close-in exoplanets

All posts tagged close-in exoplanets

Twelve multi-planet systems where the innermost member is very close to the host star, that is, has an orbital period less than 1 day. From Adams et al. (2016).

Twelve multi-planet systems where the innermost member is very close to the host star, that is, has an orbital period less than 1 day. From Adams et al. (2016).

Big research news today: our research group SuPerPiG, led by the inimitable Dr. Elisabeth Adams, announced the discovery of two new planets, EPIC 220674823 b and c.

Using data from the K2 Mission, we found these planets by looking for the shadows of the planets as they passed in front of their host stars, a planet-hunting technique known as the transit method.

These new planets are very different from planets in our solar system in several surprising ways.

First, they’re both bigger than Earth but smaller than Neptune – planet b is 50% larger, and planet c is 2.5 times larger. They inhabit a strange nether-region of planets where they’re known as super-Earths or sub-Neptunes, planets somewhere between Earth and Neptune. The reason there’s no specific name for such planets is because astronomers don’t understand this new class of planet at all.

An artist's conception of CoRoT-7 b, another ultra-short-period planet.

An artist’s conception of CoRoT-7 b, another ultra-short-period planet.

Second, both planets are MUCH closer to their Sun than the planets in our solar system. In fact, planet b is so close to its sun that it takes less time to orbit (14 hours) than all the playtime it took the Cubs to go from 3 games down to tying up the World Series. By comparison, planet c circles at the glacial pace of once every 13 days.

Another thing that’s interesting about our planets: they’re yet another system of with an ultra-short-period planet (USP) in which there is more than one planet, i.e. a multi-planet system. In fact, as we argue in our paper,  most of the known systems with ultra-short-period planets are probably multi-planet systems and that fact might help explain the origin of these chthonic planets.

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Beautiful Sky Pilot Mountain, south of Quest University.

The last two days of Exoclimes 2016 were as engaging as the first two — lots of great talks, discussion, and coffee break snacks.

The day 3 talks that really grabbed me were the first talks, focused on atmospheric mass loss from exoplanets since I’m currently working on that problem myself.

Ruth Murray-Clay gave a nice review talk about the variety of different mechanisms and regimes for atmospheric escape, while Eric Lopez suggested that, because escape should preferentially remove lighter elements from atmospheres, short-period exoplanets might retain water-rich envelopes, which could help us constrain their atmospheric compositions. Patricio Cubillos picked up on an idea previously explored by Owen and Wu and suggested that we could use mass-loss considerations to constrain the overall properties (density, etc.) of some short-period planets.

Other talks that stood out for me on day 3 included Eric Gaidos‘s talk about looking for geoengineering efforts by alien civilizations and Mateo Brogi‘s talk about measuring the spin rates of distant exoplanets, including GQ Lup b, a brown dwarf/high-mass exoplanet with a spin period of 3 days.

Day 4 of the conference whizzed by with a variety of talks regarding clouds and hazes in exoplanet atmospheres. Sarah Hörst taught us we should use the term ‘aerosol‘ instead of ‘clouds and/or hazes’ (since we’re not sure which of the two we’re seeing in exoplanet atmospheres).

Joanna Barstow and I rounded out the conference. She talked about her work analyzing exoplanet spectra and constraining aerosol (not clouds and/or haze) properties. Drawing upon the liturgical texts from the dawn of exoplanet science, I talked about my group’s work looking at Roche-lobe overflow of hot Jupiters (I’ve posted my talk below).



Artist's conception of tidal disruption of a gas giant planet.

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.

IMG_3637Had 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.

Artist's conception of cloudy GJ 1214 b. From http://www.nytimes.com/2014/01/07/science/space/the-forecast-on-gj-1214b-extremely-cloudy.html.

Artist’s conception of cloudy GJ 1214 b. From http://www.nytimes.com/2014/01/07/science/space/the-forecast-on-gj-1214b-extremely-cloudy.html.

At journal club this week, we discussed the recent discovery using data from the K2 mission of the sub-Neptune planet K2-28.

This planet, roughly twice the size of Earth, circles a very small M-dwarf star so closely that it only takes two days to complete one orbit. Even though the planet is very close to its star, the star is so cool (3000 K) and so small (30% the size of our Sun) that the planet’s temperature is only 600 K. (A planet in a similar orbit around our Sun would be 1200 K.)

The authors of the discovery paper point out that this planet is similar in many ways to another famous planet, GJ 1214 b. Like GJ 1214 b, K2-28 b is member of this puzzling but ubiquitous class of sub-Neptune planets — planets that fall somewhere between the Earth and Neptune in size and composition and do not exist in our solar system*. Both planets also orbit relatively nearby M-dwarfs, which means, like GJ 1214 b, K2-28 b might be amenable to follow-up observations.

Previous follow-up observations of GJ 1214 b indicated that planet’s atmosphere is very cloudy or hazy. So K2-28 b could provide another very important toehold along the road toward understanding this strange class of hybrid planet.

*unless Planet Nine turns out to be real

 

Last week, I had a lovely visit to the Astronomy Dept at New Mexico State University in beautiful Las Cruces. I was invited to give one of the dept’s weekly colloquia about our research group’s work on very short-period exoplanets. While there, I talked dust devil science with my host Prof. Jim Murphy, his student Kathryn Steakley, and Lynn Neakrase.

I also enjoyed some excellent Mexican food at the Double Eagle Restaurant, which has been haunted by the ghosts of two young lovers since just after the Mexican-American War.

The International Space Station passing over Mesilla, NM on 2015 Dec 4.

The International Space Station passing over Mesilla, NM on 2015 Dec 4.

Just before dinner, the ISS also passed directly over our heads, and I got a very poor photo of it (left).

So, all in all, a great visit.

 

 

I’ve posted my abstract and presentation below.

On the Edge: Exoplanets with Orbital Periods Shorter Than a Peter Jackson Movie


From wispy gas giants to tiny rocky bodies, exoplanets with orbital periods of several days and less challenge theories of planet formation and evolution. Recent searches have found small rocky planets with orbits reaching almost down to their host stars’ surfaces, including an iron-rich Mars-sized body with an orbital period of only four hours. So close to their host stars that some of them are actively disintegrating, these objects’ origins remain unclear, and even formation models that allow significant migration have trouble accounting for their very short periods. Some are members of multi-planet system and may have been driven inward via secular excitation and tidal damping by their sibling planets. Others may be the fossil cores of former gas giants whose atmospheres were stripped by tides.

In this presentation, I’ll discuss 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.

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.

A hot Jupiter being ingested by its host star. From http://sen.com/thumbs/1024x576/img/47b3082d767346e8bebdb5ad99f8f33d.jpg.

A hot Jupiter being ingested by its host star. From http://sen.com/thumbs/1024×576/img/47b3082d767346e8bebdb5ad99f8f33d.jpg.

In journal club today, we discussed the recent study by Matsakos and Königl that investigated the possibility that hot Jupiters can be ingested by their host stars.

The idea that stars might ingest hot Jupiters has been around since the planets were first discovered. The putative accomplice in this type of astrophysical murder is tidal interaction between the planet and host star (the same kind of interactions that cause the Moon to recede from the Earth).

Tides cause the hot Jupiters to slowly spiral into their host stars, while spinning up the host star, but the strength of the interactions drops off rapidly with distance between the planet and star. The first hot Jupiters were far enough from their stars that they are probably safe from tidal destruction.

However, astronomers have continued to find planets closer and closer to their host stars, raising again the specter of planetary tidal destruction.

These same tidal interactions also align a host star’s equator to its planet’s orbital plane. So stellar equators that start out highly inclined to their hot Jupiter’s orbit (and there are a surprisingly large number) can end up completely aligned, but, as Matsakos and Königl argue, only at the cost of the planet’s orbital angular momentum.

The upshot of this is that many of the exoplanet host stars with equators aligned to their planets’ orbital planets may have eaten a hot Jupiter early in their lives. Under some reasonable assumptions, Matsakos and Königl show that the observed distribution of inclination angles for host star equators is consistent with about half of the stars having eaten a hot Jupiter.

Fortunately, the planets in our solar system will not suffer the same fate — at least not for a few billion years. But once the Sun leaves the Main Sequence and enters stellar senescence in a few billion years, its radius will blow up, destroying Mercury and Venus. Whether the Earth is also consumed by the approaching cloud of plasma is not clear, but if exoplanet studies have taught us anything, it’s that the universe is a tough place to be a planet.

Today’s attendees included Jennifer Briggs, Emily Jensen, and Tyler Wade.

Artist's conception of Kepler-452 b. From https://en.wikipedia.org/wiki/Kepler-452b#/media/File:Kepler-452b_artist_concept.jpg.

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.

*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.

Artist's conception of a hot Jupiter shedding mass.

Artist’s conception of a hot Jupiter shedding mass.

At journal club today, we discussed a recent paper by Valsecchi et al. (2015) that looks at mass loss from hot Jupiters. These planets are so close to their host stars that the stars can blast away and rip the planets’ atmospheres apart.

By employing the sophisticated star/planet evolution model MESA, Valsecchi and colleagues found that the planets can shed most of their atmospheres, leaving behind a small sub-Neptune planet in a short period orbit. However, gravitational interactions between the planet and escaping gas actually push the planet away from the star as the planet is shedding mass, potentially out to orbital periods of a few days.

The upshot of this is that, based on these calculations, the recently discovered population of small ultra-short period planets probably did NOT originate from atmospheric stripping of more massive planets. So it’s not totally clear how these little planets originated, although Kevin Schlaufman suggested one still viable possibility.

Today’s attendees included Jennifer Briggs, Emily Jensen, Charlie Matthews, Jacob Sabin, and Tyler Wade.