Artist’s conception of a hot Jupiter shedding mass.
A martian dust devil observed by the HiRISE instrument, in orbit around Mars.
Dust devils, whirling columns of fine particulates, have captured imaginations going back at least to the ancient Greeks, but their inner workings continue to confound and surprise scientists. Dust devils are common in arid regions on Earth, and on Mars, where “arid” doesn’t begin to describe the climate, dust devils are ubiquitous.
Especially puzzling, dust devils are better at lifting dust into the air than they ought to be. For example, in lab-simulated vortices, even when the winds are barely above a breeze, small dust grains seem to miraculously levitate and dance. So it seems that some force other than just wind must be important for lofting dust in devils. During our research group meeting today, we discussed a recent study by Gabriele Franzese and colleagues looking at one possibility: electric fields within dust devils.
As dust grains clatter around within the turbulent body of a devil, they can collide over and over again, which can transfer charge between the grains similar to the process that generates static electricity. And, for reasons that aren’t well understood, small grains like to collect negative charge.
Since small grains can be more easily lifted than large ones, small, negatively charged grains end up at higher altitude than large grains in dust devils, resulting in charge separation and a electric field. In the same way static electricity can lift small pieces of paper, these electric fields can draw in more dust grains and help explain the surprising ability of devils to lift shrouds of dust.
From Figure 5 of Franzese et al. (2018), the electric field associated with a dust devil.
For their field study of active dust devils, Franzese and colleagues set up meteorological equipment in the deserts of Morroco and left it there, steadily measuring wind speeds, dust loading, and electric fields. As dust devils skittered past their instruments, they registered as dips and spikes in the data logs. After recovering the instruments and analyzing the data, Franzese and colleagues found more than 500 dust devils had visited their instruments over a three month time-span.
The dust devils displayed a wide variety of shapes, sizes, and, most importantly, electric fields. The picture at left shows the electric field measured for one particularly strong dust devil. In this case, the devil exhibited an enormous electric field, 12,000 V/m. For comparison, such strong electric fields usually seen within storm clouds. Franzese and colleagues show that the strength of field measured for a devil correlates one-to-one with the amount of dust within the devil, so it seems likely electric fields do play some role in the lofting the dust.
Since humidity in the air can wick away static charge, dust devils on arid Mars probably exhibit even stronger electric fields than on Earth, which may help explain why martian devils are so much more common there: even faint whirlwinds manage to lift dust. These same electric fields could also present a danger to human exploration of Mars, though, potentially damaging sensitive electronics. Or at the very least, making a case of the Mondays even worse.
At our research group meeting on Friday, we discussed an interesting paper from Dr. Tom Barclay and colleagues, which explored how many and what kinds of planets we might find with the TESS mission, launched in April this year.
As Barclay et al. argue, trying to estimate the planet yield from an upcoming survey provides several benefits. For instance, knowing how many planets TESS may find can help astronomers figure out how much time to allot for follow-up observations at large observatories. Also, thinking about TESS discoveries is like staying awake on Christmas eve, anticipating all the presents – it’s just plain exciting.
And make no mistake – TESS will be another game-changer. Kepler focused on figuring out how many of each kind of planet there is in our galaxy, but as one of the trade-offs to facilitate this kind of statistical study, most of the systems found by Kepler are much too far away and dim for us to conduct follow-up studies and learn more about the systems.
TESS takes a different tack, focusing on bright, nearby stars for which additional characterization of the planets will be easier. For instance, some of the planets discovered by TESS will be observed by NASA’s next behemoth, the James Webb Space Telescope, which will reveal the planets’ atmospheres in exquisite detail.
Barclay’s paper lets us shake the presents under the TESS tree, hinting at the goodies inside. By modeling a wide variety of planets in orbit around the 3+ million stars that TESS will see, they try to simulate the kinds of observations the mission will collect and figure out which planets TESS can find easily and which ones it will struggle with.
For example, they find TESS may discover nearly 300 planets with radii smaller than twice the Earth’s. Among these potentially Earth-like planets, roughly ten will orbit in the temperate zone, making them possible oases for extraterrestrial life.
After reading these results about potentially habitable planets, I was also excited about their prediction that TESS may find 12,000+ giant (i.e. Jupiter-sized) planets. Barclay et al. caution that these objects will be especially hard to distinguish from astrophysical false positives. But these planets may also reveal some of the most interesting astrophysical phenomena, so if there are any clever tricks to extricate these planets, the effort might prove worthwhile.
A pine grove on Puget Sound’s campus.
Just recently returned from the meeting of the Northwestern Section of the American Physical Society, which took place on the lovely campus of the University of Puget Sound.
It was a cozy meeting of physicists and student physicists from throughout the northwest, and there was a variety of talks and posters on topics from gravitational waves to DNA computers to diversity in science.
One of the talks that really stuck out in my mind was the banquet presentation from Puget Sound’s Prof. James Evans about the antikythera mechanism, a mysterious barnacle-encrusted gearwork recovered from an ancient Grecian shipwreck.
Constructed by an unknown artisan in about 200 BC (according to Evans), the machine could track the date, follow lunar phases, predict solar eclipses, and even maybe show planetary motions — all with the turn of a single crank.
For my presentation, I gave a brief overview of exoplanet astronomy, with a focus on how these discoveries have begun to hone our ideas about alien life and extraterrestrial civilizations. I’ve posted by presentation below.
(top) WASP-12 b’s orbit. (bottom) The Roche lobe around WASP-12 b.
WASP-12 b is a planet in crisis.
One of the hottest of the hot Jupiters, the gas giant circles its Sun-like host star in a blistering 1.09-day orbit, giving its an atmosphere hot enough to vaporize rubies.
In fact, WASP-12 b is so close to its star that it very nearly fills its Roche lobe, the teardrop-shaped region inside of which material is bound to the planet. Anything on the other side of the Roche lobe falls into the gravitational clutches of the host star and can either tumble into the star or leave the system altogether.
Because WASP-12 b’s atmosphere is so hot, it is very puffy and extended, and a study from several years ago pointed out that some of the atmosphere can probably spill over the Roche lobe and escape the planet. Indeed, several groups have seen indications of outflow from WASP-12 b, meaning the planet is falling apart in front of our eyes.
Knowing what happens to the gas after escaping the planet is important for understanding the fate of the planet. If the gas goes into orbit around the star, forming an accretion disk, the planet might have billions of years before it’s destroyed. On the other hand, if the gas quickly escapes from the system or is otherwise prevented from forming a disk, the star’s gravity could rip the planet apart in an astronomical blink-of-an-eye.
A recent study from Alex Debrecht and colleagues from University of Rochester Physics and Astronomy explored what happens to gas escaping WASP-12 b. For their study, they constructed hydrodynamic models using the AstroBEAR code and found that a substantial torus of hot gas could build up around the host star in about a decade, potentially enough gas to explain the observations showing some kind of spectral absorption from the system. Such tori may commonly form in systems with ultra-hot Jupiters, so this study is probably relevant to lots of exotic exoplanets.
Three-dimensional simulation of gas torus in the WASP-12 system.
As interesting as these results are, though, they leave some important issues unaddressed. For instance, Debrecht and colleagues didn’t seem to include any accretion onto the host star, which might happen proceed at a fairly high rate. And the balance between outflow from the planet and accretion onto the star will go a long way to determining the amount of material in the accretion disk.
And knowing the amount of material in the disk on orbits interior and exterior to the planet’s orbit is critical for understanding the fate of WASP-12 b and other similar ultra-hot Jupiters. The accretion disk can gravitationally tug on the planet – material interior can push the planet out, while material exterior can push the planet in, potentially dooming the planet to rapid disruption.
But as is true for even the most seminal scientific work, more research is needed.
There are currently only about half a dozen Earth-sized planets orbiting stars that you can see without a telescope or binoculars, called “naked eye stars“. Among this select class, the 55 Cancri system stands out as unique – located in the constellation Cancer, 55 Cancri, a star very similar to our Sun, hosts five planets, ranging from Earth-sized to bigger than Jupiter.
Artist’s conception of the views from four of the 55 Cancri planets. From https://en.wikipedia.org/wiki/File:55cnc.jpg.
The innermost planet, 55 Cancri e, is the one most similar to our own planet, with a mass eight times Earth’s and radius twice Earth’s.
In some big ways, though, 55 e is a far cry from Earth – it’s almost a hundred times closer to its star than we are to ours, meaning its year is only about 18 hours long. Recent observations from the Hubble Space Telescope also suggest it has a hydrogen and helium atmosphere with a pressure comparable to Earth’s. But one thing 55 e might have in common with Earth: active volcanoes.
A recent study from Patrick Tamburo of Boston University analyzed infrared observations of the 55 Cancri system collected by the Spitzer Space Telescope and found evidence for some sort of dramatic change on 55 e.
The observations were collected just as 55 e plunged behind the star as seen from the Earth. Such a configuration is referred to as a planetary eclipse or occulation.
A comparison between transits and secondary eclipses (also sometimes called occultations). In a planetary transit, the planet crosses in front of the star (see lower dip) blocking a fraction of the star’s brightness. In a secondary eclipse, the planet crosses behind the star, blocking the planet’s brightness (see dip in the middle). The latter dip in brightness is fainter due to the faintness of the planet. Image credit: Josh Winn.
During an eclipse, the planet’s host star blocks out any light coming from the planet, which can produce a tiny dip in the total amount of light coming from the system.
The depth of that eclipse dip tells us how bright the dayside of the planet is – a very bright dayside would produce a big dip, indicating a hot and bright atmosphere, while a dark dayside would produce no dip, meaning a very cool atmosphere. But what if the dip is shallow during some eclipses and deep during others?
That’s exactly what Tamburo and colleagues found. In 2012, the planet exhibited eclipses with little to no depth. But when Spitzer revisited the system in 2013, it found whopping eclipses, with the planet emitting about 0.02% of the star’s light. This change corresponds to an increase in the planet’s apparent temperature of more than 1,000 degrees Kelvin (about 2,000 F).
What could cause such a dramatic change? As in the original study of these data, Tamburo and colleagues explore the possibility that an enormous volcanic eruption on 55 e could have injected dust high into the atmosphere (about 100 km up) in 2012, shrouding the lower and hotter atmosphere and surface. By 2013, the dust cloud could have settled out, raising the planet’s apparent temperature.
How plausible is this idea? Surprisingly, plausible actually. Previous studies of 55 e have shown that interactions between the planet and its sibling planets could induce enormous amounts of tidal heating within 55 e, similar to Jupiter’s moon Io, and potentially powering tremendous geophysical activity.
Eruption column rising over Redoubt Volcano, Alaska.
Large terrestrial eruptive plumes have reached 40 km height in Earth’s atmosphere, so perhaps such altitudes are not unreasonable on 55 e. However, 55 e has a surface gravity more than twice Earth’s and its atmospheric temperatures are likely much higher than Earth’s, both of which would inhibit ascent of a volcanic plume. So it’s not totally clear this exciting idea could pan out in detail.
In any case, these new results may represent the emergence a new field of study, observational exoplanetary volcanology, and maybe scientists a few generations from now will be trying to predict volcanic activity on Kilauea and Cancer.
