Boise State’s research computing group is hosting a conference today and tomorrow on scientific computing. Along with several others, I was invited to give a 7-min, lightning talk about our research group‘s use of computing.
One of the research computing things we do is time-series analysis to look for new planets in data from the Kepler/K2 Mission. So I talked about the new planets our group has helped find – my talk is below.
Planetary radii and orbital periods for planets (black circles) and planetary candidates (red circles) discovered by the Kepler mission. The dashed curves shows how close different planets can get to their host stars before they would be tidally disrupted. Taken from Jackson et al. (2016).
Exciting news – just today, my research group had another paper accepted for publication, so we’ve added one more tiny brick to the edifice of human knowledge.
This paper explored tidal disruption of gaseous exoplanets. Over the last few decades, astronomers have discovered thousands of planets outside of our solar system, so-called “exoplanets”.
Most of the planets do not resemble planets in our solar system, and owing to biases in the way we find the planets, many of them are big, gas balls like Jupiter but orbiting much closer to their host stars than planets in our solar system – these planets are called “hot Jupiters“. The figure at left shows how close some of these planets are to their host stars.
Many of these hot Jupiters are doomed to spiral in toward their host stars, and when they get too close, the star’s gravity can rip them apart in a process called “tidal disruption“.
In our recent paper, we studied that process to try to understand how close a planet can get before it’s ripped apart and what might happen as it’s being ripped apart. The upshot of our study is that planets might get ripped apart a little farther from their stars than is often assumed BUT that ripping-apart process might proceed fairly slowly, over billions of years.
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.
Artist’s conception of tidal disruption of a gas giant planet.
In the last few decades, astronomers have discovered thousands of extrasolar planets, and there seems to be, on average, one planet for every star in the galaxy. Some of the planets are like those in our solar system, but many are not.
In fact, there’s a huge number of gas giant planets, like Jupiter, but on such short-period orbits they are nearly skimming the surfaces of their stars. These hot Jupiters are actually so close to their host stars, they are in danger of being torn apart by the stars’ gravity.
In a study just accepted for publication, my research group investigated what happens to a giant planet when it is ripped apart. We found that, over a few billion years, these planets can lose their entire atmospheres, leaving behind the little rocky core deep in the planet’s interior.
It also turns out that, as the planets lose their atmospheres, they can also get pushed out away from the star, and our study found that how much the planet gets pushed out depends pretty sensitively on the size of the rocky core.
That’s pretty neat because it means we can compare the masses and orbits of known rocky exoplanets to what we would expect if those little planets were actually the fossil cores of bigger gas giants that had their atmospheres ripped off.
The figure below shows how the current orbital periods of known planets P compares to what we’d expect if they were fossil cores, P_(Roche, max). In some cases, there’s a decent match, but in lots of cases, there’s not. So we’ve still got some work to do.
Had a wonderful visit to London, Ontario last week, home of the University of Western Ontario. Weather wasn’t quite as nice as here in Boise, but the city was just as beautiful.
My friend and colleague Catherine Neish arranged for me to give three talks while there — one on our crowd-funding effort, one on my exoplanet research, and one on our dust devil work.
I’ve posted two of the talks and abbreviated abstracts below. The dust devil talk, “Summoning Devils in the Desert”, is a reprise of a previous talk, so I didn’t include it below.
Crowdfunding To Support University Research and Public Outreach
In this presentation, I discussed my own crowdfunding project to support the rehabilitation of Boise State’s on-campus observatory. As the first project launched on PonyUp, it was an enormous success — we met our original donation goal of $8k just two weeks into the four-week campaign and so upped the goal to $10k, which we achieved two weeks later. In addition to the very gratifying monetary support of the broader Boise community, we received personal stories from many of our donors about their connections to Boise State and the observatory. I’ll talk about our approach to social and traditional media platforms and discuss how we leveraged an unlikely cosmic syzygy to boost the campaign.
On the Edge: Exoplanets with Orbital Periods Shorter Than a Peter Jackson Movie
In this presentation, I discussed the work of our Short-Period Planets Group (SuPerPiG), focused on finding and understanding this surprising new class of exoplanets. We are sifting data from the reincarnated Kepler Mission, K2, to search for additional short-period planets and have found several new candidates. We are also modeling the tidal decay and disruption of close-in gaseous planets to determine how we could identify their remnants, and preliminary results suggest the cores have a distinctive mass-period relationship that may be apparent in the observed population. Whatever their origins, short-period planets are particularly amenable to discovery and detailed follow-up by ongoing and future surveys, including the TESS mission.
UPDATE (2016 Mar 24): The paper is now available for free on astro-ph.
The Astrophysical Journal published today a paper by my colleagues and myself investigating in detail a way to look for moons around transiting exoplanets.
The discoveries of thousands of planets and planetary candidates over the last few decades has motivated a parallel effort to find exomoons. In addition to providing a base of operations for the Empire, exomoons might actually be a better place to find extrasolar life than exoplanets in some ways.
This technique for finding exomoons, called the Orbital Sampling Effect, was developed by René Heller and involves looking for the subtle signature of a moon’s shadow alongside the shadow of its transiting planet host, as depicted in the image below.
The dark cloud shown around the planet represents the exomoon’s shadow, averaged over several orbits. At epoch (1), a satellite transits just before the planet. At epoch (2), the planet’s transit begins, inducing a large dip in the measured stellar brightness. At epoch (3), the satellite modifies the planet’s transit light curve slightly but measurably.
This simple technique has advantages over alternative exomoon searches in that it doesn’t require significant computational resources to implement. It can also use data already available from the Kepler and K2 missions. However, on its own, the technique can’t provide a moon’s mass, only its size, and it requires many transits of the host planet to find the moon’s quite subtle transit signature.
No exomoon has been found yet in spite of tremendous efforts to find them, so the search continues.
Artist’s conception of Kepler-452 b. From https://en.wikipedia.org/wiki/Kepler-452b#/media/File:Kepler-452b_artist_concept.jpg.
Exciting discovery reported last week of a planet a little bigger than Earth orbiting a star very like our Sun.
The planet, Kepler-452 b, was discovered by the Kepler mission and has a radius 60% larger than the Earth’s. It receives about 10% more light from its star than we do here on the Earth, and it’s probably about 2 billion years older. Together, these qualities mean it may be the most Earth-like exoplanet found to date (although there are lots of other similar planets).
Unfortunately, the host star is so distant, 1,400 lightyears from Earth*, that the usual method for directly estimating the planet’s mass, radial velocity observations, is not feasible. Instead, the planet’s discoverers constrain the planet’s mass by considering a range of compositions, calculating the radius expected for each of those compositions, and comparing it to the observed radius. Based on this analysis, they estimate at least a 49% probability that the planet is rocky, like the Earth.
Based on the amount of light it receives from its host star, there’s a good chance Kepler-452 b is habitable. This means, given a long-list of assumptions about the planet and its atmosphere, liquid water would be stable on its surface. Thus, Kepler-452 b joins a short but rapidly growing list of planets that might host life.
With our success finding potentially habitable planets, it’s probably only a matter of time (maybe just a few more years) before we find a planet that’s not just habitable but inhabited. Children in school right now might be the first generation to grow up in a universe where they know we’re not alone.
Today’s journal club attendees included Jennifer Briggs, Hari Gopalakrishnan, and Jacob Sabin.
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*This website is the only reference I can find that gives the distance to Kepler-452 b from Earth. The paper itself doesn’t say 1,400 light years. The exoplanet.eu catalog gives a stellar magnitude V = 13.7 (also not given in the discovery paper). Converting that V magnitude to a flux and then using the stellar parameters given in the paper, I estimate a distance of 2,400 light years.
The radii of planets found in the Kepler dataset by Dressing & Charbonneau (2015) as a function of the amount of star light (insolation) they receive. The pink and green lines show ranges of insolation we think might allow the planets to be habitable.
In journal club today, we discussed a recent study by Dressing & Charbonneau (2015) that used the Kepler dataset to search for possibly habitable planets around small (M-dwarf) stars.
Dressing and Charbonneau applied a sophisticated and comprehensive search scheme to look for habitable planets and estimate how effective their search was in finding such planets. Based on their analysis, they estimated that about 1 in 4 M-dwarf stars have planets about the size of Earth in their habitable zones and that the nearest such planet is about 8.5 light years away.
This is far enough that we’d probably still need a generation ship to reach it but a lot closer than one case, 20 light years to the nearest habitable planet, considered by Hein et al. (2012) in their analysis of interstellar colonization. This reduction in travel time could reduce the minimum population required to make the trip from maybe 7,000 to 4,000 people.
Of course, the habitable planet sought in such a trip would orbit a much cooler, redder star than the Sun, so the colonists should be prepared to plant very different crops than we have on Earth.
Attendees of today’s journal club included Simon Pintar, Nathan Grigsby, Jacob Sabin, Tyler Wade, Liz Kandziolka, Jennifer Briggs, and Emily Jensen.
At Friday’s journal club, we discussed on two papers. The first, Webber et al. (2015), investigated the effects of clouds on the phase curves for hot Jupiters. Webber et al. found that planet’s phase curve may depend sensitively on whether clouds are distributed uniformly or heterogenously throughout the atmosphere. They also found that the amount of light reflected by an exoplanet depends on the composition of the clouds — clouds made of rocky minerals like MgSiO3 and MgSi2O4 are much brighter than Fe clouds.
From Ballard & Johnson (2015), this figure compares the number of stars with a certain number of planets detected by Kepler (blue diamonds) to our expectations (in red) if single planet systems actually had more planets hidden from Kepler’s view. The disagreement between the blue and red curves suggests that many of those apparently singleton planets really are only children and single and multi-planet systems are inherently different.
The second paper, Ballard & Johnson (2014), investigated the frequency of exoplanets around M-dwarf stars observed by the Kepler mission. Because the Kepler mission found planets by looking for transits, there’s always a good chance that a system with only one detected planet actually has more that just don’t pass in front of their host star as seen from Earth. But we know exactly how to account for this geometric effect.
By accounting for it, Ballard and Johnson showed that Kelper actually found a lot more systems with only one planet than we would expect if there were just more planets in those systems hidden from Kepler‘s view. So there are two distinct kinds of planetary systems around M-dwarfs: those with only one planet (or possibly several planets with large mutual inclinations) and those with several.
Why the difference? Ballard and Johnson find tantalizing hints that stars hosting only one detected planet are older on average. One simple explanation: given enough time, systems with many planets become unstable, and the lonely planets we see today originally had siblings that were gravitationally cast out of the system, to wander the void between the stars. Or the siblings were accreted by their parent stars, like Saturn eating his children. Along with many others, this study helps show that planetary systems can be much more violent places than astronomers originally thought.
Journal club attendees included Jennifer Briggs, Nathan Grigsby, Jared Hand, Tanier Jaramillo, Emily Jensen, Liz Kandziolka, and Jacob Sabin.
The tightly packed system, named Kepler-444, is home to five small planets in very compact orbits. The planets were detected from the dimming that occurs when they transit the disc of their parent star, as shown in this artist’s conception. From http://www.nasa.gov/ames/kepler/astronomers-discover-ancient-system-with-five-small-planets/.
In journal club on Friday, we discussed a fascinating paper from Campante and colleagues announcing discovery of one of the oldest planetary systems ever discovered — Kepler-444. The system comprises five planets, ranging from roughly Mercury- to Venus-sized with orbital periods from about 3 to 9 days.
Studying the frequencies of oscillations within the K-dwarf host star (an approach known as asteroseismology), Campante et al. estimate the host star, and therefore probably the planets, is about 11 billion years old, almost as old as the Milky Way galaxy itself.
To put that age into perspective, by the time our solar system formed, about 5 billion years ago, the Kepler-444 was already a billion years older than our solar system is now.
The existence of such an old system tells us that rocky planets began forming almost as soon as the Milky Way itself formed, which allows for the exciting possibility of very ancient life in the galaxy.
Present at journal club were Jennifer Briggs, Trent Garrett, Nathan Grisgby, Emily Jensen, Liz Kandziolka, Brenton Peck, and Jacob Sabin.
