“What is this fascination of yours with small red stars?” a friend asked in a recent lunch encounter, having seen something I wrote a few years back about TRAPPIST-1 in one of his annual delvings into the site. “They’re nothing like the Sun, to quote Shakespeare, and anyway, even if they have planets, they can’t support life. Right?”
Hmmm. The last question is about as open as a question can get. But my friend is on to something, at least in terms of the way most people think about exoplanets. My fascination with small red stars is precisely their difference from our familiar G-class star. An M-dwarf planet bearing life would be truly exotic, in an orbit lasting mere days rather than months (depending on the class of M-dwarf), and perhaps tidally locked, so inhabitants would see their star fixed in the sky. How science fictional can you get? And we certainly don’t have enough data to make the call on life around any of them.
Let’s talk a minute about how we classify small red stars, because this bears on the interesting project called SPECULOOS and its latest discovery that I want to get into today. SPECULOOS is of course an acronym (Search for Planets EClipsing ULtra-cOOl Stars), but in parts of northern Europe and especially Belgium it’s a word that conjures up the spiced shortcrust biscuits that are traditional on St. Nicholas’ Day (December 6). It’s always good to have something baking while you’re parsing exoplanet data.
The scientific parameters for SPECULOOS involve a transit search of the 40 parsecs nearest Earth to study the 1650 or so very low-mass stars and brown dwarfs found within this volume. Of note today is that category of stars known as ultracool dwarfs (UDS). Some 900, more or less, of these are found here in spectral types M6.5 to L2, the former being M-class dwarfs at the low end of the temperature range, the latter being even cooler than the M-dwarfs but at the high-end of the L range. We’re talking about stars with a mass between 0.07 and 0.1 solar masses and sizes not far from Jupiter’s.
I’ll send you to today’s paper for further details on the robotic, and international, network of observatories that make up SPECULOOS, and mention in passing that the remarkable TRAPPIST-1, with its seven Earth-sized transiting planets, was the network’s first discovery. A recent super-Earth has also been announced around the star LP 890-9, but the latest find, dubbed SPECULOOS-3b, orbiting an M6.5 dwarf some 16.75 parsecs out, merits special attention. This one has useful implications for our studies of exoplanet atmospheres and, as the authors point out, should be a prime target for the James Webb Space Telescope. The paper notes that “The planet’s high irradiation (16 times that of Earth) combined with the infrared luminosity and Jupiter-like size of its host star make it one of the most promising rocky exoplanets for detailed emission spectroscopy characterization.”
SPECULOOS-3 turns out to be the second-smallest main-sequence star found to host a transiting planet (it’s just a bit larger than TRAPPIST-1). The tiny host provides an excellent transit depth for detecting the Earth-sized planet. While its mass has not yet been determined, the likelihood is that it is a rocky world (all planets known to be Earth-sized in the NASA exoplanet archive have masses that imply a rocky composition). Making the definitive call will involve analyzing its composition, which would include Doppler studies and a relatively short observing program that the authors describe in the paper.
But another kind of investigation makes this find significant. Beyond radial velocity methods, we can put emission spectroscopy to work by measuring the combined light of star and planet just before the planet goes behind the star (secondary eclipse), and the star’s light just after it does so, using JWST’s Mid-InfraRed Low-Resolution Spectrometer (MIRI/LRS). The difference between the two yields the light emitted by the planet. Note the difference here from transmission spectroscopy, which examines the star’s light as it passes through the planet’s atmosphere. Emission spectroscopy is preferable here, as the paper explains:
…the interpretation of emission spectra is not dependent on the mass of the planet. Secondly, emission spectra provide the energy budget of the planet, which is essential to understand its atmosphere’s chemistry, its dynamics and can be used to constrain the planet’s albedo. Finally, in the absence of an atmosphere, emission spectroscopy instead directly accesses the planetary surface where its mineralogy can be studied, something impossible to achieve with transmission spectroscopy. For all these reasons, emission spectroscopy is a more reliable method to assess the presence of an atmosphere and study the nature of terrestrial planets around UDS. And… SPECULOOS-3 b is one of the smallest terrestrial planets that is within reach of the JWST in emission spectroscopy with MIRI/LRS.
Image: Emission spectroscopy, the secondary eclipse method, measures changes in the total infrared light from a star system as its planet transits behind the star, vanishing from our Earthly point of view. The dip in observed light can then be attributed to the planet alone. The spectrum is taken first with star and planet together, and then, as the planet disappears from view, a spectrum of just the star (second panel). By subtracting the star’s spectrum from the combined spectrum of the star plus the planet, it is possible to get the spectrum for just the planet (third panel). Credit: NASA/JPL-Caltech/R. Hurt (SSC/Caltech).
And here’s the transmission method:
Image: This is a transmission spectrum of an Earth-like exoplanet. The graph, based on a simulation, shows what starlight looks like as it passes through the atmosphere of an Earth-like exoplanet. As the exoplanet moves in front of the star, some of the starlight is absorbed by the gas in that exoplanet’s atmosphere and some is transmitted through it. Each element or molecule in the atmosphere’s gas absorbs light at a very specific pattern of wavelengths. This creates a spectrum with dips that show where the wavelengths of light are absorbed, as seen in the graph. Each dip is like a “signature” of that element or molecule. Credit: NASA, ESA, CSA, STScI, Joseph Olmsted (STScI).
As to my friend’s speculations about habitability, we can keep SPECULOOS-3b out of the mix, at least judging from its equilibrium temperature of 553 K, which works out to roughly 280°C or 535°F. Granted, we can speculate about extremophilic life or subsurface habitats, but there’s almost no point in doing that without reams of data that we do not yet possess. SPECULOOS-2b would be a better bet, being in the habitable zone of its M6-dwarf host, but there we have to bear in mind that the planet is a super-Earth. I think the question of life is a bit misplaced in the study of these dim stars. What we first have to find out is how accurately we can assess them with tools like JWST and its successors, and then begin cataloging the data. SPECULOOS-3b looks to be an early testing ground for what that future will bring.
The paper is Gillon et al., “Detection of an Earth-sized exoplanet orbiting the nearby ultracool dwarf star SPECULOOS-3,” for which the preprint is now available. I also want to give a nod to the TESS discovery of a planet transiting an M2.5 dwarf that is roughly Mars-sized. Quite a catch! The discovery paper of that one is Tey et al., “GJ 238 b: A 0.57 Earth Radius Planet Orbiting an M2.5 Dwarf Star at 15.2 pc,” Astronomical Journal Volume 167, Issue 6 (June, 2024), id.283, 13 pp. Abstract / Preprint.
The illustrations you reproduce for this piece are outsatnding!
Thank you, sir! Glad they proved useful.
Absolutely. I like that Earthlike Exoplanet Transmission Spectrum diagram. Direct imaging spectroscopy will certainly give us a solid rock chemical composition and also an absorption emission spectrum if there is any. According to open AI Chat GPT, the jury is still out on the exoplanet atmospheres of the Trappist 1 system as of mid 2024. “There have been detections of flat transmission spectra for some planets, which might indicate the absence of a substantial atmosphere or the presence of high-altitude clouds or hazes that obscure atmospheric features.” Chat GPtT
Maybe more attempts with the JWST spectrometer might be more successful.
Paul, please tell your inquiring friend that one should not always assume that any ETI in a particular system were evolutionarily native to those worlds. They could be visitors from other star systems.
Red dwarf systems may be especially attractive to such interstellar societies because of their lack of native life forms, eliminating one potential roadblock for settlement and mining resources.
Well said, Larry. The issue did come up recently with that BLC1 ‘signal’ from Proxima, after all. We can always ponder ancient home planets for cultures that spread throughout the Orion Arm, etc.
There are so many studies which exploit planetary transits of stellar disks that we tend to forget just how many planets must be out there. For the plane of an exosystem to be placed in such a way we can observe transits is a happy geometrical accident. The fact they are so common tells us that planets are very common around stars, even though we can’t see eclipses for the vast majority of them.
There are lots of stars in the galaxy but the number of planets orbiting them must greatly exceed the number of stars. Even if only a tiny fraction of them can support life, the number of potential life bearing worlds (and satellites, too!) must be truly enormous.
Its a comforting thought for those of us engaged in this business. The evidence for the ubiquity of life is circumstantial, but there is every reason to believe that life will arise wherever conditions are suitable. Even if we rule out exotic forms of alternate chemical biology, our familiar carbon-water life must be rather common in the cosmos.
The evidence for complex multicellular life is much less convincing, as is the possibility of long-lived tool-making, species capable of a high physics-based technology. But so far it appears we are on the right track.
I am appalled that so many of our friends and colleagues are convinced that astrobiology, and especially, the search for extra-terrestrial intelligence, is a waste of time. I can’t think of any better way to spend it.
Actually there could be a plethora of variant templates in just the C – H₂O system.
As with xeno nucleic acids and also xenobiology.
There may very well be alternate forms of biochemistry in the cosmos.
And there may be other ways nature can evolve complex structures capable of metabolism and other characteristics we have come to associate with life. These alternate biologies may be possible, but that does not necessarily mean they exist in nature, or if they do exist, that they do so in significant numbers.
If I am a carbon-water chauvinist it is because the chemical precursors to life we have so far detected in space are the ones we associate with our own planet and biology. We also know water/carbon life appeared to establish itself very early in Earth’s history, and has managed to flourish here for a very long time in a variety of environments. That suggests this is the type of biology we can expect to arise spontaneously wherever conditions allow, and which will be able to evolve to meet the challenges of changing conditions. The evidence is circumstantial, but nonetheless compelling.
Other types of metabolisms may indeed exist, and that would certainly expand the possibilities for life. But we need not rely on that possibility to
be confident that life is out there, and that it is common.
Now, as for the possibility of tool-using intelligence, especially one capable of being detected over interstellar distances, that is another question altogether. We have evidence of only one. Although I am skeptical of that possibility, I cannot rule it out either. That’s why I follow this discipline.
Henry Cordova said on July 31, 2024 at 10:47:
“I am appalled that so many of our friends and colleagues are convinced that astrobiology, and especially, the search for extra-terrestrial intelligence, is a waste of time. I can’t think of any better way to spend it.”
If it isn’t outright ignorance on the topic and its related fields, then it is due to fear of the unknown and related reactions. While things have improved in the past few decades, there is still something of a stigma regarding alien life, especially if it is intelligent.
This will all change the moment real ETI are found. Those who dismissed it in the past will suddenly claim they supported it all along, of course. However, we already know who the real brave and forward-thinking folks are when it comes to life elsewhere.
“While things have improved in the past few decades, there is still something of a stigma regarding alien life, especially if it is intelligent.”
It is hard (and meaningless) to build a hypothesis that you cannot test nor confirm through observation.
@Tesh
That’s a bit harsh, isn’t it?
The hypothesis can be tested and confirmed through observations. For centuries we suspected the existence of atoms, but did not have the technical means to demonstrate their existence. The same can be said for the germ theory of disease, or Darwinian evolution. In either case, the question was settled when the required technology became available. And that is what we are working on here right now!
I personally believe life is common throughout the universe, and intelligent life is extremely rare. But we may never prove either of those statements conclusively, for obvious regions, as you point out. The existence of extra terrestrial life can be demonstrated with technology we now have available, extraterrestrial intelligence has not been proven, but it certainly CAN be proven, providing certain reasonable conditions can be satisfied..
Astrobiology is hard, but not impossible.
But that doesn’t mean we should abandon the search, or give up on developing methodologies to detect either. Think of it as thought experiments: They’re fun, they’re cheap, and the probability of success is non-zero. And the benefits, if we do succeed, are potentially incalculable.
I like to think that the search for life (or intelligence) in the cosmos is comparable to proving (or disproving) the existence of God. But in the case of the Deity, there is no physical experiment or observation that could settle the question (unless God Himself chooses to cooperate).
Astrobiologists are often accused by the philosophically inclined as substituting ET for the Creator. Well, what’ s wrong with that? At least with the extra terrestrials, we have some chance of success.
The questions of astrobiology are open to rational inquiry and scientific method. We can answer them with telescopes and spacecraft. No miraculous violation of natural law as we know it is required. It is a perfectly valid, and thoroughly legitimate field of inquiry. It doesn’t require a guaranteed resolution delivered in a finite time.
@Henry
I heartily concur with your comment.
I do have a nit with this:
Darwin used artificial selection, e.g. pigeons, as demonstrable proof that organisms can be changed by selective breeding. As humans have been both accidentally and deliberately artificially selecting both plants and animals for millennia, the mechanism of evolution by selection was always evident. Darwin was able to use this, plus his observations, to put together his theory of evolution by natural selection. Later, “genes” were inferred by breeding experiments and then directly by genome sequencing after the structure of DNA was elucidated.
I don’t think there is anything comparable for inferring life is ubiquitous, although Paul Davies’ suggestion of a “shadow biosphere” would increase the probability if it was proven. If we found Martian life, particularly if its biology was different to terrestrial biology, that would strengthen the likelihood that life is ubiquitous. If we stumbled upon a non-terrestrial artifact in our system, perhaps on the Moon, this would certainly be proof that ETI has existed. If we don’t look, we will never know.
While we engage in deliberate search, I expect serendipity while making very different observations might lead to clues to the hunt. SETI is leveraging astronomy by piggybacking on data to look for signals in various parts of the spectrum.
“That’s a bit harsh, isn’t it?”
Agreed and apologies. I posted it and immediately saw if came across as such.
I agree that life is probably common in space but not possibly in time, with the emergence of intelligent life skewing those poles likely even further.
I guess my barb as such was a poor way of agreeing and supporting the “why these studies are/have been frowned upon”. Things will change but I feel like we need a breakthrough on many fronts – it may not be that far off.
Though I think that life is common, how common may be answered once we get to Mars. If there is nothing there (past or present) I would say that we will like die alone.
@Henry : “The existence of extra terrestrial life can be demonstrated with technology we now have available”
…and therefore the importance of always clearly specifying in our speculations : “in the current state of our knowledge and technology”.
Hi Paul
I share your interest and passion for these stars too.
This is another interesting star and planet.
I wounder if there are further planets in this system?
Cheers Edwin
The friend you met up with wouldn’t be named, “Andre” by any chance, would he!?
Either way sounds like it was a fascinating dinner discussion!
No, not Andre. He’s a very good guy, I’ll add, but one with only a glancing interest in astronomy. He likes to catch up now and then hoping I’ll have something interesting to tell him, and then we don’t meet again for a year or so.
Contrary to the idea of creating hypotheses to test, this conversational gambit seems more intent on shutting down the search to test the hypothesis that M_Dwarf exoplanets do not support life. The same applies to arguing about ETI. It is one thing to suggest that any ETIs are [totally] separated from us in both time and space, never to be detected, but that should not preclude searching for ETI, through em signals and other technosignatures. If we ever become starfaring explorers, we should continue to look for evidence of extant, quiet ETI, technological artifact ruins, and even geologic evidence of their presence in the past.
Just 2 posts ago, the far more positive approach was Chris Lintott’s “Accidental Astronomy” which emphasized search which almost always finds new phenomena with each new instrument. Galilleo’s telescope perhaps being the first.
This article indicates that M_Dwarf planets are ideal targets for emission spectroscopy that could indicate the conditions and composition of the atmosphere or surface, which is a good example of a search. If water was detected on such a planet, and temperatures suggested it could be liquid, that would increase the probability that life could be present and detectable.
I would note that even my skepticism for life in the Europan and Enceladen subsurface oceans [ not unlike your friend’s suggestion that M_Dwarf planets are sterile] does not mean that I would not want to bother searching, initially with probes sampling the plumes, to later landers that can drill down through the ice crust and explore the ocean below. It is expensive today, especially with the technology we have, but still well worth carrying out as any discovery of life would be momentous and have important implications.
We can speculate and argue endlessly whether or not M_Dwarfs are suitable stars for habitable and inhabited planets. but there is no substitute for looking. And as LJK notes in a comment here, life need not be native. Furthermore, ETI need not be biological. Searching with an open mind is key to new discoveries. As Isaac Asimov once said “The most exciting phrase to hear in science, the one that heralds new discoveries, is not ‘Eureka!’ but ‘That’s funny…’”
For M dwarf planets, where tidally locked, I might not even look to stellar flux HZ for a hard floor on temperate-temperature. Such a planet might be temperate on the sun-facing side or temperate on some band on the dark side.
I feel a better criterion has been raised Anthony S. Atkinson, David Alexander, and Alison O. Farrish; “Exploring the Effects of Stellar Magnetism on the Potential Habitability of Exoplanets” 9 July 2024: the stellar Alfven recombination radius. Beneath that, the planet is at the mercy of the stellar radiation, and likely (especially if tidally locked) has insufficient dynamo to protect it.
A very good point there David.
A large moon on a rather close orbit, essentially a double planet, could have kept the planet spinning and the internal dynamo running. But such a pair would not be dynamically stable at an M star with several planets on tight orbits. Not even if the planet have no close planetary neighbours. The same tidal forces that cause bound rotation will disrupt the pair over time.
While most such planets will have no atmosphere, such as the two innermost in the Trappist system, there still seem to be some exceptions.
Gliese 1132 b appear to be one such, which while being close in and hot, somehow have managed to get a secondary atmosphere after the one from the formation had been lost.
And while many M-class star is thought to have a wild youth, this give some small hope for worlds around the stars that have quieted down.
So the 3 planets in the habitable zone of Teegarden’s Star (where the new one at 0,5 M🜨 in a 7 day orbit is not confirmed, but the data look promising.) Really should be looked into, while it’s not a system where the planets transit, the distance of ~12,5 ly could be an advantage. And we finally could scrap the ‘earth similarity index’ which gives a false impression on the habitability of exoplanets. By adding details like if the planet in question got any atmosphere at all, and if so the composition of gases are such that there’s even a remote chance that anything might be found.