by Paul Gilster | Oct 22, 2020 | Astrobiology and SETI |
We’re a long way from knowing what is going on in terms of possible life in the clouds of Venus, but one thing is already clear: The phosphine signature, as well as its implications, is going to be thrashed out in the journals, as witness a new study from Rakesh Mogul (Cal Poly Pomona, Pomona, CA) and colleagues that looks at data from the Pioneer-Venus Large Probe Neutral Mass Spectrometer (LNMS), dating back to the Pioneer Venus Multiprobe mission in 1978. These data seem to support the presence of phosphine, while leaving its origin unknown.
But Clara Sousa-Silva (Harvard-Smithsonian Center for Astrophysics), who was involved in the earlier phosphine work led by Jane Greaves at Cardiff University (see What Phosphine Means on Venus), subsequently examined data collected in 2015 at Mauna Kea and found no sign of phosphine. And now we have another paper, this one submitted to Science by Ignas Snellen and team (Leiden University), that carries its message in the title: “Re-analysis of the 267-GHz ALMA observations of Venus: No statistically significant detection of phosphine.”
Finally, let me mention a study led by Arijit Manna (Midnapore City College, West Bengal, India) reporting on a possible detection of the amino acid glycine in the Venusian clouds. I’ve given citations for all of these papers below.
There are all kinds of reasons for data discrepancies on phosphine depending on its possible distribution in Venus’ atmosphere, so until we get further information, we’re left to speculate. But let’s welcome Venus back into the spotlight. I’m glad to see the re-emergence of public interest, and the fortunes of Venus in terms of future mission desirability are obviously on the rise, something the Venus science community must welcome as parched desert-crossers welcome an unexpected flowing spring.
Image: Venus from the perspective of the Japanese Akatsuki probe. Credit: JAXA/ISAS/DARTS/Damia Bouic.
Some kind of biology in the Venusian clouds might even implicate Earth and the possibility of biological spread through rocky debris. While speculation continues, I’m interested in the orbital movements of the Mercury-bound BepiColombo probe, which happened to be approaching Venus in October, using the planet to bleed off velocity as it nudges into the innermost system. It will hardly resolve the matter, but Venus and BepiColombo are intimately connected not only gravitationally but thanks to the opportunity Venus offers to check out key onboard systems.
BepiColombo is actually a combination mission, including the Mercury Planetary Orbiter (MPO), constructed by the European Space Agency, and the Mercury Magnetospheric Orbiter (MMO), a product of the Japan Aerospace Exploration Agency (JAXA). The duo are currently joined but will separate into individual orbits once Mercury is attained.
Two Venus flybys are in the works, as we recently saw in these pages, and the first of these, which took place on October 15, brought the craft within 11,000 kilometers of the planet. BepiColombo is swapping some of its kinetic energy to Venus as it in turn reduces speed, with the second Venus flyby planned for August of 2021 and six close Mercury flybys before the craft enters orbit around the planet at the end of 2025. We now get the chance to test BepiColombo’s MErcury Radiometer and Thermal Infrared Spectrometer (MERTIS), which has already been tested in an earlier Earth/Moon flyby that took place in our COVID spring.
Designed to measure the spectra of rock-forming materials on Mercury’s surface, MERTIS can likewise use its infrared sensors to probe the Venusian atmosphere, with a closer approach pending in the second flyby. According to German aerospace center DLR, MERTIS is sensitive to wavelengths of 7 to 14 and 7 to 40 micrometers respectively in its two uncooled radiation sensors. Both sensors were used during the approach to Venus and approximately 100,000 individual images are expected. At the same time, the Japanese Venus orbiter Akatsuki conducted its own observations, along with Earth-based instruments both professional and amateur.
Image: BepiColombo on the long journey to Mercury. Credit: ESA/ATG Medialab.
Phosphine is a short-lived molecule, suggesting that a source on Venus or in its atmosphere is replenishing it, and scientists are still trying to find out if abiotic factors could be in play here, including the possibility of volcanism or reactions following meteorite strikes, even lightning discharges. MERTIS and five other activated instruments on the Mercury Planet Orbiter will not be able to detect phosphine from the flyby distance, but the BepiColombo team has reasons for making these observations that do not involve the gas, says Gisbert Peter, MERTIS project manager at the DLR Institute of Optical Sensor Systems, where the instrument was built:
“During the Earth flyby, we studied the Moon, characterising MERTIS in flight for the first time under real experimental conditions. We achieved good results. Now we are pointing MERTIS towards a planet for the first time. This will allow us to make comparisons with measurements taken prior to the launch of BepiColombo, to optimise operation and data processing, and to gain experience for the design of future experiments.”
Peter Wurz is project leader on STROFIO, which is a mass spectrometer designed at the University of Bern to record the atmosphere of Mercury and examine its composition. Wurz anticipates the results at Venus using not just data from STROFIO but also the spacecraft’s MIPA and PICAM instruments, likewise developed at the university.
“We are expecting data from the ionized particles in Venus’ atmosphere from these two instruments, which are switched on during the Venus flyby. The amount of particle loss and its composition can be determined using the two instruments.”
All of which is an excellent workout for BepiColombo, but MERTIS can also be expected to examine sulphur dioxide concentrations, a reduction of which was recorded about ten years ago. Meanwhile, scientists can study Venus’ atmospheric composition, with the instruments aboard the Japanese Mercury Magnetospheric Orbiter tracking its structure and dynamics. Expect no answers to the astrobiological riddle, but helpful data about Venus otherwise.
And isn’t it interesting seeing how tricky it is to get into the inner system? All those flybys point back to Mariner 10, which used an initial Mercury flyby to enable additional close passes at the planet — these calculations came from Giuseppe ‘Bepi’ Colombo, the Italian physicist for whom the current mission is named. The recent maneuver reduced the craft’s relative speed compared to Mercury to 1.84 kilometers per second, the goal being to orbit the Sun at close to Mercury’s speed and eventually become captured by the gravity of the small world.
The paper referenced in the first paragraph is Mogul et al., “Is Phosphine in the Mass Spectra from Venus’ Clouds?” available as a preprint. The Sousa-Silva paper is “A stringent upper limit of the PH3 abundance at the cloud top of Venus,” in press at Astronomy & Astrophysics (abstract). The paper on glycine is Manna et al., “Detection of simplest amino acid glycine in the atmosphere of the Venus,” submitted to Science (preprint). The “Snellen paper is “Re-analysis of the 267-GHz ALMA observations of Venus: No statistically significant detection of phosphine,” submitted to Science (abstract).
by Paul Gilster | Oct 21, 2020 | Asteroid and Comet Deflection |
A spacecraft about the size of an SUV continues operations at an asteroid the size of a mountain. The spacecraft is OSIRIS-REx, the asteroid Bennu, and yesterday’s successful touchdown and sample collection attempt elicits nothing but admiration for the science team that offered up the SUV comparison. They’re collecting materials with a robotic device 321 million kilometers from home. Yesterday’s operations seem to have gone off without a hitch, the only lingering question being whether the sample is sufficient, or whether further sampling in January will be needed.
If all goes well, we will acquire the largest surface sample from another world since Apollo. TAGSAM is the Touch-And-Go Sample Acquisition Mechanism aboard the craft, a 3.35-meter sampling arm extended from the spacecraft as OSIRIS-REx was descending roughly 800 meters to the surface. The ‘Checkpoint’ burn occurred at 125 meters as the craft maneuvered to reach the sample collection site, dubbed ‘Nightingale.’ The ‘Matchpoint’ burn followed ‘Checkpoint’ by 10 minutes to match Bennu’s rotation at point of contact. A coast past the ‘Mount Doom’ boulder was followed by touchdown in a crater relatively free of rocks.
This is dramatic stuff. The image below is actually from August during a rehearsal for the sample collection (images of yesterday’s touchdown are to be downlinked to Earth later today), but it’s an animated view that gets across the excitement of the event. Mission principal investigator Dante Lauretta (University of Arizona. Tucson) had plenty of good things to say about the result:
“After over a decade of planning, the team is overjoyed at the success of today’s sampling attempt. Even though we have some work ahead of us to determine the outcome of the event – the successful contact, the TAGSAM gas firing, and back-away from Bennu are major accomplishments for the team. I look forward to analyzing the data to determine the mass of sample collected.”
Image: Captured on Aug. 11, 2020 during the second rehearsal of the OSIRIS-REx mission’s sample collection event, this series of images shows the SamCam imager’s field of view as the NASA spacecraft approaches asteroid Bennu’s surface. The rehearsal brought the spacecraft through the first three maneuvers of the sampling sequence to a point approximately 40 meters above the surface, after which the spacecraft performed a back-away burn. Credit: NASA/Goddard/University of Arizona.
The goal is 60 grams of material, with the first indication of sample size being new images of the surface to see how much material was disturbed by the TAGSAM activities. Michael Moreau (NASA GSFC) is OSIRIS-REx deputy project manager:
“Our first indication of whether we were successful in collecting a sample will come on October 21 when we downlink the back-away movie from the spacecraft. If TAG made a significant disturbance of the surface, we likely collected a lot of material.”
Images of the TAGSAM head, taken with the camera known as SamCam, should provide evidence of dust and rock in the collector, with some possibility of seeing inside the head to look for evidence of the sample within. Beyond imagery, controllers will try to determine the spacecraft’s moment of inertia by extending the TAGSAM arm and spinning the spacecraft about an axis perpendicular to the arm. Comparison to data from a similar maneuver before the sampling should allow engineers to measure the change in the mass of the collection head.
Between the imagery and the mass measurement, we should learn whether at least 60 grams of surface material have been collected. Once this has been verified, the sample collector head can be placed into the Sample Return Capsule (SRC) and the sample arm retracted as controllers look to a departure from Bennu in March of 2021. If necessary, a second maneuver, at the landing backup site called ‘Osprey,’ could take place on January 12, 2021.
Image: These images show the OSIRIS-REx Touch-and-Go Sample Acquisition Mechanism (TAGSAM) sampling head extended from the spacecraft at the end of the TAGSAM arm. The spacecraft’s SamCam camera captured the images on Nov. 14, 2018 as part of a visual checkout of the TAGSAM system, which was developed by Lockheed Martin Space to acquire a sample of asteroid material in a low-gravity environment. The imaging was a rehearsal for a series of observations that will be taken at Bennu directly after sample collection. Credit: NASA/Goddard/University of Arizona.
Sample return is scheduled for September 24, 2023, with the Sample Return Capsule descending by parachute into the western desert of Utah. So far so good, and congratulations all around to the OSIRIS-REx team!
by Paul Gilster | Oct 20, 2020 | Exoplanetary Science |
The SAINT-EX telescope, operated by NCCR PlanetS, produces a nice resonance as I write this morning. The latter acronym stands for the National Centre of Competence in Research PlanetS, operated jointly by the University of Bern and the University of Geneva. The former, SAINT-EX, identifies a project called Search And characterIsatioN of Transiting EXoplanets, and the team involved explicitly states that they shaped their acronym to invoke Antoine de Saint-Exupéry, legendary aviator and author of, among others, Wind, Sand and Stars (1939), Night Flight (1931) and Flight to Arras (1942).
I’ve talked about Saint-Exupéry now and again throughout the history of Centauri Dreams, not only because he was an inspiration for my own foray into flying, but also because for our interstellar purposes he is credited with this inspirational thought:
“If you want to build a ship, don’t drum up the men to gather wood, divide the work and give orders. Instead, teach them to yearn for the vast and endless sea.”
I say Saint-Exupéry is ‘credited’ with this because it seems to be a condensation of a longer passage that appeared in his 1948 title La Citadelle and I’ve never been able to track the oft-quoted short version down to an original in this specific form. No matter, it’s a grand thought and it plays to our passions as explorers and mappers of new worlds.
So good for NCCR PlanetS for the nod to a favorite writer, and thanks for its recent work at TOI-1266, a red dwarf now known to host at least two planets. The confirmation was achieved with the SAINT-EX robotic 1-meter telescope that the project operates in Mexico; the paper was recently published in Astronomy & Astrophysics. TOI identifies a TESS Object of Interest, meaning the object had been considered promising for follow-up work to validate the candidate signatures as planets. The paper on TOI-1266 provides the needed confirmation.
Here’s what we know: TOI-1266 b is an apparent sub-Neptune about two and a half times the diameter of the Earth that orbits the primary in 11 days. TOI-1266 c is closer in size to a ‘super-Earth’ in being about one and a half times the size of our planet in a 19-day orbit.
The orbits are circular, co-planar and stable. Referring to the more than 3000 transiting planets thus far identified, which includes 499 with more than a single transiting world, the authors discuss distinctive planetary populations. Both planets at TOI-1266 are found at what lead author Brice-Olivier Demory calls the ‘radius valley,’ for reasons explained in the paper:
This large sample of transiting exoplanets allows for in-depth exploration of the distinct exoplanet populations. One such study by Fulton et al. (2017) identified a bi-modal distribution for the sizes of super-Earth and sub-Neptune Kepler exoplanets, with a peak at ?1.3 R? and another at ?2.4 R?. The interval between the two peaks is called the radius valley and it is typically attributed to the stellar irradiation received by the planets, with more irradiated planets being smaller due to the loss of their gaseous envelopes.
Image: Size of TOI-1266 system compared to the inner solar system at a scale of one astronomical unit, the distance between the Earth and the Sun. The orbital distances of the two exoplanets discovered around the star TOI-1266, which is half of the size of the Sun, are smaller than Mercury’s orbital distance. TOI-1266 b, the closest planet to the star at a distance of 0.07 astronomical units, has a diameter of 2.37 times that of Earth’s and is therefore considered a sub-Neptune. TOI-1266 c, at 0.01 astronomical units from its star and with 1.56 times the Earth’s diameter, is considered a super-Earth. For each planetary system, the star’s diameter and the orbital distances to its planets are shown in scale. The relative diameter of all planets of both systems are on scale, being TOI-1266 b the largest planet and Mercury the smallest. The zoom-in to TOI-1266, in the lower part of the image, shows that the irradiation received by TOI-1266 c from its star is 21% larger than the irradiation received by Venus from the Sun in the upper part of the image. © Institute of Astronomy, UNAM/ Juan Carlos Yustis.
It’s useful to have a super-Earth and a sub-Neptune in the same system because this allows for tighter constraints on formation models. The paper argues that this will be “a key system to better understand the nature of the radius valley around early to mid-M dwarfs.”
It’s also interesting that TOI-1266’s small size and relative proximity (about 117 light years) make it a possible target for atmospheric studies with future instruments. The authors show that the James Webb Space Telescope should be able to operate well above the observatory’s noise floor level in performing transmission spectroscopy here (i.e., viewing changes in the star’s light as the either planet moves first in front of and then is eclipsed by the star). The TOI-1266 planets compare favorably to TRAPPIST-1b on this score, and the host star shows no evidence of significant starspots that might distort the signal of the exoplanet atmospheres.
Image: Direct comparison among the planets of TOI-1266 system with the interior planets of the solar system. © Institute of Astronomy, UNAM / Juan Carlos Yustis.
The paper is Brice-Olivier Demory et al., “A super-Earth and a sub-Neptune orbiting the bright, quiet M3 dwarf TOI-1266,” Astronomy & Astrophysics Volume 642 (2 October 2020). Abstract.
by Paul Gilster | Oct 16, 2020 | Culture and Society |
Peter Coles, who is a professor of theoretical physics at Maynooth University in Ireland, tells an anecdote about John Barrow, who died recently at the age of 67. Barrow had been Coles’ thesis supervisor and a profound influence on his work as well as a good friend. As Coles tells it in his In the Dark blog, Barrow had an engaging and sometimes slightly morbid sense of humor, dry enough to tease out the ironies abundant in life’s accomplishments. Thus his reaction to being made a Fellow of the Royal Society, which was to point out in an email that his joy was tempered by having received as his first communication from the Society not only a fat bill for his subscription, but also a form upon which to enter the details of his future obituary.
How saddening that Barrow’s obituary materials had to be put to use so soon. The man was 67, felled by cancer. As Coles notes, he was “one of cosmology’s brightest lights.” I can glance across my office to the nearest of many bookshelves where I see various Barrow titles, including of course The Anthropic Cosmological Principle (Oxford University Press, 1986) and The Left Hand of Creation (Oxford, 1983), the latter of which was my introduction to Barrow and the reason I snapped up the former. Along the way, Barrow produced numerous popular science titles and, of course, hundreds of papers in the journals.
Image: Cosmologist, mathematician and physicist John D. Barrow, who died on September 26. Credit: Tom Powell.
You can learn an extraordinary amount of physics from reading (and re-reading) The Anthropic Cosmological Principle, which Barrow wrote with Frank Tipler, and while theoretical astrophysics dominates the tome, you’ll come out of it steeped not only in cosmology but also philosophy and the history of ideas, from Johann Fichte to Henri Bergson, with a healthy dose of Teilhard de Chardin. None of that means you have to buy into anthropic cosmological principles in the way Barrow and Tipler framed their consequences, although the case is made with vigor and verve. The book is a treasure house of accumulated ideas; the reader can move among them learning, questioning, arguing. I wish I had had the chance to talk to Barrow, preferably over a leisurely dinner, for he was both a raconteur and a wit who spoke with a polymath’s elegance.
As to the anthropic cosmological principle, let’s say that parts of it are self-evident though sometimes productive. Drawing on Brandon Carter, the authors laid out the fact that the physical constants we study in the cosmos have to be such that life can exist within the universe they support. But let me quote Ethan Siegel’s recent piece on anthropic matters, as he’ll say it better. Two forms of the principle emerge in The Anthropic Cosmological Principle:
1) The observed values of all physical and cosmological quantities are not equally probable but they take on values restricted by the requirement that there exists sites where carbon-based life can evolve and by the requirement that the Universe be old enough for it to have already done so.
2) The Universe must have those properties which allow life to develop within it at some stage in history.
All of which seems self-evident until you absorb the implication in the second item: A universe without life is not allowed. Now we’re in the realm of teleology, making this take on the principle controversial, to say the least. To push the pedal down all the way, Barrow and Tipler introduce a Final Anthropic Principle (FAP) which points to the necessity for information processing to emerge and insists that once present, it will never disappear. Tipler would take these ideas and run with them in his own The Physics of Immortality (Doubleday, 1994). Martin Gardner was less impressed, calling the notion of an ‘Omega Point’ the Completely Ridiculous Anthropic Principle (CRAP). I would say it is entertaining metaphysics and leave it at that.
But the weak anthropic principle still resonates. Paul Davies calls it “a familiar part of the theorist’s arsenal” and Siegel discusses its use in Stephen Weinberg’s 1987 calculation of the energy density of empty space, which must be at least 118 orders of magnitude smaller than what scientists had derived from quantum field theory or we would not be here to calculate it (measurements now indicate the actual figure is 120 orders of magnitude smaller). How it can be used and sometimes abused is, in fact, the subject of the Siegel essay I referenced above. I’m not comfortable enough going through the intellectual thickets here to say more. I always wanted to ask Barrow about his current thinking on the matter to satisfy my questions, though as fate would have it, I never had the chance to meet the man.
What appealed about the dinner idea was Barrow’s interest in wine and cuisine. In the course of my checkered career, I labored as a wine critic and restaurant reviewer for a local paper, and the thought of engaging this wide-ranging intellect on issues ranging from early music to the fine structure constant while sipping, say, a Vosne-Romanée (I would have pulled out all the stops for Barrow!) was enticing. But I have to read the reminiscences of others to reinforce my sense of the man. Paul Davies’ has noted his sartorial elegance (all those trips to Italy), his quick wit, his grasp of literature and the visual arts. Much of which I had more or less inferred from his books.
I mention Italy because Barrow loved the country and I learned from Davies that his stage play Infinities saw its premiere in Milan (it went on to win the Italian Premi Ubu award for the best play in Italian theatre in 2002), and it was consoling to find out that Barrow and his wife Elizabeth had made one last trip there no more than weeks before he died. While we of course remember him here as a scientist, and an extraordinarily influential one at that, style manifests itself in many ways, from the elegance of a new suit to the figures on a physicist’s chalkboard. On his defining intellectual style, let’s turn back to Davies:
More recently, Barrow was interested in the possibility that the fine-structure constant—an unexplained number that describes the strength of the electromagnetic force—might not be constant at all but rather vary over cosmological scales. He produced a theoretical basis for incorporating such a phenomenon in physical law while also remaining open-minded on the observational evidence. His adventurous choices of research problems typified Barrow’s intellectual style, which was to challenge the hidden assumptions underpinning cherished mainstream theories. Fundamental problems in physics and cosmology may appear intractable, he reasoned, because we are thinking about them the wrong way. It was a mode of thought that resonated with many colleagues, this writer included, who are drawn to reflect on the deepest questions of existence.
Indeed, Davies’ many books display the kind of intellectual bandwidth that Barrow tapped so effortlessly, and which is on display in Barrow’s numerous books. He wrote, both in his professional work and his popular science titles, with a crisp, unaffected clarity. I appreciated, as I do with Davies, his willingness to engage with the public, to whom he communicated an obvious joy in the acquiring of knowledge. That’s quite a range, to move from analysis of the early universe and the nature of its physical constants to the reader on the underground, riding home from a harried day with book in lap explaining how mathematics gives shape to the cosmos. How cosmologists slide from the micro to the macro scale will always astonish me.
But practicality has played a hand in this. Go back to the early 1980s and consider, as Barrow often did, the changes that have occurred in how we communicate science. Public discourse on their research and its consequences was relatively rare among researchers until more recent times, when university science departments began to determinedly reach out not only for new students but for the visibility that attracts good faculty. Barrow would win the Royal Society’s Michael Faraday Prize for excellence in communicating science to UK audiences, a subject he further embraced through his involvement as director of the Millennium Mathematics Project, which seeks to spread mathematical skills broadly outside the boundaries of the academy. The latter seems to have inspired his 100 Essential Things You Didn’t Know You Didn’t Know About Maths and the Arts (Bodley Head, 2014), a series of short riffs on the subject.
Image: John Barrow receiving the Queen’s Anniversary Prize on behalf of the Millennium Mathematics Project. Credit: Centre for Theoretical Cosmology News.
Barrow could never have been accused of being a blinkered academic. Even so, he would write over 500 research papers in astrophysics and cosmology, much of the time asking how astronomical observations could inform our understanding of fundamental physics. I’ve mentioned the fine-structure constant and the possibility of variation in earlier cosmological epochs. But he also explored cosmic inflation and probed extremes of nature, the singularities suggested by the nature of the Big Bang. His honors and awards are too numerous to list in the space of this article. His over 20 books have been translated into 28 languages.
What a fine spirit he inspired in those around him. I caught that sense of quest and possibility early, from The Left Hand of Creation, which Barrow wrote with Joseph Silk. The book’s final paragraph is worth quoting:
Could there be any shortcuts to the answers to the cosmological questions? There are some who foolishly desire contact with advanced extraterrestrials in order that we might painlessly discover the secrets of the universe secondhand and prematurely extend our understanding. Such a civilization would surely resemble a child who receives as a gift a collection of completed crossword puzzles. The human search for the structure of the universe is more important than finding it because it motivates the creative power of the human imagination. About 50 years ago a group of eminent cosmologists were asked what single question they would ask of an infallible oracle who could answer them with only yes or no. When his opportunity came, Georges Lemaître made the wisest choice. He said, “I would ask the Oracle not to answer in order that a subsequent generation would not be deprived of the pleasure of searching for and finding the solution.”
Dinner with Barrow would have been just the ticket. A toast, then, to the man who said “A universe simple enough to be understood is too simple to produce a mind capable of understanding it.” Here’s to a universe of complexity so rich that it could inspire the life of such a man.
by Paul Gilster | Oct 15, 2020 | Exoplanetary Science |
Gas giant planets in orbits similar to Jupiter’s are a tough catch for exoplanet hunters. They’re far enough from the star (5 AU in the case of Jupiter) that radial velocity methods are far less sensitive than they would be for star-hugging ‘hot Jupiters.’ A transit search can spot a Jupiter analogue, but the multi-year wait for the proper alignment is obviously problematic.
Still, we’d like to know more, because the gravitational influence of Jupiter may have a crucial role in deflecting asteroids and comets from the inner system, thus protecting terrestrial worlds like ours. If this is the case, then we need to ask whether we just lucked out by having Jupiter where it is, or whether there is some mechanism that makes the presence of a gas giant in a kind of ‘protective’ outer orbit likely when rocky worlds inhabit the inner system.
Enter the computer simulations run by Martin Schlecker at the Max Planck Institute for Astronomy (MPIA) in Heidelberg. Working with scientists at the University of Bern and the University of Arizona, Schlecker has run simulations of 1000 planetary systems evolving around Sun-like stars. In this case, ‘Sun-like’ means solar-type stars with the same mass as our own Sun. The simulated planetary systems are derived through a model known as the Generation III Bern global model of planetary formation and evolution, and the team has used the results to make a statistical comparison with a sample of observed exoplanet systems.
Schlecker calls gas giants in more distant orbits ‘cold Jupiters.’ They’re beyond the snowline, so that water can exist in the form of ice. The paper homes in on ‘super-Earths’ more massive than our own planet but far less massive than gas giants, which are estimated to orbit as many as 50% of stars in class F, G and K. In question is how cold Jupiters affect the formation of these inner super-Earths, and if in fact they make it easier for super-Earths to survive. Some recent studies have found a positive correlation between super-Earths and cold Jupiters. Is it sound?
Image: Artistic impression of a planetary system with two super-Earths and one Jupiter in orbit around a Sun-like star. Simulations show that massive protoplanetary disks in addition to rocky super-Earths with small amounts of ice and gas often form a cold Jupiter in the outer regions of the planetary systems. Credit: MPIA graphics department.
The team’s goal is a “population synthesis of multi-planet systems from initial conditions representative of protoplanetary disks in star forming regions.” The computer model tracks how systems starting from random initial conditions evolve over several billion years as planets coalesce and interact with other objects in the system, with the possibility of collisions or ejections as orbits change due to the gravitational interactions factored in. You can see that with the difficulty in observing actual cold Jupiters in such settings, computer simulations are the way to proceed as the tools of observation are tuned up for future detections.
A hot Jupiter would be likely to disrupt planetary orbits, perhaps preventing the formation of some inner worlds altogether, but a cold Jupiter is far enough from the star that we can expect systems with inner super-Earths to exist, and indeed, some studies suggest that systems with a cold Jupiter often contain a super-Earth. But the simulations of Schlecker’s team indicate that only about a third of cold Jupiters occur in systems accompanied by at least one super-Earth. The correlation of inner super-Earths and cold Jupiters is there, but somewhat weaker than has been found through actual observation.
A possible explanation for the discrepancy between observation and simulation: Gas giant migration involving ‘warm Jupiters’ on intermediate orbits, causing super-Earth collisions or ejections. Here we can look to the parameters of the simulations. If the tendency of the simulated gas giants to migrate is slightly lowered, more inner super-Earths remain in numbers that are compatible with what we have recorded with our telescopes and spectrographs.
Here the limited number of actually observed systems with gas giant and super-Earths becomes an issue. Whereas the study’s simulations find a tendency to produce systems with both a cold Jupiter and at least one dry super-Earth (“with little water or ice, and a thin atmosphere at most,” as Schlecker notes), we only have a small number of systems (24) out of the 3200 planetary systems currently known that follow this configuration. Schlecker also notes that the Earth is comparatively dry — “…the Earth is, despite the enormous oceans and the polar regions, with a volume fraction for water of only 0.12% altogether a dry planet.” What we don’t find observationally are ice-rich super-Earths with a cold Jupiter in the same system.
Image: Schematic diagram of the scenarios of how according to the analyzed simulations icy super-Earths (a) or rocky (ice-poor) super-Earths form together with a cold Jupiter (b). The mass of the protoplanetary disk determines the result. Credit: Schlencker et al./ MPIA.
The mass of the protoplanetary disk may be the key. A massive enough disk can form inner rocky planets with cold gas giants beyond the snowline, with the rocky planets being poor in ice and gas. Outer super-Earths that form ice-rich would be unable to migrate inward because of the giant planet. In disks of medium mass, by contrast, material to produce warm super-Earths is limited, and gas giants beyond the snowline give way to super-Earths abundant in ice.
In the authors’ words:
We find a difference in the bulk composition of inner super-Earths with and without cold Jupiters. High-density super-Earths point to the existence of outer giant planets in the same system. Conversely, a present cold Jupiter gives rise to rocky, volatile-depleted inner super-Earths. Birth environments that produce such dry planet cores in the inner system are also favorable for the formation of outer giants, which obstruct inward migration of icy planets that form on distant orbits. This predicted correlation can be tested observationally.
and…
…low-mass solid disks tend to produce only super-Earths but no giant planets. Intermediate-mass disks may produce both super-Earths and cold Jupiters. High-mass disks lead to the destruction of super-Earths and only giants remain.
The mass of the protoplanetary disk is thus crucial, with super-Earth occurrence in combination with cold Jupiters rising with increasing disk mass, then dropping for disks massive enough to become dominated by giant planets. Interestingly, the study indicates that host stars of high-metallicity tend to have planetary systems with warm giant planets that can wreak havoc on inner planets in the system.
We’re in that region of the discovery space where we’re building theories that can be tested against future observations. Thus the next generation of space telescopes like the James Webb Space Telescope, as well as the Extremely Large Telescope from the European Southern Observatory. Do the simulations of Schlecker and team hold up? Do they show a disposition for planets of Earth size, rather than the larger super-Earths, to exist in systems with cold Jupiters?
We won’t know, the scientist acknowledges, until we have the ability to detect planets like ours in large numbers. For now, the only numbers we can begin to run are those of the larger super-Earth population, and even here, we’re still building the statistical sample. What the new instrumentation shows in coming years will help us fine-tune these simulated results.
The paper is Schlecker et al., “The New Generation Planetary Population Synthesis (NGPPS). III. Warm super-Earths and cold Jupiters: A weak occurrence correlation, but with a strong architecture-composition link,” in process at Astronomy & Astrophysics (abstract).
