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.
Artist’s conception of a hot Jupiter shedding mass.
