Although it seems so long ago as to have been in another century (which it actually almost was), the first detection of an exoplanet atmosphere came in the discovery of sodium during a transit of the hot Jupiter HD 209458b in 2002. To achieve it, researchers led by David Charbonneau used the method called transmission spectroscopy, in which they analyzed light from the star as it passed through the atmosphere of the planet. Since then, numerous other compounds have been found in planetary atmospheres, including water, methane and carbon dioxide.
Scientists also expect to find the absorption signatures of metallic compounds in hot Jupiters, and these have been detected in brown dwarfs as well as ultra-hot Jupiters. Now we have new work out of SRON Netherlands Institute for Space Research and the University of Groningen. Led by Marrick Braam, a team of astronomers has found evidence for chromium hydride (CrH) in the atmosphere of the planet WASP-31b, a hot Jupiter with a temperature of about 1200° C in the twilight region where light from the star passes through the atmosphere during a transit.
This is an interesting find because it has implications for weather. The temperature in this region is just where chromium hydride transitions from a liquid to a gas at the corresponding pressure in the atmosphere, making it a potential weather-maker just as water is for Earth. Braam speaks of the possibility of clouds and rain coming out of this, but hastens to add that while his team found chromium hydride with Hubble and Spitzer data, it found none in data from the Very Large Telescope (VLT). That would place the find in the realm of ‘evidence’ rather than proof.
Even so, it gives us something interesting to work with, assuming we do get the James Webb Space Telescope into space later this year. Co-author and SRON Exoplanets program leader Michiel Min notes the significance of the orbital lock hot Jupiters fall into:
“Hot Jupiters, including WASP-31b, always have the same side facing their host star. We therefore expect a day side with chromium hydride in gaseous form and a night side with liquid chromium hydride. According to theoretical models, the large temperature difference creates strong winds. We want to confirm that with observations.”
Image: A hot Jupiter crossing the face of its star as seen from Earth. Credit: ESA/ATG medialab, CC BY-SA 3.0 IGO.
WASP-31b orbits an F star at a distance of 0.047 AU and is one of the lowest density exoplanets yet found, with a mass of 0.478 that of Jupiter and a radius 1.549 times Jupiter’s. Potassium has already been found in its atmosphere as well as evidence for aerosols in the form of clouds and hazes, with some evidence for water vapor and ammonia. Braam and team analyzed previously available transmission data from Hubble and Spitzer using a software retrieval code called TauRex (Tau Retrieval for Exoplanets), reporting “the first statistical evidence for the signatures of CrH in an exoplanet atmosphere.” The paper goes on to note:
The evidence for CrH naturally follows from its presence in brown dwarfs and is expected to be limited to planets with temperatures between 1300 and 2000 K. Cr-bearing species may play a role in the formation of clouds in exoplanet atmospheres, and their detection is also an indication of the accretion of solids during the formation of a planet.
The paper is Braam et al., “Evidence for chromium hydride in the atmosphere of hot Jupiter WASP-31b,” accepted at Astronomy & Astrophysics. Abstract / Full Text.
At the Princeton Plasma Physics Laboratory (PPPL) in Plainsboro, New Jersey, physicist Fatima Ebrahimi has been exploring a plasma thruster that, on paper at least, appears to offer significant advantages over the kind of ion thruster engines now widely used in space missions. As opposed to electric propulsion methods, which draw a current of ions from a plasma source and accelerate it using high voltage grids, a plasma thruster generates currents and potentials within the plasma itself, thus harnessing magnetic fields to accelerate the plasma ions.
What Ebrahimi has in mind is to use magnetic reconnection, a process observed on the surface of the Sun (and also occurring in fusion tokamaks), to accelerate the particles to high speeds. The physicist found inspiration for the idea in PPPL’s ongoing work in fusion. Says Ebrahimi:
“I’ve been cooking this concept for a while. I had the idea in 2017 while sitting on a deck and thinking about the similarities between a car’s exhaust and the high-velocity exhaust particles created by PPPL’s National Spherical Torus Experiment (NSTX). During its operation, this tokamak produces magnetic bubbles called plasmoids that move at around 20 kilometers per second, which seemed to me a lot like thrust.”
In magnetic reconnection, magnetic field lines converge, separate, and join together again, producing energy that Ebrahimi believes can be applied to a thruster. Whereas ion thrusters propelling plasma particles via electric fields can produce a useful but low specific impulse, the magnetic reconnection thruster concept can, in theory, generate exhausts with velocities of hundreds of kilometers per second, roughly ten times the capability of conventional thrusters.
Image: Magnetic reconnection refers to the breaking and reconnecting of oppositely directed magnetic field lines in a plasma. In the process, magnetic field energy is converted to plasma kinetic and thermal energy. Credit: Magnetic Reconnection Experiment.
Writing up the concept in the Journal of Plasma Physics, Ebrahimi notes that this thruster design allows thrust to be regulated through changing the strength of the magnetic fields. The physicist describes it as turning a knob to fine-tune the velocity by the application of more electromagnets and magnetic fields. In addition, the new thruster ejects both plasma particles and plasmoids, the latter a component of power no other thruster can use.
If we could build a magnetic reconnection thruster like this, it would allow flexibility in the plasma chosen for a specific mission, with the more conventional xenon used in ion thrusters giving way to a range of lighter gases. Ebrahimi’s 2017 work showed how magnetic reconnection could be triggered by the motion of particles and magnetic fields within a plasma, with the accompanying production of plasmoids. Her ongoing fusion research investigates the use of reconnection to both create and confine the plasma that fuels the reaction without the need for a large central magnet.
Image: PPPL physicist Fatima Ebrahimi in front of an artist’s conception of a fusion rocket. Credit: Elle Starkman, PPPL Office of Communications, and ITER.
Thus research into plasmoids and magnetic reconnection filters down from the investigation of fusion into an as yet untested concept for propulsion. It’s important to emphasize that we are, to say the least, in the early stages of exploring the new propulsion concept. “This work was inspired by past fusion work and this is the first time that plasmoids and reconnection have been proposed for space propulsion,” adds Ebrahimi. “The next step is building a prototype.”
The paper is Ebrahimi, “An Alfvenic reconnecting plasmoid thruster,” Journal of Plasma Physics Vol. 86, Issue 6 (21 December 2020). Abstract.
‘Xallarap’ is parallax spelled backward (at least it’s not another acronym). And while I doubt the word will catch on in common parlance, the effect it stands for is going to be useful indeed for astronomers using the Nancy Grace Roman Space Telescope. This is WFIRST — the Wide Field Infrared Survey Telescope — under its new name, a fact I mention because I think this is the first time we’ve talked about the mission since the name change in 2020.
Image: High-resolution illustration of the Roman spacecraft against a starry background. Credit: NASA’s Goddard Space Flight Center.
While a large part of its primary mission will be devoted to dark energy and the growth of structure in the cosmos, a significant part of the effort will be directed toward gravitational microlensing, which should uncover thousands of exoplanets. This is where the xallarap effect comes in. It’s a way of drawing new data out of a microlensed event, so that while we can continue to observe planets around a nearer star as it aligns with a background star, we will also be able to find large planets and brown dwarfs orbiting the more distant stars themselves.
Let’s back up slightly. The gravitational microlensing we’ve become familiar with relies on a star crossing in front of a more distant one, a chance alignment that causes light from the farther object to bend, a result of the curvature of spacetime deduced by Einstein. The closer star acts as a lens, making light from the background star appear magnified, and the analysis of that magnified light can also show the signature of a planet orbiting the lensing star. It’s a method sensitive to planets as small as Mars.
Moreover, gravitational microlensing makes it possible to see planets in a wide range of orbits. While the alignment events are one-off affairs — the stars from our vantage point are not going to be doing this again — we do have the benefit of being able to detect analogs of most of the planets in our own system. What Shota Miyazaki (Osaka University) and colleagues have demonstrated in a new paper is that ‘hot Jupiters’ and brown dwarfs will also be detectable around the more distant star. Xallarap is their coinage.
David Bennett leads the gravitational microlensing group at NASA GSFC:
“It’s called the xallarap effect, which is parallax spelled backward. Parallax relies on motion of the observer – Earth moving around the Sun – to produce a change in the alignment between the distant source star, the closer lens star and the observer. Xallarap works the opposite way, modifying the alignment due to the motion of the source.”
Image: This animation demonstrates the xallarap effect. As a planet moves around its host star, it exerts a tiny gravitational tug that shifts the star’s position a bit. This can pull the distant star closer and farther from a perfect alignment. Since the nearer star acts as a natural lens, it’s like the distant star’s light will be pulled slightly in and out of focus by the orbiting planet. By picking out little shudders in the starlight, astronomers will be able to infer the presence of planets. Credit: NASA’s Goddard Space Flight Center.
The Roman telescope will be digging into the Milky Way’s central bulge in search of such objects as well as the thousands of exoplanets expected to be found through the older microlensing method, which Xallarap complements. And while microlensing works best at locating planets farther from their star than the orbit of Venus, xallarap appears to be better suited for massive worlds in tight orbits, which produce the biggest tug on the host star.
This will include the kind of ‘hot Jupiters’ we’ve found before but still have problems explaining in terms of their formation and possible migration, so finding them through the new method should add usefully to the dataset. In a similar way, the Roman instrument’s data on brown dwarfs found through xallarap should extend our knowledge of multiple star systems that include these objects, which are more massive than Jupiter but roughly the same radius.
Given that the view toward galactic center takes in stars that formed as much as 10 billion years ago, the Roman telescope will be extending the exoplanet search significantly. Until now, we’ve homed in on stars no more than a few thousand lights years out, the exception being those previously found through microlensing. The xallarap effect will complement the mission by helping us find older planets and brown dwarfs that fill in our knowledge of how such systems evolve. How long can a hot Jupiter maintain its tight orbit? How frequently will we find objects like these around ancient stars?
We should start getting useful data from both xallarap and more conventional microlensing some time in the mid-2020s, when the Roman instrument is due to launch. Nice work by Miyazaki and team.
The paper is Miyazaki et al., “Revealing Short-period Exoplanets and Brown Dwarfs in the Galactic Bulge Using the Microlensing Xallarap Effect with the Nancy Grace Roman Space Telescope,” Astronomical Journal Vol. 161, No 2 (25 January 2021). Abstract.
A planet like GJ 3512 b is hard to explain. Here we have a gas giant that seems to be the result of gravitational instabilities inside the ring of gas and dust that circles its star. This Jupiter-like world is unusual because of the ratio between planet and star. The Sun, for example, is about 1050 times more massive than Jupiter. But for GJ 3512 b, that ratio is 270, a reflection of the fact that this gas giant orbits a red dwarf with about 12 percent of the Sun’s mass. How does a red dwarf produce a debris disk that allows such a massive planet to grow?
I bring up GJ 3512 b (discovered in 2019) as an example of the kind of anomaly that highlights the gaps in our knowledge. While we often talk about red dwarf systems in these pages, the processes of planet formation around these stars remain murky. M-dwarfs may make up 80 percent of the stars in the Milky Way, but they host a scant 10 percent of the exoplanets we’ve found thus far. To tackle this gap, a team of scientists led by Nicolas Kurtovic (Max Planck Institute for Astronomy, Heidelberg) has released the results of its analysis of six very low mass stars n the Taurus star-forming region. These are stars with mass less than 20% that of the Sun.
The new work relies on data from the Atacama Large Millimeter/submillimeter Array (ALMA), taken at a wavelength of 0.87 millimeters in order to trace dust and gas in the disk around these stars at an angular resolution of 0.1 arcseconds. What’s intriguing here are the signs of ring-like structures in the dust that extend between 50 and 90 AU from the stars, which reminds us of similar, much larger disks around more massive stars. Half of the disks the Kurtovic team studied showed structures at these distances. The consensus is that such rings are markers for planets in the process of formation as they accumulate gas and dust.
Image: Artistic representation of a planet-forming disk of dust and gas around a very low-mass star (VLMS). The inner dust disk contains a ring structure that indicates the formation of a new planet. The dust disk resides inside a larger gas disk whose thickness increases towards the edge. Credit: MPIA graphics department.
In the case of the M-dwarfs under study, the gaps in the rings being cleared by these planets would require worlds about as massive as Saturn, an indication that the material for gas giant formation is available. But time is a problem: Inward-movng dust evaporates close to the star, and in the case of red dwarfs, the migration is twice as fast as for more massive stars. There is little time in such a scenario for the planetary embryos needed for core accretion to form.
This is, I think, the key point in the paper, which notes the problem and explains current thinking:
The core accretion scenario for planet formation assumes collisional growth from sub-µm-sized dust particles from the interstellar medium (ISM) to kilometer-sized bodies or planetesimals… The collisions of particles and their dynamics within the disk are regulated by the interaction with the surrounding gas. Different physical processes lead to collisions of particles and their potential growth, such as Brownian motion, turbulence, dust settling, and radial drift…
Much depends upon the star in question. The paper continues (italics mine):
All of these processes have a direct or indirect dependency on the properties of the hosting star, such as the temperature and mass. For instance, from theoretical calculations, settling and radial drift are expected to be more efficient in disks around VLMS [Very Low Mass Stars] and BDs [Brown Dwarfs], with BD disks being 15-20% flatter and with radial drift velocities being twice as high or even more in these disks compared to T-Tauri disks.
The authors estimate that the ringed structures they’ve found around three of the red dwarfs under study formed approximately 200,000 years before the dust would have migrated to the central star. So planet formation must be swift: The planetary embryos need to accumulate enough mass to create gaps in the disk, which effectively block the dust from further inward migration. Without these gaps, the likelihood of planet formation drops.
Image: Observational data and model of the dust disk around the VLMS MHO6. Left: Image of the dust disk. Middle: The disk model with a 20 au wide central hole, which is consistent with a Saturn-mass planet located at a distance of 7 au from the star, accreting disk material. Right: Radial profile of the model (blue) and after convolving it with the telescope’s angular resolution (red). The black symbols represent the data obtained from the measured brightness distribution. The grey bar corresponds to the angular resolution of the observations. Credit: Kurtovic et al./MPIA.
Remember, we only have six stars to work with here, and the other three stars under investigation are likewise problematic. They appear to show dust concentrations between 20 and 40 AU from their stars, but in all three cases lack structure that can be defined by ALMA. Better resolution would, the authors believe, tease out rings inside even these smaller disks.
But the paper acknowledges that disks around stars that are still lower in mass demand that inward migration be reduced through ‘gas pressure bumps’ that can trap dust efficiently. Here’s the process (once again, the italics are mine):
The presence of pressure bumps produces substructures, such as rings, gaps, spiral arms, and lopsided asymmetries, with a different amplitude, contrasts, and locations depending on the origin of the pressure variations… Currently, due to sensitivity limitations, most of our observational knowledge about substructures comes from bright (and probably massive) disks, such as the DSHARP sample… A less biased sample of ALMA observations of disks in the star-formation region of Taurus has demonstrated that at least 33% of disks host substructures at a resolution of 0.1”, and the disks that do not have any substructures are compact (dust disk radii lower that ?50 au…). It remains an open question if compact disks are small because they lack pressure bumps or because current observations lack the resolution to detect rings and gaps in these disks.
Such pressure bumps are a workable explanation but one without the kind of observational evidence we’ll need as we continue to investigate planet formation around low-mass stars and brown dwarfs. We are a long way from having a fully developed model for planet formation in this environment, and the Kurtovic et al. paper drives the point home. On a matter as fundamental as how common planets around red dwarfs are, we are still in the early stages of data gathering.
The paper is Kurtovic, Pinilla, et al., “Structures of Disks around Very Low Mass Stars in the Taurus Star-Forming Region,” Astronomy & Astrophysics, 645, A139 (2021). Abstract / Preprint.
In Centauri Dreams, Paul Gilster looks at peer-reviewed research on deep space exploration, with an eye toward interstellar possibilities. For many years this site coordinated its efforts with the Tau Zero Foundation. It now serves as an independent forum for deep space news and ideas. In the logo above, the leftmost star is Alpha Centauri, a triple system closer than any other star, and a primary target for early interstellar probes. To its right is Beta Centauri (not a part of the Alpha Centauri system), with Beta, Gamma, Delta and Epsilon Crucis, stars in the Southern Cross, visible at the far right (image courtesy of Marco Lorenzi).
If you'd like to submit a comment for possible publication on Centauri Dreams, I will be glad to consider it. The primary criterion is that comments contribute meaningfully to the debate. Among other criteria for selection: Comments must be on topic, directly related to the post in question, must use appropriate language, and must not be abusive to others. Civility counts. In addition, a valid email address is required for a comment to be considered. Centauri Dreams is emphatically not a soapbox for political or religious views submitted by individuals or organizations. A long form of the policy can be viewed on the Administrative page. The short form is this: If your comment is not on topic and respectful to others, I'm probably not going to run it.
