I’m collecting a number of documents on gravitational wave detection and unusual concepts regarding their use by advanced civilizations. It’s going to take a while for me to go through all these, but as I mentioned in the last post, I plan to zero in on the intriguing notion that civilizations with abilities far beyond our own might use gravitational waves rather than the electromagnetic spectrum to serve as the backbone of their communication system. It’s a science fictional concept for sure, though there may be ways it could be imagined for a sufficiently advanced culture.
For today, though, let’s look at a new survey that targets highly unusual planets. Binaries Escorted by Orbiting Planets has an acronym I can get into: BEBOP. It awakens the Charlie Parker in me; I can almost smell the smoky air of a mid-20th century jazz club and hear the clinking of glasses above Parker’s stunning alto work. I was thinking about the great sax player because I had just watched, for about the fifth time, Clint Eastwood’s superb 1988 film Bird, whose soundtrack is, of course, fabulous.
On the astronomy front, the BEBOP survey is a radial velocity sweep for circumbinary planets, those intriguing worlds, rare but definitely out there, that orbit around two stars in tight binary systems. Beginning in 2013, BEBOP targeted 47 eclipsing binaries, using data from the CORALIE spectrograph on the Swiss Euler Telescope at La Silla, Chile. This is intriguing because what we know about circumbinary planets has largely come from detections based on transit analysis. Radial velocity work has uncovered planets orbiting one star in a wide binary configuration but until now, not both.
Image: Artist’s visualization of a circumbinary planet. Credit: Ohio State University / Getty Images.
The new work adds data from the HARPS spectrograph at La Silla and the ESPRESSO spectrograph at Paranal to confirm one of two planets at TOI-1338/BEBOP-1. Thus we have radial velocity evidence for the gas giant BEBOP-1 c, massing in the range of 65 Earth masses, in an orbit around the binary of 215 days. A second world, referenced as TOI-1338 b because it shows up only in transit data from TESS, complements the RV find, making this only the second circumbinary system known to host multiple planets. TOI-1338 b is 21.8 times as massive as the Earth and as a transiting world could well be a candidate for atmospheric studies by the James Webb Space Telescope.
But BEBOP-1 c is the planet that stands out. I think I am safe in calling a co-author on this paper, David Martin (Ohio State University), a master of understatement when he describes the problems in extracting radial velocity data on a circumbinary world. After all, we’re relying on the tiniest gravitational effects flagged by minute changes in wavelength, and now we have to factor in multiple sets of stellar spectra. Here’s Martin:
“When a planet orbits two stars, it can be a bit more complicated to find because both of its stars are also moving through space. So how we can detect these stars’ exoplanets, and the way in which they are formed, are all quite different. Whereas people were previously able to find planets around single stars using radial velocities pretty easily, this technique was not being successfully used to search for binaries.”
Nice work indeed. Circumbinary planets are what the paper describes as ‘harsh environments’ for planet formation given the gravitational matrix in which such formation occurs, and thus we should be able to use the growing number of such systems (now 14 including this one) in the study of how planets form and also migrate. BEBOP should be a useful survey in providing accurate masses for planets in systems we’ve already discovered with the transit method.
Image: This is Figure 3 from the paper, offering an overview of the BEBOP-1 system. Caption: The BEBOP-1 system is shown along with the extent of the system’s habitable zone (HZ) calculated using the Multiple Star HZ website. The conservative habitable zone is shown by the dark green region, while the optimistic habitable zone is shown by the light green region. The binary stars are marked by the blue star symbols in the centre. The red shaded region denotes the instability region surrounding the binary stars as described by Holman and Wiegert. BEBOP-1 c’s orbit is shown by the red orbit models…shaded from the 50th to 99th percentiles. TOI-1338 b’s orbit is shown by the yellow models, and is also based on 500 random samples drawn from the posterior in its discovery paper. Credit: Standing et al.
Learning more about how planets in such perturbed environments emerge should advance the study of planet growth around single stars. It’s likely that the increased transit probabilities of circumbinary planets should play into our efforts to study planetary atmospheres as well. And while transits should provide the bulk of new discoveries in this space, radial velocity follow-ups should expand our knowledge of individual systems, being less dependent on orbital periods and inclination. BEBOP presages a productive use of these complementary observing methods.
The paper is Standing et al., “Radial-velocity discovery of a second planet in the TOI-1338/BEBOP-1 circumbinary system,” Nature Astronomy 12 June 2023 (full text). See also Martin et al., “The BEBOP radial-velocity survey for circumbinary planets I. Eight years of CORALIE observations of 47 single-line eclipsing binaries and abundance constraints on the masses of circumbinary planets,” Astronomy & Astrophysics Vol. 624, A68 (April 2019), 45 pp. Abstract.
The possibility of habitable planets orbiting a close binary system has SETI implications that are rarely considered. Both stars will have formed simultaneously, so they will be of the same age, and their initial chemical composition will be identical. But they will probably differ considerably in other ways. Even if neither star has directly influenced the other through mass transfer, they will almost certainly differ in their initial mass. Initial mass is the most influential parameter in a star’s history and evolution.
The more massive star will evolve faster, so even if one of the pair is an ideal “Goldilocks” site for the evolution of life, the more rapid evolution of the other, going through red giant, planetary nebula, or even nova stages, would be lethal for living things evolving on its companion. Only if the two are separated by sufficient distance that the evolution of one does not cook the other at some time during their joint history will there be a chance of life arising on the lighter, more stable member. In this circumstance, the system will simply wind up looking much like our own sun, but with a distant companion.
If one of the stars is much more massive than the other, it may rapidly go through its evolution so that the other star can then settle down and generate a biosphere in due time. Or we may stumble upon a pair of very similar stars, but eventually one will evolve off the main sequence first, destroying all life on the other. In general, a close binary may shelter life, but not for long.
The life-bearing world will orbit the more evolved star, and its orbit will not interact with the other. The Luke Skywalker vision of two close, multicolored suns of comparable size setting in the west, probably does not exist anywhere.
Attempting to visualize such a complex system, I immediately, and seemingly incorrectly, presumed that the planetary orbit could not possibly be stable. My as the article goes on talking about stellar timescales.
But still, wouldn’t the orbiting planet experience a sort of ‘gravitational pulse’, the result of varying distances from the barycenter to the planet?
Asked in a different way: What would be the variability in gravitational attraction felt by the planet? Orbiting the binary barycenter, as the stars eclipse each other from the planet’s point of view, wouldn’t there be some change in gravitational effect? And wouldn’t this disturb the planetary orbit, over sufficient time?
Slightly rephrased: wouldn’t the barycenter appear to shift from the point of view of the planet? Even if the shift is slight, the universe is a very patient environment.
You have a point, but although I cant reproduce the math to prove it, I suspect the orbiting planet will be far enough away from the barycenter of the binary it will see it as one gravitational point, especially if the stars are very close, or one is much more massive than the other.
Still, even should this be the case in most scenarios, I suspect that it cannot be ruled out . If there are multiple bodies in the system, perhaps some orbital resonance will emerge that will result in a stable configuration.
I would be more interested in the climate changes as the planet orbits the 2 stars. While the 2 HZs look circular, is the insolation from the stars, especially if they differ in output (almost surely they must), going to change the climate on the planet? While the axial tilt of the Earth creates regular annual changes, in the case of circumbinary orbits, the position of the 2 stars will differ for each annual orbit. Indeed, they will constantly change during the year, creating relatively rapid warmer and cooler conditions, as well as possible different combined spectral output, which becomes extreme at each eclipse.
How would a biosphere meet these challenges? More rapid reproduction and growth cycles? Differing forms best adapted to different conditions (c.f. Aldiss’ “Helliconia”, or the more extreme changes on Trisolaris in Cixin Lui’s “Three Body trilogy”).
If there is life, especially complex life on planets in circumbinary orbits, it would be fascinating to study how life adapts to any changes in insolation.
Thank you.
The orbital stability of circumbinary planets was proved many times since the ’90s.
They are stable if the planet semi-major axis is larger than 3 times the separation of the central binary star.
Thank you.
I wondered about this while reading about planetary migration observations in our own solar system. If I understand current thinking, this particular orbiting planet would be some type of a gas giant, rocky planets not being expected so far out.
Here at home, as I understand it, our early solar system was a chaotic place with rocky planetesimals (and resonance) playing a huge role in the migration of these gas giants. A young binary system adds much more chaos, particularly at much shorter radii, leading me to wonder if such a gas giant must necessarily have formed beyond the three radii figure that you mentioned.
Again, thank you. Period.
Although I have only scanned the paper, especially the section on stability, I was thinking slightly differently. While there is a barycenter there are 2 gravitational loci orbiting the barycenter. Wouldn’t a planet experience some acceleration in the direction of its orbit as each star passed it? If so, the planet’s orbital radius would increase, while that of the 2 stars’ orbital radii decrease? This may well be subtle but over billions of years…
The paper’s supplementary data suggests that there is a wide band of possible conditions between very stable orbits and chaotic orbits. It seems quite possible that a planet might well experience quasi-stable orbits yet still migrate, most likely outwards.
In the best scenario, this outward migration might track the increasing stellar output maintaining the orbit in the HZ, or it might migrate faster pushing it out beyond the HZ. OTOH, a binary with 2 different stellar masses may result in the more massive star increasing its output relatively quickly, pushing the HZ outwards until the planet becomes a Venus analog.
While reading this post, I had to listen to Steve Lawrence singing Fabulous and his life-long love and wife Eydie Gorme singing Fly Me to the Moon.
The labelling of the respective habitable zones got me, I’m obviously unfamiliar with the field. I would have thought that an ‘optimistic’ HZ would be the larger one.
Interesting study.
What you are looking at is the inner edge of the HZ, not the full range. The optimistic HZ includes the conservative HZ and is extending the inner edge which is increasing its range. Hope this helps.
Of course, thanks Alex. Optimistic inwards rather than what I was thinking – outwards.