BSU Journal Club

The Shadows of Extrasolar Comets

Posted by admin on September 1, 2017
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Very neat paper recently published about the possible discovery of comets orbiting a distant star found using data from the Kepler mission.

To find the elusive exocomets, a group led by Prof. Saul Rappaport at MIT conducted an exhaustive search of more than 200,000 lightcurves collected by Kepler over its 3.5 year nominal mission.

By meticulously sifting these lightcurves by hand, Rappaport’s group were able to spot strangely and non-periodic signals, the kinds of signals that computers, with their rigid predictability, have trouble finding.

The shadow of an exocomet.

The figure at left shows the shadows of exocomets orbiting a very bright star, a little bigger than the Sun, as they pass between the star and the Earth. The cometary signal is asymmetric and doesn’t occur on a regular schedule, totally unlike an exoplanetary transit.

Rappaport’s group reports seeing six of these strange signals coming from the Kepler target star. Probably most of the signal is coming from a cloud of dust ejected by the exocomet. Such dust ejections are common for comets in our solar system, giving rise to one of the two lustrous tails usually seen for comets (the other tail is made of a stream of ionized plasma).

By fitting a simple dust model to the shadow signals, Rappaport and colleagues estimate that their exocomet is shedding dust at a rate of about 20,000 tons per second, roughly equivalent to the total mass of meteors that burn up in the Earth’s atmosphere every year.

The shadow signals appear six times, separated by tens to hundreds of days. Assuming the two dips separated by about 200 days are due to one comet (they don’t have to be), the comet would have to contain about as much mass as Halley’s comet and probably more.

Babylonian Astronomy and the Spin of the Earth

Posted by admin on June 2, 2017
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Ancient Babylonian astronomy textbook.

Today, in the first summer meeting of our research group, we discussed a recent paper from Prof. Guillermo Gonzalez of Ball State that explored observations from Babylonian clay tablets to estimate changes to the spin rate of the Earth.

Even though it seems like there’s never enough time in the day, it turns out that the Earth’s rotation rate has been slowing over thousands (and even billions) of years. Numerous effects, including tidal interactions with the Moon, reshaping of Earth via earthquakes, and the melting of glaciers, all contribute to slow down or speed up Earth’s rotation.

One good way to measure the change in the length of the day is to measure the positions of stars, planets, and the Moon in the sky and compare their positions with where you’d expect them to be based on the time. And going back to about 1600, astronomers have used telescopes to accurately measure the timing of celestial configurations to good enough precision that changes in the length of the day can be seen – for instance, the day was about 22 minutes longer when Galileo first pointed his telescope to the heavens.

But the change in length of the day is pretty small – the Moon’s tides slow the Earth about 2.3 milliseconds per day each century – and the change isn’t necessarily constant over time. Fortunately, even thousands of years of ago, humans were keeping track of the positions of objects in the sky, even though they didn’t know what the objects were.

In Gonzalez’s recent paper, he analyzes reports from ancient Babylonian astronomers of lunar occultations and appulses (i.e., close approaches). These reports extend back to 400 BC, giving an observational baseline of nearly 2500 years, and are replete with lovely and ancient descriptions:

“Year 7. Month IV, the 22nd, 64 degrees before sunrise, Saturn came out from the northern horn of the Moon.”

Gonzalez’s results mostly agree with previous work, although he finds that the Earth’s rotation has slowed a bit more slowly than other estimates suggest since the time of Babylon.

Hot Planets and Heavy-Metal Stars

Posted by admin on April 11, 2017
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Artist’s conception of the ultra-short-period planet Kepler-78 b, discovered by Sanchis-Ojeda and colleagues in 2013.

An eyebrow-raising paper emerged recently from Prof. Josh Winn and colleagues about a type of planet near and dear to my heart, ultra-short-period planets, or USPs for short.

These planets are roughly the size of Earth and probably rocky but are hundreds of times closer to their host star than the Earth is to the Sun. These planets are so hot some have melted daysides and others are evaporating. Because they’re so much closer to their stars, ultra-short-period planets zip around their stars in just hours – hence the clunky name.

Our group, along with others, has suggested USPs might be the remnants of hot Jupiters (gas-giant planets close to their stars) that had their atmospheres ripped off. If so, we’d expect systems hosting USPs to resemble systems hosting hot Jupiters.

One distinctive feature of stars with hot Jupiters is that they have more iron (Fe) and other heavy elements in their atmospheres. Astronomers call the amount of heavy elements (“metals”)  stellar metallicity. Hot-Jupiter host stars are heavy in metals probably because planets form from the same materials as the star and big planets need large amounts of metals to form. The same trend doesn’t seem to hold for small, roughly Earth-sized planets, though – small planets don’t seem to be as picky. So, if USPs are hot Jupiters that lost their atmospheres, their stars should also be metal-rich.

Figure 4 from Winn et al. (2017), showing the distribution of stellar metallicities for USP-hosting stars (red), hot Jupiter-hosting stars (orange), and stars hosting small but slightly longer period planets (blue).

But the recent paper from Winn and colleagues throws this origin story for USPs into doubt. In their study, they looked at metallicities for stars hosting USPs, stars hosting hot Jupiters, and those hosting small planets a bit farther out than USPs, all discovered by the Kepler Mission. The figure at left shows their results.

As expected, the orange curve for hot Jupiter hosts peaks toward higher metallicity (that is, toward bigger [Fe/H]-values), and if USPs are former hot Jupiters, the red histogram should look like the orange one.

Instead, it looks a lot like the blue one for smaller, farther out planets. This result suggests that USPs are just like their longer-period cousins – planets that have always been small, just with very short periods.

What to make of this? There’s some statistical wiggle room, allowing some, but not all, USPs to have been hot Jupiters, but Winn’s analysis says no more than 46%. It’s also possible that the boundaries between what Winn calls “hot Jupiters” and what he calls “hot small planets” could be refined by additional analysis, shifting the orange curve down a bit (or maybe shifting the blue curve up).

But the chances that USPs experienced a dramatic and brutal origin are a little slimmer now. Maybe that’s a good thing – it says the universe might be a little bit less violent than we thought.

Fast Radio Bursts and Alien Spaceships

Posted by admin on March 14, 2017
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Artist’s illustration of a light-sail powered by a radio beam (red) generated on the surface of a planet. (M. Weiss/Center for Astrophysics)

A fascinating study published last week out of Harvard CfA exploring the possibility that mysterious astronomical radio signals are actually alien spotlights used to accelerate enormous ships sailing on photonic winds.

Since 2007, astronomers have observed about 20 events involving highly energetic bursts of radio waves, either live or in archival data, called fast radio bursts or FRBs. These bursts lasted for only milliseconds. (For reference, it takes a few hundred milliseconds to blink your eye.) In some cases, the bursts were isolated incidents; in others, they consisted of a few bursts separated by seconds or days.

FRB signatures suggest they originate billions of lightyears away, somewhere outside our galaxy. Given these large distances and the fact that we can see them, the radio bursts must come from some enormously powerful source. Explanations for FRBs range from merging black holes or neutron stars to hyperflares from stars with magnetic fields so strong they would blank out the magnetic strip on your credit card at the distance of the Moon. However, as with most strange happenings in astronomy, the most exciting but controversial explanation involves aliens.

In their recent paper, Drs. Lingam and Loeb of Harvard suggest that FRBs are actually beams from enormous alien spotlights used to accelerate football-field-sized spaceships sailing on immense light sails. This idea isn’t as crazy as it sounds. There have been several recent stories about using light sails and lasers to explore the solar system or even other planetary systems.

Lingam and Loeb combine simple but compelling energetic and engineering constraints to explain the mysterious FRBs using their alien spotlight hypothesis.

  • Why do FRBs appear and disappear quickly? The spotlights are fixed to the surface of a spinning planet and rotate in and out of view.
  • How big and powerful would these spotlights have to be for us to see them from Earth? A little bigger than the Earth and about as bright as the Sun. Such a bright spotlight would probably be very hot, and so to keep it from melting, the aliens would probably have to make it very big to spread out the heat.
  • How big would the spotlights have to be not melt from their own heat? Also a little bigger than the Earth.

So, of course, the idea is still highly speculative, but different aspects seem to hang together, forming a coherent picture. Given how frequently FRBs have been seen, Lingam and Lobe estimate there could be something like 10,000 FRB-producing civilizations in a galaxy similar to our own, roughly consistent with some estimates using the Drake Equation.

The authors even suggest we could test their idea that these spotlights are used to accelerate alien sailing ships by looking for the subtle telltales of shadows cast the ships’ sails in the FRBs.

Automating the Red Buttes Observatory

Posted by admin on March 9, 2017
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Wyoming’s Red Buttes Observatory

As part of rich tapestry of American astronomy, lots of universities across the US have small but highly serviceable observatories, with histories going back many decades. For instance, Boise State’s own on-campus observatory was installed in the late 70s, and although it’s hardy and still functional, like a lot of these observatories, it lacks capabilities that would allow its use as an active research facility. Such small, older facilities have primarily been used for outreach and teaching, rather than up-to-date research.

In the last few years, tremendous improvements in hardware and software have dramatically reduced the costs and expanded the availability of research-grade instrumentation and computational capabilities. Many of these observatories are now roaring to life and contributing to research efforts at the razor-edge of astronomy – characterizing new exoplanets, contributing to rapid-response gamma-ray burst surveys, among other projects.

However, the process of upgrading these observatories is challenging, as we’ve discovered in trying to refurbish Boise State’s observatory, and there is not a lot of guidance out there about best practices.

Recently, David Kasper of University of Wyoming Physics and Astronomy led an effort to automate Wyoming’s Red Buttes Observatory (RBO), located about 15 miles south of Laramie. Thankfully, he documented their work in a paper published late last year.

The paper provides lots of very specific technical details and even the source code the group used to make RBO a facility capable of supporting undergraduate and graduate research. For instance, RBO’s new weather station determines meteorological conditions at the observatory and posts them, every 60-s, to a public website. Automated monitoring of these conditions allows the observatory itself to decide whether the weather is conducive to observing and even to close up the dome in the case of inclement weather.

The new observatory has collected transit observations of hot Jupiters, a project right up my alley. And so, their work will provide an important roadmap for Boise State’s efforts to renew our observatory. We hope, soon, to see the shadows of distant worlds right from downtown Boise.

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If you’d like to donate to help with refurbishing our observatory, please visit this website.

The Origins of Short-Period Super-Earths

Posted by admin on March 7, 2017
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This artist’s impression shows the super-Earth exoplanet orbiting the nearby star GJ 1214.

Eve Lee and Eugene Chiang of Berkeley Astronomy just posted a very interesting paper about the origins of super-Earths in ultra-short-period orbits.

The topic I’ve been interested in most in recent years is the origin and fate of these ultra-short-period planets. These little guys orbit very close to their host stars, taking, in some cases, only a few hours to circle their host stars. In other words, the year for some of these planets is shorter than a feature-length movie.

Such planets were completely unexpected before astronomers began discovering them, and it’s not at all clear where they came from – naïvely, we’d expect that they can’t form where we find them. And many of them are so small (less massive than the Earth in some cases) that tidal interactions, which can cause bigger planets to death spiral into their stars, probably don’t have much effect.

In their paper, Lee and Chiang explored the origins of short-period super-Earths, planets somewhat, but not much, bigger than Earth. This population declines the closer you get to the host star – there are more super-Earths with periods of several days (short-periods) than of several hours (ultra-short-periods), which probably tells us something about the planets’ origins.

It was thought that such planets might originate via gas disk migration. This is the gravitational give-and-take between a nascent planet and the maternal gas disk from whence it arises.

Lee and Chiang found that, surprisingly, this migration on its own would not have made enough ultra-short-period planets but too many short-period planets. Next, they tried to include tidal interactions, which made enough ultra-short-period planets but too many short-period planets.

Instead, Lee and Chiang found that they could explain the short-period super-Earths if they assumed the planets formed near where we find them (and included a little tidal migration). That’s a little surprising since the standard model of planet formation posits that the grains of dust and ice that eventually coalesce to form planets cannot exist within a few days of their host star.

So, if Lee and Chiang are right, these super-Earths, instead of growing up from tiny grains, may have grown from the collisions of 1000-km planetesimals that themselves migrated close to the host star. In this case, the origins of short-period super-Earths may have been particularly violent.

The TRAPPIST-1 System

Posted by admin on February 28, 2017
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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.

“Characterizing Exoplanets for Habitability” by T. Robinson

Posted by admin on January 24, 2017
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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.

The Snow on Pluto Stays Mainly in the Lows

Posted by admin on September 23, 2016
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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.

W&S on the WTF Star

Posted by admin on September 16, 2016
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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.