Centauri Dreams

It will surprise few Centauri Dreams readers that at least some brown dwarfs have bands of clouds, just as we see similar bands on our Solar System’s largest planet. In fact, three brown dwarfs have recently shown signs of cloud banding, and today’s subject, Luhman 16, has previously been analyzed in terms of large cloud patches. I think new work based on data from the European Southern Observatory’s Very Large Telescope (VLT) in Chile may be less significant for what it says about brown dwarfs than what it says about how we study them.

Image: Illustration comparing the masses of planets, brown dwarfs, and stars. Credit: NASA/JPL-Caltech/R. Hurt (IPAC).

For the work in question, reported in a paper from Maxwell Millar-Blanchaer (Caltech) and colleagues, is the first time polarimetry has been put to work to infer bands in brown dwarf clouds. Polarization tells us the direction that a light wave oscillates. Millar-Blanchaer likens polarimetric instruments to polarized sunglasses — here, one direction of polarization is blocked to reduce glare — but adds that astronomers are less concerned with reducing glare than measuring it. Light reflecting off cloud droplets can favor a measurable angle of polarization.

We’re beginning to see methods to analyze polarized light cropping up more and more in astronomy, with implications for exoplanets. Thus Dimitri Mawet (Caltech), a senior research scientist at the Jet Propulsion Laboratory:

“Polarimetry is receiving renewed attention in astronomy. Polarimetry is a very difficult art, but new techniques and data analysis methods make it more precise and sensitive than ever before, enabling groundbreaking studies on everything from distant supermassive black holes, newborn and dying stars, brown dwarfs, and exoplanets, all the way down to objects in our own solar system.”

Image: Astronomers have found evidence for a striped pattern of clouds on the brown dwarf called Luhman 16A, as illustrated here in this artist’s concept. The bands of clouds were inferred using a technique called polarimetry, in which polarized light is measured from an astrophysical object much like polarized sunglasses are used to block out glare. This is the first time that polarimetry has been used to measure cloud patterns on a brown dwarf. The red object in the background is Luhman 16B, the partner brown dwarf to Luhman 16A. Together, this pair is the closest brown dwarf system to Earth at 6.5 light-years away. Credit: Caltech/R. Hurt (IPAC).

Luhman 16 draws attention for being the closest known brown dwarf binary, approximately 6.5 light years away. Both brown dwarfs weigh in at roughly 30 times Jupiter’s mass, examples of ‘failed stars’ that formed from collapsing clouds of gas but lacked the mass to ignite hydrogen fusion. Millar-Blanchaer and team used the VLT’s NaCo instrument (Nasmyth Adaptive Optics System, coupled with the Near-Infrared Imager and Spectrograph called CONICA) to study the polarized light from both of the Luhman brown dwarfs. Luhman 16A’s polarization can only be explained by bands of clouds, while the data on bands are inconclusive for Luhman 16B. The latter’s variability in brightness seems to be the result of irregular, patchy clouds instead.

Consider what we’re doing here. It took NASA’s Wide-field Infrared Survey Explorer (WISE) to find Luhman 16 in 2013, and Millar-Blanchaer’s team now detects clouds through polarization that cannot be directly resolved. The scientists used atmospheric modeling run against measurements of the polarized light to infer the presence of the cloud bands on Luhman 16A.

“To determine what the light encountered on its way we compared observations against models with different properties: brown dwarf atmospheres with solid cloud decks, striped cloud bands, and even brown dwarfs that are oblate due to their fast rotation. We found that only models of atmospheres with cloud bands could match our observations of Luhman 16A,” says Theodora Karalidi (University of Central Florida), a member of the discovery team.

The beauty of using polarimetry this way is that it is sensitive to cloud properties, a trait that should serve us well not only with brown dwarf atmospheres but those of exoplanets. A follow-up study is on the way that will examine later observations looking for short- and long-term variability in the polarization. Meanwhile, the current work confirms the existence of banded clouds near the L/T transition in brown dwarfs, an evolution in spectral type that ultimately results in relatively cloud-free regions across the brown dwarf disks. The previously studied cloud bands have all been in similar L/T transition dwarfs. From the paper:

We have demonstrated that polarimetry provides a promising avenue to answer this question; variability occurrence rates and amplitudes decline outside of the L/T transition, and therefore many targets are unsuitable to cloud mapping via variability monitoring. High-accuracy targeted polarimetric studies sensitive to cloud bands similar to those found in this study may therefore be critical to furthering our understanding of the cloud dynamics in substellar objects. The characterization presented herein relies heavily on previous characterization of Luhman 16. Similar polarimetric studies should be pursued for other brown dwarfs with comparable levels of characterization (e.g., brown dwarfs with well-constrained masses and/or inclinations).

So we have a path forward in studying brown dwarf atmospheres and, potentially, the atmospheres of giant exoplanets. Follow-up work on Luhman 16’s two brown dwarfs is complicated by the fact that the NaCo instrument package has just been decommissioned , but other near-infrared polarimeters are out there, and we should anticipate new atmospheric studies growing out of the data they collect. Bear in mind that the WFIRST mission will be able to conduct polarimetry, which may be its first application using reflected light to study giant exoplanet atmospheres.

The paper is Millar-Blanchaer et al., “Detection of Polarization due to Cloud Bands in the Nearby Luhman 16 Brown Dwarf Binary,” Astrophysical Journal Vol. 894, No. 1 (5 May 2020). Abstract.

Transit timing variations are useful to astronomers trying to learn what forces are acting upon a known exoplanet. They could eventually help us ferret out the existence of a sufficiently large moon, for example, though we have yet to confirm one. But they also show us how much impact other planets in the same system can have upon the planet being observed.

All this is why the Kepler-88 system has been high on the list of interesting targets for astronomers. Before the recent discovery of a new gas giant, we knew about Kepler-88 b and c, one of them (the outer world Kepler-88 c) about 20 times more massive than Kepler-88 b, a planet less massive than Neptune. The story here was the mean motion resonance, in which planet c, a Jupiter-mass world, orbits the star in 22 days while Kepler-88 b orbits in 11: Two orbits of b in the time it takes c to make a single orbit. Planet b is the only transiting planet in this system; Kepler-88 c was confirmed by radial velocity methods.

The mass differential is telling, so that the more massive Kepler-88 c causes notable transit timing variations in Kepler-88 b. In fact, the Kepler mission was able to detect TTVs of up to half a day forced by Kepler-88 c. These are to my knowledge the largest transit timing variations yet observed. All of that makes the system interesting in its own right, but now we have word of another planet here, Kepler-88 d, which turns out to be about three Jupiter masses.

The radial velocity discovery, which also confirmed the existence, mass and orbital period of Kepler-88 c, was made by a team led by Lauren Weiss (University of Hawaii Institute for Astronomy), using the High-Resolution Echelle Spectrometer (HIRES) instrument on the 10-meter Keck I telescope in Hawaii. The discovery paper appears in The Astronomical Journal.

Image: An artist’s illustration of the Kepler-88 planetary system. Credit: W. M. Keck Observatory / Adam Makarenko.

“At three times the mass of Jupiter, Kepler-88 d has likely been even more influential in the history of the Kepler-88 system than the so-called king, Kepler-88 c, which is only one Jupiter mass,” says Weiss. That influence is a reference to the effects of a gas giant in a Jupiter-like orbit (the planet is in an elliptical orbit around Kepler-88 with a period of four years). Jupiter is assumed to have affected cometary orbits in the early days of the Solar System, drawing materials rich in volatiles into the inner system where they would have been part of the mechanism for producing oceans on Earth.

But this is a stellar system that will have a much different fate than our own. The host star Kepler-88 is a massive B-class object, highly luminous and with a lifetime lasting only in the low millions of years. No habitable zone worlds to nourish with oceans or, at least, no such worlds with a billion-year timeframe for life to flourish.

ADDENDUM See andy’s comment below, and also Mike Fidler’s — the B-class statement in the paragraph above is is incorrect, and the result of discrepancies between the sources. It looks as though this is a G-class object and I’m trying to confirm that.

Later: I’m going with andy’s assessment — this is a G-class star, as given by the paper’s data on it: 5466 K, 0.985 solar masses and 0.900 solar radii. Other sources also peg it as a G, so I think we have to assume that the B-class identification that appears in some online sources is incorrect.

Even so, it’s friendly to planet formation. According to the paper, finding multiple giant planets is not surprising, because of the high metallicity of the star. The paper goes on to speculate about planet formation and the likelihood of early migration:

Since both planets c and d are gas giants, they must have formed early in the disk lifetime, when gas was abundant… Perhaps additional giant planets were present earlier, or are still present. Planets c and d likely underwent viscous (Type I) migration in the proto-planetary disk. As the gas disk dissipated, planet-planet scattering would likely have increased, and low and high-eccentricity migration likely became important at this time. The high eccentricity of planet d probably arose due to a significant exchange of angular momentum with another gas-giant planet.

The smaller Kepler-88 b, then, may have formed when gas was less abundant in the early disk, and the authors believe that the world could have been caught in its current mean motion resonance with Kepler-88 c during the period of early inward migration of planet c.

The paper is Weiss et al., “The Discovery of the Long-Period, Eccentric Planet Kepler-88 d and System Characterization with Radial Velocities and Photodynamical Analysis,” Astronomical Journal Vol. 159, No. 5 (29 April 2020). Abstract / Preprint.

We may be measuring planetary temperatures with less than optimum tools. Calling it a “new phenomenon,” Cornell University’s Nikole Lewis described the background of a just published paper looking into hot Jupiter temperatures. Lewis had been increasingly puzzled by earlier work on the matter, which produced temperatures colder than scientists expected. The deputy director of the Carl Sagan Institute, Lewis joined colleagues Ryan MacDonald and Jayesh Goyal in looking for the reason, reporting their results in Astrophysical Journal Letters.

What emerged was the need to fine-tune our analysis of exoplanet atmospheres, as delivered by the technique called transmission spectroscopy, in which the light of a parent star is filtered through a planetary atmosphere during a transit. Have a look, for example, at an illustration of the hot-Jupiter WASP-43b as it transits its star. Scientists have been able to construct temperature maps for the planet as well as probing its atmosphere to understand the molecular chemistry within. But the data on atmospheric composition have to be interpreted correctly.

Image: In this artist’s illustration the Jupiter-sized planet WASP-43b orbits its parent star with a year lasting just 19 hours. The planet is tidally locked, meaning it keeps one hemisphere facing the star, just as the Moon keeps one face toward Earth. The color scale on the planet represents the temperature across its atmosphere. This is based on data from a 2014 study (not the Lewis paper) that mapped the temperature of WASP-43b in more detail than had been done for any other exoplanet at that time. Credit: NASA, ESA, and Z. Levay (STScI).

Hot Jupiters orbit close enough to their star to become tidally locked, while the intense gravitational forces at work can cause the planet to bulge, making it egg-shaped. What we would expect is a wide range of temperatures, varying by thousands of degrees, between the blistering ‘day’ side of the star and the frigid side turned away from the star. Averaging temperatures like these can be a problem, according to the Cornell researchers, and the range of temperatures likewise can promote entirely different chemistry between the two sides.

“When you treat a planet in only one dimension, you see a planet’s properties – such as temperature – incorrectly,” Lewis said. “You end up with biases. We knew the 1,000-degree differences were not correct, but we didn’t have a better tool. Now, we do.”

Let’s dig into this. We have over 40 examples of hot Jupiters with transmission spectra, meaning that the science of exoplanet atmospheres is becoming established — eventually, we’ll drill down to smaller rocky worlds to learn about possible biosignatures, but for now, we’re detecting various atoms and molecules in the atmospheres of close-in gas giants. Observations of high enough precision can use transmission spectroscopy to learn about temperatures at the terminator, the zone where day meets night and the temperature contrast can be huge.

Both optical and near-infrared data are needed to draw accurate conclusions. The anomaly that the authors are addressing is that almost all of the retrieved temperatures for hot Jupiters are cooler than planetary equilibrium temperatures derived for the planet. In fact, the retrieved temperatures for most hot Jupiters are between 200 and 600 K cooler than the equilibrium temperature ought to be. The equilibrium temperature is the point at which the planet emits as much thermal energy as it receives from the star. It is commonly denoted as T_eq.

The authors propose that the colder temperatures being found via transmission spectra are the result of the use of 1-dimensional models. The atmosphere at the terminator is more complex than the 1D model can show. They call for more complex 3D general circulation models (GCM) to replace them. Sets of differential equations are put to work in a GCM, with the planet divided into a 3-dimensional grid and analyzed in terms of winds, heat transfer, radiation and other factors, taking into account their interactions with other parts of the grid. The difference is striking, as the image below, drawn from the paper, demonstrates.

Image: This is Figure 1 from the paper. Caption: Schematic explanation of the cold retrieved temperatures of exoplanet terminators. Left: a transiting exoplanet with a morning-evening temperature difference (observer’s perspective). Differing temperature and abundance profiles encode into the planet’s transmission spectrum. Right: the observed spectrum is analysed by retrieval techniques assuming a uniform terminator. The retrieved 1D temperature profile required to fit the observations is biased to colder temperatures. Credit: MacDonald et al.

Thus the earlier models bias the results toward colder temperatures. And this can be significant in our evaluation of planetary atmospheres, as the paper notes:

Those [chemical] species exhibiting compositional differences have retrieved 1D abundances biased lower than the true terminator-averaged values. Even species uniform around the terminator (here, H2O) are biased, though to higher abundances. Compositional biases become more severe as the retrieved P-T profile deviates further from the true terminator temperature. In the most extreme case, the retrieved H2O abundance is biased by over an order of magnitude, such that one would incorrectly believe a solar-metallicity atmosphere was 15 × super-solar at > 3σ confidence.

We need to resolve this matter, then, to conduct more accurate atmospheric work. The paper continues:

The retrieved cold temperatures of exoplanet terminators in the literature can be explained by inhomogenous morning-evening terminator compositions. The inferred temperatures arise from retrievals assuming uniform terminator properties. We have demonstrated analytically that the transit depth of a planet with different morning and evening terminator compositions, when equated to a 1D transit depth, results in a substantially colder temperature than the true average terminator temperature. This also holds for state-of-the-art retrieval codes, with the added complication that retrieved chemical abundances can also be significantly biased.

Using the older models has meant that the temperatures of hot Jupiters thus far measured may be biased by hundreds of degrees below their true value. The figure reaches 1000 K in the case of ultra-hot Jupiters. An implication here is that the chemistry derived from the older models is less reliable. The authors call for the use of more sophisticated retrieval tools, acknowledging the increased computer overhead involved with 3D approaches but arguing that such models will produce more accurate data on atmospheric temperatures and composition.

The paper is MacDonald et al., “Why Is it So Cold in Here? Explaining the Cold Temperatures Retrieved from Transmission Spectra of Exoplanet Atmospheres,” Astrophysical Journal Letters Vol. 893, No. 2 (23 April 2020). Abstract / preprint.