Tony Greicius, editor
“Precise measurements reveal that the exoplanets have remarkably similar densities, which provides clues about their composition.

“The red dwarf star TRAPPIST-1 is home to the largest group of roughly Earth-size planets ever found in a single stellar system. Located about 40 light-years away, these seven rocky siblings provide an example of the tremendous variety of planetary systems that likely fill the universe….”

“…A new study published today in the Planetary Science Journal shows that the TRAPPIST-1 planets have remarkably similar densities. That could mean they all contain about the same ratio of materials thought to compose most rocky planets, like iron, oxygen, magnesium, and silicon. But if this is the case, that ratio must be notably different than Earth’s: The TRAPPIST-1 planets are about 8% less dense than they would be if they had the same makeup as our home planet. Based on that conclusion, the paper authors hypothesized a few different mixtures of ingredients could give the TRAPPIST-1 planets the measured density….”

That article’s “today” was January 22, 2021; so if there hasn’t been more exact data gathered yet, there soon will be. But that article in The Planetary Journal is the most recent I’ve found, so I’ll run with that. Plus whatever I find elsewhere.

Briefly, since it’s Friday afternoon as I write this.

First, about the three possible interiors shown in that illustration:

“Three possible interiors of the TRAPPIST-1 exoplanets. The more precisely scientists know the density of a planet, the more they can narrow down the range of possible interiors for that planet. All seven planets have very similar densities, so they likely have a similar compositions. (NASA/JPL-Caltech (January 22, 2021))”

Next, what the scientists said about looking forward to more precise data:

“…We forecast JWST timing observations and speculate on possible implications of the planet densities for the formation, migration, and evolution of the planet system.”
Refining the Transit-timing and Photometric Analysis of TRAPPIST-1: Masses, Radii, Densities, Dynamics, and Ephemerides“, Summary; Eric Agol et al., The Planetary Science Journal (January 22, 2021) via

Although the TRAPPIST-1 planets are all nearly the same density, they’re not identical.

TRAPPIST-1c, for example might be a rocky world with a Venus-like atmosphere; TRAPPIST-1d might be covered in a very deep ocean while TRAPPIST-1e could have been all iron and rock.

Those informed speculations made sense in 2018. Like I’ve said, we keep getting better data, which lets us fine-tune what we know.

Maybe at least some of the TRAPPIST-1 planets have a mantle and core, although not as proportionately large a core as Earth’s. If so, they might have their own significant magnetic fields. How their fields would interact with their sun’s? That, I don’t know.

Which reminds me. TRAPPIST-1 has a strong magnetic field: with a mean strength of about 600 gauss. That’s a lot stronger than the Sun’s overall 10 gauss.8

Living in “a Grain From a Balance”, and Admiring the View

Hubble/ESA's image: NGC 4848 and other galaxies. Text: Wisdom 11:22.

Maybe, between lower-than-Earth density, stronger-than-Solar magnetic fields and being (probably) tidally locked, with one face always facing their sun, none of the TRAPPIST-1 planets are habitable.

It’s even possible, based on what we know today, that none of them have more than a close-to-vacuum atmosphere.

But, also based on what we know today, some of them might.

Not TRAPPIST-1 b, though. Observations from the Webb telescope showed that the system’s innermost world is hot and airless.9

But much as I’d like us to find another world with life, and people, I won’t insist that we must have neighbors. Or that we must not. Either way, it’s not my decision.

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

Part of my job is admiring the view. I’ve talked about that before:

1 Ideas, newfangled and otherwise:

2 Three and nine tenths centuries of informed speculation:

3 Goodby clashning stars, hello nebular hypothesis (again):

4 Solar System analog, sort of:

5 A sort-of-Solar size selection, and pulsar planets:

6 Pea pod planet patterns:

7 TRAPPIST-1 planetary system, mostly:

8 Magnetism, mass and Mercury:

9 TRAPPIST-1 b, a recent status update:

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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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