BSU Journal Club

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.




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

Artist’s conception of Vanderburg’s disintegrating body. From

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.



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.

Screen Shot 2015-10-30 at 5.00.32 PMI’m gearing up for the K2 Science Conference next week and preparing my presentation. So this week at journal club, I thought it would be fun for everyone to give short presentations on their research projects.

Jennifer Briggs talked about looking at secondary eclipses of the hot Jupiter HAT-P-7 b and how we’re trying to use variations in the eclipses to look for meteorological variability.

I presented some preliminary results from our SuPerPiG search for very short-period exoplanets using data from the K2 mission. The practice talk was very helpful to me because I learned that I had way too many slides.

We spent a little time talking about good presentation style and techniques, and it reminded me that Emily Lakdawalla of the Planetary Society put together a very good blog post about how to give a presentation.

This week’s attendees included Jennifer Briggs, Emily Jensen, Karan Davis, Tyler Gordon, Hari Gopalakrishnan, Ahn Hyung, and Jake and Steven (whose last names I still don’t know).

The Kepler-11 planetary system, with at least 6 planets in short orbits. From

The Kepler-11 planetary system, with at least 6 planets in short period orbits. From

Following on last week’s journal club where we discussed a paper in which collisions removed planetary atmospheres, this week we looked at a new paper by Aaron Boley and colleagues in which collisions promoted accretion of an atmosphere.

Boley and colleagues modeled gravitational interactions in tightly packed planetary systems, like the Kepler-11 system, 6 planets packed into a space smaller than Venus’ orbit.

Not surprisingly, when so many planets are packed into such a tight space, bad things can happen, and Boley and colleagues showed that such planets often collide with one another, sticking together to form even larger planets. In some cases, the newly formed planet can be large enough that it can accrete gas from its maternal protoplanetary disk and form a gas giant planet.

The standard model for planet formation suggests gas giants shouldn’t form close to their host stars, but Boley and colleagues argue that their collisional scenario could explain the presence of so many hot Jupiters and Neptunes found around Sun-like stars in the last few decades. Their work could help resolve the puzzle of hot Jupiters, an exoplanet mystery older than some of my students.

Journal club attendees included Jennifer Briggs, Emily Jensen, Karan Davis, Tyler Gordon, and Jacob Sabin. (Physics majors Jake and Steve also attended, but I don’t know their last names.)

Artist's depiction of a collision between two planetary bodies. From

Artist’s depiction of a collision between two planetary bodies. From

We read a fun paper in journal club today, written by Inamdar and Schlichting of MIT that looks at the impact of large impactors on the atmospheres of gas-rich exoplanets.

Among the surprising discoveries of exoplanet searches is a huge class of  gas-rich planets between Neptune and Earth in size. Called sub-Neptunes or super-Earths, standard models for planet formation predict these planets shouldn’t exist — either they should have remained as small as the Earth as they accreted or they should have quickly grown to the size of Jupiter or Saturn. We don’t have planets like these in our solar system, but they may be one of the most abundant type of planet in the galaxy.

Even harder to understand, sub-Neptunes display a very broad range of densities, with some having densities greater than Earth’s and others with the density of wind-packed snow. This diversity indicates some planets have large rocky/icy cores with just a little gas on top, while others have tiny cores with bloated hydrogen/helium atmospheres. Since we think gaseous planets all form more-or-less the same way, it’s hard to explain this wide range of internal structures.

Inamdar and Schlichting explore the possibility that giant impacts between young planets in these systems could account for this diversity. By applying a simple 1-D hydrodynamic model, they show that these massively violent collisions could easily remove large amounts of atmosphere from the young planets.

Whether a certain planet experienced such a collision depends in a stochastic way on the initial conditions and gravitational interactions in these chaotic young planetary systems. So some planets would have experienced large collisions that removed a lot of their atmospheres, giving a high mean density, while others didn’t, leaving them low-density.

These same kind of planetary collisions shaped the diversity of planets in our own solar system. For example, the Earth’s Moon formed as the result of a collision between the proto-Earth and Mars-sized object, named Theia. Uranus probably got its unusual tilt from a collision with an Earth-sized object early in its history.

So even though most extrasolar planetary systems we know about don’t resemble our own, the results from this study show the same processes shaped them, and planets everywhere probably experienced a violent adolescence.

Journal club attendees today included Jennifer Briggs, Karan Davis, Hari Gopalakrishnan, Tyler Gordon, Emily Jensen, and Jacob Sabin.


The geysers near Enceladus' south pole. From

The geysers near Enceladus’ south pole. From

Saturn’s moon Enceladus has inspired fascination since Herschel found it in the late 1700s. The discovery of active cryovolcanoes geysers** on its surface by the Cassini mission in 2006 raised that fascination to a feverous intensity.

Although the source of energy powering the volcanoes geysers** was not (and is still not) understood, their implication was clear: Enceladus could have a sub-surface ocean, like his big sister Europa.

The follow-up discovery of salty particles in the geysers all but clinched the existence of a sub-surface water source. Salt in the eruptions would probably require liquid water to dissolve rocky materials to produce the salt. But it wasn’t clear whether the sub-surface source is a small pocket of water directly beneath the geysers or a more global-scale ocean.

In our journal club today, we discussed a paper that points to the existence of a global ocean. The recently published study from Peter Thomas and colleagues analyzed Enceladus’ rotation to study its internal structure. By tracking the motion of hundreds of control points on Enceladus’ surface, they found that its outer icy crust oscillates back and forth during its rotation much more than it should if the moon were solid all the way through.

Thomas and colleagues show that the large oscillations they found (called libration) require a large layer of fluid within Enceladus to lubricate the space between its outer, icy shell and its rocky interior. Otherwise, the moon would oscillate a lot less. Difficult to say exactly, but Thomas and colleagues estimate the ocean could be as thick as 30 km beneath an 20-km thick icy crust.

Just as Europa’s ocean, a sub-surface ocean in Enceladus could represent an enormous harbor for life. Even on this tiny moon, such a deep ocean is only about a tenth the size of the Earth’s oceans*.

Although Enceladus’ surface would be a tough neighborhood for life, sub-surface biota (if they ever evolved) would be protected by a thick layer of ice from the vacuum of space and interplanetary radiation. In fact, this same radiation could impinge on the surface and produce a steady supply of biologically useful oxidants, which could then trickle down into the subsurface ocean and help power the alien biosphere.

Thomas and colleagues suggest a more detailed analysis of Enceladus’ surface geology, newly inspired by their discovery, might help unravel the history of the ocean. Similar analyses of Europa’s complex tangle of surface ridges and cracks helped piece together that moon’s geological history.

Journal club attendees today included Jennifer Briggs, Hari Gopalakrishnan, Tyler Gordon, and Jacob Sabin.

*Earth’s oceans are, on average, 4 km deep, much smaller than Earth’s radius of 6,400 km. Therefore, I approximated their volume as pi*(6,400 km)^2 (4 km) ~ 500 Mkm^3. A similar calculation for Enceladus’ ocean — pi*(50 km)^2 (30 km) ~ 24 Mkm^3.

**Dr. Thomas kindly pointed out that there is some debate over the nature of the eruptions on Enceladus, and volcanoes is probably not the term to use.