Good talks today on recent developments in our understanding of planet formation by Christophe Mordasini. Improved models for the dynamics of grains in the accretion streams for growing gas giants have helped solved some of the mysteries associated with the planets’ formation.
Aurelian Crida gave a very informative talk on developments in planetary migration. Turns out that migration can be very complicated.
And some good talks on the dynamics of mature planetary systems. Christa Van Laerhoeven reviewed classical secular theory and discussed how the orbital architectures of some systems can be determined, even in the absence of detailed information about the planets’ orbits.
Day 2 of the IAU meeting was very busy, with lots of great talks and presentations. Two events, in particular, stood out to me, though.
The first was a session on Highlights from Space Missions, which had a focus on results from the Dawn and Rosetta missions.
The Dawn mission visited the asteroid Vesta and is currently in orbit around Ceres, the largest asteroid in the asteroid belt and a world in its own right. By way of highlighting the recent results, the mission PI, Prof. Chris Russell, presented breath-taking images and movies from the mission. I’ve included some below.
The first movie is of a bright mountain on the surface of the asteroid Ceres called The Pyramid.
The second movie shows Occator crater with its mysteriously bright central … thing. The Dawn mission team is speculating that the bright spot is some kind of exotic salt deposit, based on its reflectance spectrum, but they’re not really sure what it is yet.
Next up, Dr. Sierks showed highlights from the the Rosetta mission, which is visiting Comet 67P and dropped the Philae lander last year onto the comet’s surface, also with mind-blowing movies.
The first movie shows the comet’s rotation, revealing its voluptuous shape.
The next movie shows how the comet’s rotation causes its jets to curve, as the icy vapor escapes into space.
And the final movie (poorly focused unfortunately) shows the cosmic snow erupted into interplanetary space by the comet’s jets. These particles actually represent a hazard to the spacecraft and make it difficult for its operators to orient the spacecraft since they use background stars to figure out how it’s oriented. As a result, the spacecraft was moved to a more distant, safer orbit after these images were collected.
In the evening, IAU hosted an event at which they invited the public to vote on names for 20 known exoplanet systems. Just since last night, the number of votes has gone from zero to more than 15,000.
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.
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.
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.
These light absorption features were originally discovered by Heger way back in 1919, only a few months after the end of World War I. Their discrete absorption peaks are pervasive throughout visible wavelengths, indicating they are not simply due light-scattering by interstellar dust. The fact that they appear unchanged no matter the nature of the star whose light they absorb also suggests they don’t arise from the star itself. Instead, they must lay somewhere in the vast space between the Earth and the star.
Astronomers proposed DIBs might arise from dust grains, carbon chains, and even floating bacteria. Bucky balls, large soccer-ball-shaped carbon molecules, had also been proposed as candidates since they were discovered in white dwarf stars.
But deciding which candidate was actually the culprit required meticulous and highly sensitive lab work to recreate the extreme conditions of outer space, where temperatures are near absolute zero and gas pressures can be 10 million times smaller than at Earth’s surface. After twenty years of work, Maier and his team in Switzerland and Germany finally managed to create a little pocket of interstellar space in their lab.
By carefully ionizing buckyballs and introducing them into a cold He gas, they showed the spectral features created by the buckyballs in association with He matched almost exactly the spectral features of some DIBs.
The upshot of this is that the spectral forest of DIBs found at other wavelengths likely points to the prevalence of other large and complex molecules self-assembling in space, so this discovery is just the tip of the chemical iceberg. It has even been suggested that the complex molecular precursors for life originated in interstellar space in the same way as the buckyballs.
Whether that’s true or not, this discovery shows that the vast and lonely spaces between the stars aren’t quite as empty as they seem.
Journal club attendees included Jennifer Briggs, Emily Jensen, and Tyler Wade.
As the only planetary scientist at Boise State, I was asked to present on the New Horizons mission to the Math REU program here, so I put together the presentation below.
At journal club, we discussed the discovery of two new hot Jupiters using data from ESA‘s CoRoT mission, with the names CoRoT-28 b and -29 b. Both systems seem a little off.
Equally puzzling is the transit light curve for CoRoT-29 b (shown below at left). Most transit curves are u-shaped, but CoRoT-29 b’s is strangely asymmetric. The asymmetry resembles what has been seen for a planet transiting a rapidly rotating star — rapid rotation reduces the gravity at the stellar equator, resulting in a cooler, darker region. Barnes et al. (2013) looked at the transit light curves for such a Kepler system and actually used the light curve to study the planet’s orbital inclination.
(left) CoRoT-29 b transit light curve. (right) Planet transiting star spot.
But CoRoT-29 doesn’t appear to be a rapid rotator. So instead Cabrera et al. suggest that perhaps the star has a large, nearly stationary star spot and that the planet transits the spot over and over again. However, this scenario would require a nearly stationary spot with a very long lifetime (~90 days), neither of which is expected.
So a few more astrophysical conundra to add to the growing list of puzzling exoplanet discoveries.
Journal club attendees included Jennifer Briggs, Emily Jensen, and Hari Gopalakrishnan.
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.
Although the authors found the camera exhibited a few peculiarities (that are apparently not described in any of Canon’s documentation), they showed that it could be used to observe exoplanet transits — a really great result.
It means that astronomers, amateur or professional, who want to do transit observations don’t need to spend $10,000 to buy a high-end CCD camera. Instead, they can spend just a few hundred to produce reasonable quality transit light curves.
One especially tantalizing result from the paper: Zhang and colleagues mention having seen exoplanet transit-like signals for four of the target stars they studied, only one of which is known to host a planet — KELT-3 b. That means they may not only have recovered known transiting with the Canon EOS 60D; they may also have found three new ones. Presumably, they’re in the process of trying to confirm whether the other three are new planets.
UPDATE: The authors kindly updated me to say that follow-up observations indicated these three candidates are all false positives. But they would have discovered KELT-3 b with their survey, if it hadn’t already been discovered. So a pretty amazing achievement.
Attendees included Jennifer Briggs, Andrew Farrar, Nathan Grigsby, Emily Jensen, and Tyler Wade.
A quick update on day 3 of the IAU conference.
