TRAPPIST-1 b Measured by Webb: Hot, Airless

NASA/JPL-Caltech/M. Gillon (Univ. of Lige, Belgium), Animator Amy Moran (GST) [Lead]'s image: TRAPPIST-1 planetary system and infrared observations from the Spitzer Space Telescope IRAC. (released March 22, 2017)
TRAPPIST-1 exoplanets: (left) infrared observations from the Spitzer Space Telescope IRAC’
(right) illustration of the TRAPPIST-1 planetary system’s orbits. (NASA/JPL-Caltech/et al. 2017)

The TRAPPIST-1 planetary system is news again, this time because we’ve taken the innermost planet’s temperature.

That, by itself, isn’t newsworthy. We’ve been using infrared observations to learn how hot exoplanets are at least since 2006.1

What makes the latest observations special is that they’re the first time scientists have measured a comparatively small, cool exoplanet’s temperature.

That’s what I’ll be talking about this week, along with whatever else comes to mind.

Top Three Multiplanetary Systems

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)
TRAPPIST-1 planetary system, enlarged 25x, and inner Solar System.
Green areas are the two star’s habitable zones, where liquid water could exist on an Earth-like planet.

This week’s news is a big deal partly because it involves the TRAPPIST-1 planetary system. Only two planetary systems have more planets than TRAPPIST-1.2

Solar System

ESO/O. Furtak's illustration: comparing the orbits TRAPPIST-1's planets with the Galilean moons of Jupiter and the inner Solar System. (February 22, 2017) see of them is our Solar System, with eight confirmed planets. We’ve got 10, if you count Ceres and Pluto.

Ceres was called a planet after its discovery in 1801, partly because it was where Johann Bode said a planet should be.

By the 1860s, astronomers had spotted Pallas and other asteroids — Herschell coined “asteroid”, “star-like”, as a generic term — and they’d decided that asteroids weren’t like the other planets.

Then the IAU (International Astronomical Union) defined “planet” as something that orbits the Sun, is massive enough to be nearly round, and has “cleared the neighborhood” around its orbit.

That was in 2006. The IAU definition reclassified Ceres and Pluto as dwarf planets.

The IAU’s definition intentionally doesn’t include exoplanets, but the IUGS (International Union of Geological Sciences) definitions do. There’s no IUGS official definition of planet. They’ve got several unofficial ones, plus the IAU definition.

Complicated? Yes. And I’m pretty sure we’re nowhere near the end of these discussions.

Now, back to TRAPPIST-1’s planetary system. Three or four of TRAPPIST-1’s planets lie in its habitable zone: e, f and g; or maybe d, e, f and g.

I’ve read that using very optimistic definitions, and applying them to limited parts of the planets, all seven might have liquid water and air somewhere on or near their surfaces.

The Solar System’s habitable zone includes one to four planets, depending on which definitions are in play: Earth, or Venus, Earth, Mars and Ceres.3

Kepler-90 Planetary System, Upsilon Andromedae d and back to TRAPPIST-1

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)
Wendy Stenzel’s illustration, comparing Kepler-90 and Solar System orbits. (2017)
NASA/Ames Research Center/Wendy Stenzel's illustration, comparing sizes of Kepler-90 and Solar System planets. (December 14, 2017)
Wendy Stenzel’s illustration, comparing Kepler-90 and Solar System planet sizes. (2017)

The other planetary system with eight confirmed planets is Kepler-90’s.

Kepler-90 is a main-sequence G or F star that’s about 2,550 or 2,800 light-years out, in the general direction of Gamma Draconis and Kappa Cygni.

Its distance depends on who’s talking, which may account for some sources saying it’s spectral class G and others class F. Kepler-90’s surface temperature is just over 6,000 K, the dividing line between spectral class G and F stars.

The Kepler-90 planetary system is just like the Solar System, except for how it’s different. The inner Kepler-90 worlds are small and presumably rocky, with gas giants farther out. On the other hand, all eight Kepler-90 planets have orbits smaller than Earth’s.

The star’s outermost known planet, Kepler-90 h, is in the system’s habitable zone. But since it’s a gas giant, with a mass and diameter a bit bigger than Jupiter’s, it’s not a place where we’ll be looking for life.

If Kepler-90 h has a large, rocky moon; that’s another story. Some scientists figure an exomoon might share a magnetic field with its planet, like Earth’s moon did at one point.

Then there’s Upsilon Andromedae d, a super-Jupiter in an odd-but-maybe-habitable orbit. And that’s another topic.

Getting back to TRAPPIST-1 and its planets, the inner worlds (b, c and d) may not be as habitable as they seemed at first glance.

In 2017, some scientists said that TRAPPIST-1’s magnetic field might set up induction heating in the three innermost worlds.4

In any case, they’re on the starside of the the habitable zone, and that finally brings me back to TRAPPIST-1 and this week’s news.

Taking TRAPPIST-1 b’s Temperature With Webb’s MIRI

NASA/ESA/CSA/Joseph Olmsted (STScI)'s illustration (Science by Thomas P. Greene (NASA Ames), Taylor Bell (BAERI), Elsa Ducrot (CEA), Pierre-Olivier Lagage (CEA)): comparing TRAPPIST-1b's dayside temperature (measured using JWST Mid-Infrared Instrument (MIRI)) to computer models. This illustration shows what the temperature would be under various conditions. The temperature of the dayside of Mercury is also shown for reference. (March 27, 2023) see
J. Olmsted (STScI)’s illustration, showing TRAPPIST-1b’s dayside temperature. (2023)

Calling TRAPPIST-1b “small and … cool” is accurate, since they’re both comparative terms.

TRAPPIST-1b is smaller and cooler than hot gas giants like KELT-9b and WASP-39b, and much cooler than WD 1145+017 b, a planet that’s being vaporized by its star.5

NASA’s Webb Measures the Temperature of a Rocky Exoplanet
Laura Betz, NASA’s Goddard Space Flight Center; Margaret Carruthers, Christine Pulliam, Space Telescope Science Institute(STSI); Solar System and Beyond, NASA (March 27, 2023)

“An international team of researchers has used NASA’s James Webb Space Telescope to measure the temperature of the rocky exoplanet TRAPPIST-1 b. The measurement is based on the planet’s thermal emission: heat energy given off in the form of infrared light detected by Webb’s Mid-Infrared Instrument (MIRI). The result indicates that the planet’s dayside has a temperature of about 500 kelvins (roughly 450 degrees Fahrenheit) and suggests that it has no significant atmosphere.

This is the first detection of any form of light emitted by an exoplanet as small and as cool as the rocky planets in our own solar system. The result marks an important step in determining whether planets orbiting small active stars like TRAPPIST-1 can sustain atmospheres needed to support life. It also bodes well for Webb’s ability to characterize temperate, Earth-sized exoplanets using MIRI….
[emphasis mine]

“…The team used a technique called secondary eclipse photometry, in which MIRI measured the change in brightness from the system as the planet moved behind the star….”

TRAPPIST-1 b is the system’s innermost planet: orbiting only 1,726,000 kilometers 1,072,500 miles, from TRAPPIST-1. Although TRAPPIST-1 is a very cool star, nobody expected the inner planet to be Earth-like in terms of temperature.

It’s quite Earth-like in other ways, though. Its diameter is around 1⅛ times our world’s, with a mass of about 1.37 Earths and about 110% Earth’s surface gravity.

But even before the new Webb data, we knew TRAPPIST-1 b wasn’t going to be Earth 2.0. An old equilibrium temperature estimate for the planet was 397 kelvins, 256° Fahrenheit.

I’m not sure how the newly-measured dayside temperature, around 500 kelvins, 227° Centigrade, 450° Fahrenheit, fits with the 2017 induction heating model. It’s hot, but not as hot as Mercury’s high temperature: 700 kelvins, 427° Centigrade, 800° F.

A couple more points before I start talking about stars, art and traffic lights.

This phrase, “…as cool as the rocky planets in our own solar system…”, jumped out at me as an example of how much we’ve learned in the last few decades. It hasn’t been all that long since Mercury and Venus ranked as very hot planets.

If TRAPPIST-1 b had a significant atmosphere, more than the wisp that Mercury has, it’d have winds: which would carry heat to its nightside, cooling the daylight part. Since that’s not (apparently) happening, the odds are very good that TRAPPIST-1 b is airless.6

Blackbody Radiation, Red Stars and Astronomical Art

NASA/ESA/CSA/J. Olmsted (STScI)'s illustration: TRAPPIST-1 b, innermost of seven known planets in the TRAPPIST-1 system, orbiting at a distance of 0.011 AU, completing one circuit in just 1.51 Earth-days. (March 27, 2023)
Artist’s illustration of TRAPPIST-1 b, orbiting an ultra-cool red dwarf star.

“Blackbody radiation” shows up pretty often when scientists talk about something that’s very hot.

The term makes sense, sort of, since it’s the spectral radiance curve of thermal radiation emitted by a black body. A black body, in this context, is an ideal object that perfectly absorbs all electromagnetic radiation.

And this makes sense???

Look, it’s Friday as I write this: it’s getting late, and here’s a sample of the academese I’m trying to translate into English:

“…The specific (radiative) intensity is a quantity that describes the rate of radiative transfer of energy at P1, a point of space with coordinates x, at time t. It is a scalar-valued function of four variables, customarily written as
I (x, t ; r1, ν)…”
(Spectral radiance, Wikipedia)

Please bear with me. This actually does relate to TRAPPIST-1 and space art.

Thermal Radiation and the Ultraviolet Catastrophe!

Darth Kule's illustration: 'Black body spectral radiance curves for various temperatures after Planck, and comparison with the classical theory of Rayleigh-Jeans'. (June 10, 2010)Blackbody radiation and black bodies, the sort physicists talk about, don’t really exist.

A black body perfectly absorbs all electromagnetic radiation.

Electromagnetic radiation is what physics buffs call waves in an electromagnetic field: from low-end radio wavelengths measured in megameters to gamma rays with wavelengths of about a picometer. I’ll call it Em radiation to save space.

A helium atom is about 62 picometers across, a megameter is a thousand kilometers. We’re talking about extremes here.

Thermal radiation is Em radiation that stuff emits when it’s above absolute zero.

The hotter something is, the more thermal radiation it emits; and the shorter the peak of its spectral radiance curve.

Knew I forgot something. A spectral radiance curve is what you get when you put a continuous spectrum on a graph with intensity along one axis and wavelength on another.

Back in 1900, someone using classical physics crunched numbers for thermal radiation. The numbers showed that something that’s as hot as the sun would emit an infinite amount of energy in the shorter wavelengths.

We’re still here, so obviously there was something off about classical physics.

Theoretical physicist Paul Ehrenfest called this doesn’t-match-observations thing the “ultraviolet catastrophe” in 1911.

“Ultraviolet Catastrophe” might make a dandy title for a disaster film, if more folks knew their history and science. And that’s yet another topic.

Classical physics is still a pretty close fit with reality for many everyday phenomena.

Folks like Ehrenfest, Plank and others you don’t hear about every day developed quantum mechanics,7 and I’d better start talking about heat, colors and stars.

Star Light, Star Not-So-Bright

NASA/JPL-Caltech/T. Pyle (IPAC)'s illustration: a possible surface of Trappist-1f (February 22, 2017) see ssc2017-01c
T. Pyle (IPAC)’s illustration, showing what TRAPPIST-1f’s surface might look like. (2017)
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)
Bhutajata’s illustration: colors emitted by a black body, from 800 to 12,200 kelvins. (2015)

Darth Kule's illustration: 'Black body spectral radiance curves for various temperatures after Planck, and comparison with the classical theory of Rayleigh-Jeans'. (June 10, 2010)Red dwarf stars are main sequence stars with surface temperatures between 2,400 and 3,700 kelvins.

That’s a lot cooler than our star’s 5,770 kelvins, so red dwarfs emit much of their energy in the infrared.

And they would look redder than our sun. Redder? More reddish? Never mind.

But I’m not convinced that even an ultra-cool red dwarf like TRAPPIST-1 — 2,566 kelvins at its surface, give or take 26 — would have the stoplight-red color its given in some astronomical art.

Some illustrations of TRAPPIST-1’s planetary system have the star looking like a traffic light, while others give it the color of a low-watt incandescent light bulb. Why? That, I don’t know.

Maybe the ‘cosmic traffic light’ illustrations happened because someone told the artist to make sure that the star looks red: so that viewers will realize it’s a “red” dwarf star.

Given a choice, I’d show TRAPPIST-1 the way it would look to someone standing on one of its planets, or in a visiting ship. Something like T. Pyle’s 2017 illustration.

We don’t know whether there’s an atmosphere or water on any of the planets. But we do know that TRAPPIST-1 is about as hot as a slightly-dim incandescent bulb’s filament.8

I figure it’d have about the same color.

Coming Next Week: Possible Interiors of TRAPPIST-1’s Planets

From ESO/M. Kornmesser/, via, used w/o permission: 'An artist's impression of the view from a planet in the TRAPPIST-1 system.'And that’s all I have time for this week.

One thing I’d meant to cover was informed speculation about what TRAPPIST-1’s planets are made of.

So unless something major comes up (yes, I have been reading headlines: but don’t have anything new to say about the usual mess), I’ll probably have another ‘TRAPPIST-1’ post ready for next week.

That’s the plan, anyway. Meanwhile, here’s the usual set of links:

1 Infrared astronomy:

2 Pluralities of planets:

3 Planets, asteroids and definitions:

NASA, ESA, and A. Feild (STScI). Artist's depiction of Upsilon Andromedae planetary system. (2010)4 Moons, magnetic fields and more:

5 Comparative syntax (I’m a recovering English teacher), and planetary extremes:

6 James Webb Space Telescope (JWST), infrared astronomy and planets:

7 More than you need or may want to know about:

8 Colors, temperatures and stars:

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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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