BSU Journal Club

The TESS Spacecraft.

At our journal club last week, we discussed the discovery of a new warm Jupiter from the TESS Mission.

TESS is the successor to the wildly successful Kepler/K2 Mission and is designed to find exoplanets using the same technique as Kepler – looking for their shadows as planets pass in front of their host stars, i.e. the transit technique.

Sadly, the Kepler spacecraft was officially shut down two weeks ag0 because it ran out of fuel, but TESS, launched last March, is off and running, having already discovered about half a dozen new planets.

One of those planets, we discussed in journal club on Friday – a planet orbiting the star HD1397. The gas giant planet is about the same size as Jupiter but half the mass, making it significantly less dense than Saturn.

The planet also has an unusually eccentric or stretched-out orbit that swings very near its host star, passing to within 8 stellar radii from its star at its closest point. By contrast, the Earth is 200 stellar radii away from the Sun.

If this planet had been discovered 20 years ago, it would have completely stumped astrophysicists, and many would likely have doubted its existence. Nowadays, though, such strange planets are practically the norm in exoplanet astronomy.

So with HD1397 b’s discovery, the exoplanet train rumbles on, and we should expect thousands upon thousands more bizzarities from TESS that will, like Kepler’s discoveries, again re-write the planetary rulebook.


At our research group meeting, we also discussed the art of scientific presentations. I’ve pasted the example presentation I gave below.

Kepler-1625 b’s transit as seen by Kepler, with the putative moon’s transit appearing as “ears” on either side. From Teachey et al. (2018a).

Last week, Alex Teachey and Dave Kipping of Columbia University presented impressive evidence for what would be the very first known moon orbiting an exoplanet.

The planet, Kepler-1625 b, is likely more massive than Jupiter but in an Earth-like orbit around an old (9 billion years old) but otherwise Sun-like star. Discovered in 2016 among data from the Kepler Mission, the planet was subjected to an intense analysis by Teachey and Kipping as part of the Hunt for Exomoons with Kepler program. In spite of their persistence, Teachey and Kipping found only hints of a moonshadow accompanying the planet’s distinct transit signal.

Hoping to corroborate their putative moon, they applied for and received 40-hours to observe the system with the Hubble Space Telescope (HST) and look for more lunar transits. In these data, Teachey and Kipping found even more convincing evidence for a moon.

Kepler-1625 b’s and the putative moon’s transits as seen by HST. From Teachey et al. (2018b).

In the figure above, the little dip just to the right of the bigger dip (the planet’s transit) shows every sign of being the shadow of an exomoon circling a star about 7000 lightyears from Earth. Look around at how lucky we are to be alive right now.

Because of the extraordinary magnitude of their claim, Teachey and Kipping peppered their paper with lots of caveats, extending even to their paper’s title (“evidence for an exomoon”, not “we found a large exomoon”).

On top of that, they deployed an flotilla of statistical tests to argue in favor of the exomoon interpretation. One test in particular figures prominently in their analysis – the Bayes factor.

In this context, this ominous-sounding number is a measure of how much more likely one scientific model is over another, given a dataset. For instance, if you found your dog guiltily hiding from a mess in your house (your dataset), you would conclude there is a higher probability your dog made the mess (one scientific model) than a ghost did (another model).

A portrait of someone who may be (but probably isn’t) Thomas Bayes.

The Bayes factor derives from work by the Rev. Thomas Bayes, a minister living in Georgian England, who developed a method to infer the underlying probability for a particular experimental outcome, given results from several actual experiments.

Later, the scientist Simon-Pierre Laplace developed Bayes’ work into a more general theory of inference that he hoped could be used, for example, by juries to judge the guilt or innocence of a defendant.

Nowadays, Bayesian inference shows up everywhere, from analyses of climate change to estimates of the frequency of orange Reese’s pieces. It’s even possible that our brains are natively wired as Bayesian-inference machines.

And so in deciding whether they’d found an exomoon, Teachey and Kipping compared the probability that their Hubble data arose from a model including a lunar transit (as well as gravitational tugs between a planet and moon) to the probability the data showed a lone transiting planet.

Although, as they caution, these probability estimates can’t account for everything, they find the planet-moon model is 400,000 times more probable than the planet-only model.

As always, more data are needed to corroborate this fantastic result, but if it holds up, Kepler-1625 would be a system with one super-sized Jupiter-like planet accompanied by a Neptune-sized moon which orbits at a distance of about 300,000 km, not too different from our own moon’s distance.

Very shortly after Teachey and Kipping’s work was published, Kollmeier and Raymond explored the question of whether this monster moon could have its own moon and found that even a moon as large as Ceres could remain stable.

This result immediately prompted a more pressing question: should we call such a body a “sub-moon” or “moonmoon”?

 

Atacama Compact Array (ACA) on the ALMA high site at an altitude of 5000 metres in northern Chile. From here.

If you haven’t heard about it, the ALMA array, a collection of sixty-six, 12-meter radio dishes situated high in the Atacama desert, is phenomenal. Using the technique of radio interferometry, it’s capable of imaging astronomical objects in infrared (and redder) light with incredibly high resolution.

HL Tauri, as seen by ALMA.

For instance, the image at left was captured by ALMA and shows the debris disk in an infant planetary system orbiting a distant star, HL Tauri. The bull’s-eye pattern is (probably) created by nascent planets still growing by scooping up dust and gas. That disk is physically larger than our whole solar system, but as seen from Earth, 450 light-years away, the disk subtends an angle about 3 micro-degrees across – about the same as the Statue of Liberty as seen from Boise.

As it turns out, Jupiter’s moon Europa, an icy body only a little smaller than our moon, is about as big seen from Earth, making a good target for the ALMA array. Moreover, since the Galileo mission‘s exploration of the Jupiter system, few detailed and high-resolution observations have been made of Europa. On top of that, Europa has a subsurface water ocean that could host alien life.

With all this in mind, Caltech graduate student Samantha Trumbo and Prof. Mike Brown (of Pluto-killing fame) collected ALMA observations of Europa in the fall of 2015. Since ALMA observes in infrared wavelengths, it’s sensitive to heat coming off Europa and essentially acts as a long-distance thermometer, allowing them to map the temperature on Europa’s surface. If certain parts of Europa are warmer than expected, that could indicate sub-surface heating, which might have big implications for any Europan life.

Temperature map of Europa, from Trumbo et al. (2018).

But instead of mysterious hotspots, Trumbo found equally strange cold spots. The color map of Europa at left (red means hot, blue means cold) compares the expected temperatures (“Model”) to what’s actually observed (“Data”), and there are big differences all across Europa.

So what does this mean? Trumbo et al. say it’s not clear but suggest one possibility. The region with the largest temperature discrepancy corresponds to the location of highest water ice abundance, where water from the sub-surface may have been volcanically extruded onto the surface. Since this region was not been imaged at high resolution by Galileo, it’s hard to identify landforms that might corroborate recent eruptions, but such features have been observed elsewhere on Europa.

As always in science, more data would help resolve the puzzle. In any case, NASA is planning a mission for launch in the 2020s that will use an ice-penetrating radar, not too different from ALMA, to probe Europa’s sub-surface ocean and, hopefully resolve the mystery of Europa’s cold spots.

 

Artist’s impression of planet alignment in 2016. From here.

Anyone who’s done some stargazing has probably noticed that the Sun and the Moon appear along nearly the same arc in the sky. This Sun’s arc, called the ecliptic, corresponds to the plane of the Earth’s orbit. Since all planets in the solar system share nearly the same orbital plane, they likewise hew close to this arc. It turns out that the ecliptic also coincides closely with the Sun’s equator.

The near alignment of all planetary orbits in the solar system is one of the most important clues to their formation – the solar system originated billions of years ago from a thin disk of gas and dust girding the young Sun’s belly like a hula hoop, an idea going back at least to Immanuel Kant in the 1700s called the Nebular Hypothesis.

Once it was accepted, this idea was so successful at explaining and predicting features of the solar system, astronomers believed all planetary systems in our galaxy would resemble our own – with small, rocky planets close to their stars and large, gassy planets farther away, but all sharing the same orbital plane.

The discoveries of thousands of exoplanets have turned all that on its head – planets around other stars have orbits oriented every which way. For example, the Upsilon Andromeda system has three Jupiter-like planets, all on orbits that are widely misaligned.

Although these topsy-turvy planetary orbits were initially puzzling, astronomers are starting to tease out the explanations for these systems. Planets probably do start out in well-aligned orbits, but, like kids in the backseat on a long car trip, jostling between the planets (due to mutual gravitational tugs) soon upsets this delicate arrangement and upends the orbits. In the case of Upsilon Andromeda, planets may even have been ejected from the system.

A recent study from Fei Dai and colleagues explored connections between orbital misalignment and the origins of one puzzling class of exoplanet – small, short-period planets. These planets range in size (and probably composition) from Neptune-like to smaller than Earth but inhabit orbits very close to their host stars, some taking only hours to circle the star. Many of these short-period planets also have sibling planets farther out, and the arrangement of these orbits might tell us how the planets got so close to their stars.

As for the Upsilon Andromeda system, the mutual inclination between the orbits, if its big, may point to a history of violence in the system. Such violence may explain how the short-period planets got so close to their stars – they could have started out far away and been thrown by their siblings toward the star. By contrast, a small mutual inclination could mean the system has always been relatively quiescent, and the short-period planets may have gently migrated inward from farther out.

By analyzing the transit light curves of the planets as observed by the Kepler spacecraft, Dai and colleagues found a pattern in the mutual inclinations for these systems. From their paper, the figure below shows that when the distance of the shortest-period planet in a system a/R* is larger, the mutual inclination ΔI between orbits tends to be less widely distributed.

Figure 3 from Dai et al. (2018).

What does this result mean? Since the short-period planets closest to their stars (small a/R*) also seem to have a very wide range of mutual inclinations, maybe they experience the same kind of gravitational jostling that took place in Upsilon Andromeda, while planets farther out, they were moved in more gracefully.

Taking a wider perspective, evidence is mounting that, while planetary systems are common in the galaxy, our own solar system is unique in many ways – there’s really no place like home.

Map of Mars’ south polar ice. The colors show how thick the ice is, and the black rectangle shows the location of the newly discovered sub-surface lake. From Orosei et al. (2018).

As summer winds down and we prepare for the fall semester, I finally found the time to read the recent announcement about finding sub-surface water on Mars using the MARSIS radar onboard Mars Express.

Although evidence for liquid water on Mars has been reported for a long time, these reports are almost always about ancient flows or very modest, salty trickles (and the presence of water often turns out to be illusory). By contrast, if this most recent report holds up to scrutiny, there could be 10 billion liters of liquid water under the martian south pole.

That’s not much on the scale of the Great Lakes (the smallest one, Lake Erie, contains 10,000 times more water), but it’s more than a thousand times the volume of tanks at the Georgia Aquarium, which hosts more than 100,000 aquatic animals. So the martian lake could easily host a microbial zoo (although no direct evidence for that as of now).

As is common in polar regions on Earth, the martian water lies under kilometers of polar ice and is probably so cold it requires some kind of geological anti-freeze to keep from freezing solid (the kinds of mineral salts that can do the job are actually pretty common on Mars). The overburden pressure from all the ice also helps keep the water liquid.

But the fact that the lake sits underneath so much ice raises an obvious question: how did the scientists spot it in the first place? The answer is related to why the recent wildfires in the west, in addition to fouling the air, have given us very lovely sunsets.

Preferential scattering of blue light by the atmosphere.

When it first leaves the surface of the Sun, sunlight is colored white. But as it passes through the atmosphere, the light (which is a wave of electric and magnetic fields, after all) interacts electromagnetically with the atmospheric gas molecules, which themselves contain electric charges.

The closer the wavelength of the light ray is to the sizes of the molecules, the stronger this electromagnetic interaction and the more the ray can be diverted from its straight path.

Since blue light has a wavelength (500 nanometers) closer to the size of the atmospheric molecules than red light (700 nanometers, it is more readily diverted or scattered. At dusk, as the sun sets, its light has to pass through more and more of the Earth’s atmosphere. So more and more blue light is scattered away, leaving behind more red light and making the Sun look red. If you sprinkle in lots of smoke from a wildfire, you can enhance the coloration.

What does all this have to do with martian lakes? The MARSIS instrument used to find the subsurface lake uses very red radar light, with wavelengths tens to hundreds of meters long. Similar to red sunlight, such long wavelengths can easily pass through even solid rock since they’re much larger than the rocky molecules that make up the martian surface.

Reflectivity of radar light from beneath the martian south pole. The bright patch at the bottom marked with “Basal reflection” is from the sub-surface lake. From Orosei et al. (2018).

This explanation simplifies things a lot, but the upshot is that MARSIS could see the lake as a very unusual bright patch underneath all that polar ice.

What’s next? It’s possible that continued data collection and analysis will turn up other subsurface lakes on Mars. If Mars’ south pole is brimming over with these icy lakes, it could be an especially good habitat for martian microbes. So maybe the effort to find martian life should explore using the same ice drilling technology being considered for exploring the oceans of Jupiter’s moon Europa.

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.

(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.