All posts for the month September, 2016

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.

osiris-rex_artists_conceptionNASA’s OSIRIS-REx asteroid sample return mission launched on September 8th to visit asteroid Bennu, a carbon-rich, near-Earth asteroid. The spacecraft will rendezvous with the asteroid in 2018 and ultimately bring samples of Bennu back to Earth in 2023. Join the Boise State Physics Department on Oct 7 at 7:30p to celebrate the launch.

The event will kick off at 7:30p in room 101 of the Multipurpose Classroom Building on Boise State’s campus, right across the street from the Brady Street Parking Garage. Alessondra Springmann, a planetary scientist at the University of Arizona, will give a public talk on the OSIRIS-REx Mission.

At 8:30p, the event will move to the top of the Brady Garage, where telescopes will be set up for gazing at the Moon, Mars, and Saturn.

For more info, or e-mail Prof. Brian Jackson (

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.

IMG_0308We had a brilliant time on Friday, talking about the recent discovery of Proxima Centauri b, even though the clouds prevented us from star-gazing. Lots of great questions from the audience, with some really good ones from the youngest audience members.

Thanks to my student volunteers to sticking it out and to KBSX for helping us advertise the event. Most of all, thanks to our wonderful audience for coming.

For the rest of the semester, Boise State Physics will host public star-gazing events on the first Friday of every month at 7:30p, so the next one will be on Oct 7. Stay tuned for details!