BSU Journal Club

Unless you were living under a very large and heavy rock last week, you probably heard about the discovery of seven planets in the TRAPPIST-1 system by Michaël Gillon and colleagues.

Although this system was already known to host three, roughly Earth-sized transiting planets, the discovery of four more throws the door wide open for habitability – all seven planets receive the right amount of starlight from their diminutive red-dwarf host that liquid water might be stable on their surfaces.

There are so many interesting questions to explore for this system – What are the planets’ atmospheres like? How did this system of tightly-packed planets form and how do their orbits remain stable? And, of course, are they habitable?

Fortunately, concerted follow-up observations and theoretical studies can probably a lot of these questions. The fact that the planets all transit their small host star means their atmospheres are ideal for study by the James Webb Space Telescope. Strong gravitational tugs among the planets caused their orbits to change visibly over the course of the observations, so we have strong constraints on how exactly the planets interact.

The last and probably most important question is going to be a lot more difficult to answer. But since a detailed understanding of this system is likely (and probably inevitable, given the enormous enthusiasm for this system), we’ll soon be very close to answering the question of whether the TRAPPIST-1 system is habitable and maybe even inhabited.

One bit of trivia: the TRAPPIST survey that discovered this system was named in honor of the contemplative Roman Catholic religious order of Trappists, and the astronomers reportedly celebrated their discovery with a round of Trappist beer. Maybe this should be the start of a new tradition of naming new planetary systems after beers.

Glint from a sea on Saturn’s moon Titan. From http://www.jpl.nasa.gov/spaceimages/details.php?id=PIA18433.

Our spring semester journal club opened with a nice review paper on finding habitable planets from Tyler Robinson, NASA Sagan Fellow at UC Santa Cruz.

The traditional definition of a habitable planet is “a world that can maintain stable liquid water on its surface”, but, as astrobiologists have explored for decades, this definition involves a vast flotilla of assumptions and very narrowly focuses our search for Earth-like life.

Even with all its limitations, this definition provides a very useful and practical starting point – at the first order, whether a world can host stable liquid water on its surface depends on the amount of sunlight it receives and whether it has a sufficiently thick (but not too thick) atmosphere.

Having found countless worlds outside our solar solar, astronomers are able to assess whether those worlds satisfy these conditions using observations we can already make, and a few dozen (probably) do.

In his review paper, Robinson discusses the observational and theoretical techniques astronomers can employ in the near future to take the next steps in deciding whether a world really has liquid water. Among the different approaches he describes, one is the most striking is the search for the glint from an alien ocean.

Robinson points out that Galileo was the first person to propose how to look for an ocean on another world. In his controversial Dialogue Concerning the Two Chief Worlds Systems, he says that, if the Moon had seas, “the surface of the seas would appear darker, and that of the land brighter”, just as on Earth.

Seas can also appear very bright compared to land, given the right observing geometry: seas exhibit specular reflection – this is the same effect you see looking out a plane’s window when the Sun reflects off the ocean. So looking for the glint provides a way to find large bodies of water on a distant world. Indeed, the first extraterrestrial oceans were found on Saturn’s moon Titan using this method, and one of the sea glints now frequently observed by the Cassini mission is shown in the figure at left.

Of course, we don’t have spacecraft orbiting any extrasolar worlds (yet), so we can’t resolve individual points on their surfaces. But, as discussed by Robinson, as they orbit their host stars, some of those worlds line up the right way that we could see a spike in the total amount of light coming from the planets. Observing such a spike over and over again whenever the planet was in the right geometry would be a strong hint that it had a large body of liquid reflecting sunlight. Given a little more information about planetary conditions, we could confidently infer such a planet had liquid water on its surface.

Amazingly, astronomers have used Earthshine reflected from the Moon to indirectly observe sea glints from the Earth. And so we’ve actually detected oceans on two worlds using distant spacecraft (if you let me call the Moon a “spacecraft”). As Robinson’s review implies, astronomers are probably on the cusp of finding oceans on extrasolar worlds. From there, it’s just a hop, skip, and a jump to finding life.

At journal club today, we discussed a recent paper in Nature from Tanguy Bertrand and François Forget that looks at how the topography and meteorology of Pluto conspire to produce the dramatic frosts and glaciers seen on the surface of Pluto during the recent New Horizons fly-by.

One of the most spectacular results from the fly-by was the discovery that Pluto has rugged mountain chains, enormous geographic basins, and flowing glaciers. The image below shows the evidence for glacial flow in Sputnik Planum, called the Heart of Pluto.

It had been suggested that the flowing nitrogen frost might have collected in Sputnik Planum from a source region connected to Pluto’s deep interior.

However, coupling a sophisticated meteorological model to a model for vaporization and condensation, Bertrand and Forget show in their study that the gathering of frost in Sputnik is likely just due to the fact that it’s a deep basin, about 4 km below the Plutoid.

As a result, the atmospheric pressure tends to be larger at the bottom of the basin than elsewhere on Pluto’s surface, which encourages frost deposition there. The authors point to a similar effect on Mars, where CO2 snows out preferentially at the south pole in Hellas Basin.

It’s worth keeping in mind that the atmospheric pressure at Pluto’s surface is one one-hundred-thousandth the pressure at Earth’s surface, but even with a dwarf atmosphere, this dwarf planet exhibits complex and fascinating meteorological and geological phenomena.

And just because it’s awesome, here’s a synthetic fly-over of Pluto’s surface, generated by the New Horizons mission.

Flux time series for Boyajian's star, showing the 4-year Kepler observations. From Boyajian et al. (2016).

Flux time series for Boyajian’s star, showing the 4-year Kepler observations. From Boyajian et al. (2016).

At journal club today, we discussed a recent study from Jason Wright and Steinn Sigurdsson at PSU astronomy on a strangely dimming star observed by the Kepler mission.

The star has been called the WTF star (‘Where’s the Flux?’), Tabby’s Star (and probably a few more colorful things by perplexed astronomers), but Wright and Sigurdsson invoke the long astronomical tradition of naming noteworthy stars with their discoverers’ last names — they call it Boyajian’s Star, after Dr. Tabetha Boyajian, astronomer royale at Yale.

The strange thing about Boyajian’s star is that the Kepler mission observed the star to dim dramatically several times over a few years, dropping by 20% over the course of a few days several times over a few hundred days. That would be like having a partial solar eclipse that lasted 96 hours every few months. Even stranger, recent analyses of 100+ year old photographic plates suggest the star has been dimming, unnoticed, for a long time.

Various explanations for this strange behavior have been proposed, from enormous swarms of comets obscuring the star to alien megastructures, and Wright does a very good job exploring the different possibilities on his blog.

But as usually happens in astronomy, the most exciting explanations are the least likely (probably not an alien Dyson sphere), and Wright and Sigurdsson favor the idea that some sort of interstellar material between the Earth and Boyajian’s star is obscuring the star. Wright and Sigurdsson point out that, by measuring the distance to the star, the Gaia mission will help us resolve the mystery.

Fig. 11 from Barnes et al. (2016) showing evolution of the HZ (blue region) of Proxima Centauri, along with the orbits of Proxima Centauri b (solid line) and Mercury (dashed line).

Fig. 11 from Barnes et al. (2016) showing evolution of the HZ (blue region) of Proxima Centauri, along with the orbits of Proxima Centauri b (solid line) and Mercury (dashed line).

As a follow-up to last week’s Proxima Centauri b event, we discussed a recent analysis of the planet’s habitability by Prof. Rory Barnes and colleagues in our weekly journal club.

In this paper, the authors consider a very wide range of evolutionary scenarios for Proxima b to explore the resulting range of outcomes and decide how habitable the planet is, really.

They incorporate lots of potentially important effects, including the evolution of the host star’s luminosity and its influence on the planet’s surface temperature.

M-dwarf stars, like Proxima Centauri, get dimmer early in their lifetimes. As a consequence, the surface temperature of a planet orbiting such a star can drop over time.

Or, put another way, the habitable zone (HZ) around the star can move inward, meaning planets that start out interior to the HZ (i.e., planets that might be too hot to be habitable) may eventually enter the HZ.

Figure 11 from Barnes et al. (2016) shows that this is probably what happened to Proxima b: it started out way too hot for habitability and, as its host star dimmed, it entered the HZ.

As Barnes et al. show, such a history could potentially drive away all the planet’s water (assuming it started with any), leaving behind a dried husk of a planet. But the fact that the planet is CURRENTLY in the HZ could fool us into thinking it’s habitable.

This result shows that planetary habitability is a complicated idea and that the current conditions on a planet can depend in a complex (and hard-to-determine) way on its history. Time (and lots more data) will tell whether Proxima b is actually an extraterrestrial oasis for life or a barren wasteland.

Raising_Super-EarthIn the three years since my daughter was born, the one lesson I’ve managed to wrench for the morass of toddler tantrums and sleepless nights is that, like all people, children are complicated and no two are exactly alike. However, decades of childhood development studies have shown at least one interesting (if sad) commonality — stress during childhood can stunt a child’s mental (and even physical) growth.

Like our understanding of child development, our understanding of the formation and evolution of planets, from the largest gas balls to the smallest icy specks, is still in its infancy. And the bewildering variety of exoplanets discovered in recent years has challenged even the fledgling comprehension we once had.

Particularly puzzling is the class of exoplanets known as sub-Neptunes or super-Earths. These planets are somewhere between the Earth and Neptune in size, many rich in hydrogen and helium. Even though they seem to be the most common type of planet in our galaxy, how they form is still an open question.

We understand a little better, though, how they grow up, and a recent paper by Chen and Rogers develops a new model to track the evolution of super-Earths after they form, as the planets age over billions of years.

The study applies this new model to investigate how we can use the size of a super-Earth to determine what the planet’s made out of. Normally, you’d need at least a planet’s mass AND size to constrain its composition, but several recent studies, including Chen and Rogers’s, show that the size of a super-Earth is mostly sensitive to the mass of its atmosphere — add just a little atmosphere to a super-Earth, and its radius blows up a lot. This result is hugely useful since most exoplanets only have their radius, not their mass, measured.

Chen and Rogers also explore the effects of atmospheric loss on super-Earths. Many known exoplanets are so close to their host stars that they are actively losing their atmospheres to space. Chen and Rogers show that this mass loss can completely remove the atmospheres of a very small super-Earth and remove a lot of the atmosphere from a large super-Earth, like childhood stress, leaving the planet stunted.

The upshot is that super-Earths with atmospheres that are small but not too small (about 1% of the planet’s mass) preferentially retain the atmosphere. Consistent with other studies, this result helps explain the otherwise puzzling frequency of super-Earths with smallish atmospheres.

In this age of open-source code, Chen and Rogers plan to make their model publicly available. So soon anyone will be able to raise a super-Earth from adolescence into adulthood and finally senility.

 

From http://www.redshift-live.com/binaries/asset/image/25908/image/Graviational_Waves.jpg.

From http://www.redshift-live.com/binaries/asset/image/25908/image/Graviational_Waves.jpg.

Nothing. They just waved.

Led by physics major Tyler Wade, this week’s astronomy journal club discussed the very exciting result from the LIGO collaboration, the first detection of gravitational waves.

Einstein predicted the existence of gravitational waves back in 1916. (If your differential geometry and German are any good, you can read the original paper here.) Essentially, gravitational waves are a consequence of that fact that mass can distort the shape of space (that’s what we call gravity).

The upshot of this is that any massive object in motion can excite gravitational waves, but only very massive objects (like, black hole-sized) produce waves big enough that we have any hope of measuring them.

And so for the last few decades, the LIGO project, along with other gravitational observatories, has been monitoring the space-time continuum, looking for tiny distortions due to rapid, oscillatory motion of massive celestial bodies.

LIGO attempts to detect these distortions by sending two laser beams, one each, out and back along two orthogonal 4-km tunnels. By measuring the travel time for each laser beam down each tunnel, they can determine their lengths to a ridiculous precision. A passing gravitational wave would VERY slightly modify the tunnel lengths in a particular way.

How slightly? The signal reported last week by LIGO corresponds to a change in the tunnel length by 0.0000000000000000000001 meters. That’s the equivalent of a change in the width of the Milky Way galaxy by 1 meter.

At two different observatory sites, one in Washington state and the other in Louisiana, the LIGO collaboration measured the distinctive signature of gravitational waves generated by two black holes, many times the mass of the Sun, as they completed their death spiral, merging into an even bigger black hole and radiating an enormous amount of energy.

Why is this important? Well, seeing gravitational waves is not going to allow us to control gravity (at least not yet), and the fact that they exist is not surprising. Instead, LIGO has provided us a brand-new way of doing astronomy.

It’s as if, up until now, we were doing astronomy colorblind, and suddenly LIGO built a color telescope. Of course, being able to see in color would open up vast and unexpected vistas on the universe. The detection of gravitational radiation is the same kind of revolutionary achievement.

NYT has a really great animation and video describing how the detection worked, which I’ve embedded below.

The red dots show the observations, with the dips due to asteroid chunks transiting the white dwarf. The inset shows an artist's conception of the disruption process.

The red dots show the observations from this study, with the dips due to asteroid chunks transiting the white dwarf. The inset shows an artist’s conception of the disruption process.

For our second journal club meeting this semester (didn’t manage to blog the first one), we discussed a study from Saul Rappaport and colleagues on observations of the white dwarf WD 1145+017, which continues to show evidence that it is eating a small asteroid.

A study last year from Vanderburg and colleagues (which we discussed last semester) presented observations from the K2 Mission showing distinctive but highly-variable transit signals coming from WD 1145+017. That group conducted follow-up observations that pointed to the presence of an asteroid very close to the star, being ripped apart by the star’s gravity.

As crazy as it sounds, the idea that some white dwarfs are eating asteroids is fairly well-established, but Vanderburg’s study was the first to present observations of the process clearly in action. The variability of the transit signals indicates that the violent process is dynamic and complicated.

This new study from Rappaport and colleagues continues the saga of WD 1145+017 and finds that the disruption process persists more than a year after the initial observations. And using the apparent drift rates of the different chunks of asteroid, Rappaport is able to constrain the mass of the parent asteroid to be about 1% that of Ceres in our solar system.

One of the most exciting aspects of this study for me is that the observations were made using a network of small, amateur telescopes. Some of the scopes used in the study were 25-cm, and so I’m hopeful that, in the near future, we will be able to use Boise State’s own Challis Observatory to conduct follow-up. Just gotta wait for a clear night.

We had our last research group meeting of 2015 on Friday since finals are coming up soon. Fairly large crowd, though, for a meeting so late in the year.

Artist's conception of Vanderburg's disintegrating body. From https://www.cfa.harvard.edu/~avanderb/page1.html.

Artist’s conception of Vanderburg’s disintegrating body. From https://www.cfa.harvard.edu/~avanderb/page1.html.

We discussed Andrew Vanderburg’s discovery of a disintegrating minor body orbiting a white dwarf star.  The body, as small as Ceres or smaller, is so close to its host star that it’s actively evaporating and falling apart, and the shadows of the resulting dust cloud is visible data from the K2 Mission. The dust then falls onto the white dwarf, polluting its atmosphere in a way we can see spectrally.

We also had a very impressive presentation from Hari Gopalakrishnan of Renaissance High School on a recent study from Jim Fuller at Caltech. Fuller and colleauges analyzed oscillations at the surface of a red giant star to infer the presence and strength of magnetic fields deep in the star’s interior. Hari kindly shared the presentation, which I’ve linked below.

Attendees at this journal club included Jennifer Briggs, Karan Davis, Emily Jensen, Tyler Gordon, Steven Kreyche, Jake Soares, and Hari Gopalakrishnan.

From http://kepler.nasa.gov/images/mws/kepler4441.jpg.

From http://kepler.nasa.gov/images/mws/kepler4441.jpg.

We had an abbreviated research group meeting today at which we discussed the recent K2 Science Conference before I head off to the DPS conference in Washington DC. Everyone was in good spirits, considering how late in the semester it is. We’re planning to meet once more before Thanksgiving and will probably go on hiatus until spring semester after that.

Today’s attendees included Hari Gopalakrishnan, Jennifer Briggs, Emily Jensen, Karan Davis, Tyler Gordon, Jake Soares, and Steven Kreyche.