Journal Club

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.

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.

A false-color, infrared map of Titan’s north pole. The black arrow points to an evaporite deposit along the shore of Ligeia Mare.

A false-color, infrared map of Titan’s north pole. The black arrow points to an evaporite deposit along the shore of Ligeia Mare.

I read a recent paper by MacKenzie et al. (2014) about evaporite deposits along the rims of lakes on Saturn’s moon Titan.

The more we learn about Saturn’s enigmatic moon Titan, the more it resembles the Earth: Titan has a thick atmosphere made mostly of nitrogen and a complex “hydrologic” cycle involving big storms, river beds, and even lakes and seas. But, because Titan’s surface temperatures are nearly -300 degrees F, unlike on Earth, the liquids involved in the cycle are methane and ethane.

And recently, observations from the Cassini spacecraft in orbit around Saturn have found evidence for what may be evaporite deposits along the rims of many of Titan’s seas and lakes. Evaporite deposits typically form on Earth when water collects in isolated basins and slowly evaporates away, leaving behind the minerals dissolved in the water. They are common throughout the Southwest in the US.

In this recent paper, MacKenzie and colleagues conducted a thorough mapping of the putative evaporite deposits on Titan to understand their extent and possible connections to climate. The study presents lots of interesting results: in particular, they find evaporites occur at a variety of latitudes, including in places that look a lot like dry lake beds. This result corroborates the suggestion that these places were indeed filled with liquid in the past but climatic changes have since dried them out.

Surprisingly, even though there are many apparent dry lake beds near the south pole on Titan, MacKenzie and colleagues find no evidence for evaporites there. They speculate either the deposits were laid down long ago (> 50,000 years ago) and have since been buried OR conditions were never suitable for evaporite formation, even when the south polar lake beds were filled. Either result could be telling us something very interesting about evolution of Titan’s climate.

The K2 mission will observe sequential ecliptic campaigns with a duration of ∼83 days, where 75 days are dedicated to science.

The K2 mission will observe sequential ecliptic campaigns with a duration of ∼83 days, where 75 days are dedicated to science.

I’m a bit late to the game on this one, but I wanted to read Howell et al.’s (2014) paper describing the planned K2 mission, what is essentially the reincarnation of the Kepler mission.

Launched in 2009, the Kepler spacecraft was happily staring at about 150,000 target stars over about 4 years, looking for the shadows of planets as they passed between the Earth and their host star (called planetary transits). Groups using Kepler data have found thousands of planets outside our solar system, revolutionizing exoplanet studies. Unfortunately, two of the reaction wheels used to keep the spacecraft pointing stably at its target field failed by May 2013, ending the nominal science mission.

However, by carefully angling the telescope (see figure at left) and keeping it pointed along its orbital plane, NASA engineers realized they could use the photon pressure from Sun as a sort of third reaction wheel, allowing the astronomical revolution to continue — thus was born the K2 mission, which NASA selected for funding May this year.

The paper from Howell et al. (2014) describes the scientific and engineering capabilities of the K2 mission, which closely match those of the Kepler mission. So a lot of what Kepler could do, K2 can, too. In fact, since mission engineers have to turn the spacecraft every 80 days to satisfy the pointing requirements, K2 will look at lots of different fields on the sky, in contrast to Kepler, which only stared at the same field.

This variable pointing will enable a wider variety of scientific investigations, as discussed by Howell et al., including looking for more transiting planets, but also studies of other galaxies, supernovae, stellar clusters, and more. In fact, the astronomical community submitted more than 100 different ideas of things to do with K2. So the scientists and engineers at NASA have really done a spectacular job salvaging what otherwise would have been a disappointing loss.

Artist’s conception of a habitable exoplanet in orbit around its red dwarf star. Credit: NASA Ames/SETI Institute/JPL-Caltech. From http://themeridianijournal.com/2014/04/big-discovery-first-earth-sized-exoplanet-habitable-zone-another-star/#more-5509.

Artist’s conception of a habitable exoplanet in orbit around its red dwarf star. Credit: NASA Ames/SETI Institute/JPL-Caltech. From http://themeridianijournal.com/2014/04/big-discovery-first-earth-sized-exoplanet-habitable-zone-another-star/#more-5509.

Interesting paper from Prof. Michael Jura at UCLA and colleagues, in which they look for the chemical signatures of plate tectonics in white dwarfs that are accreting planetary and asteroidal material. The paper presents a really neat idea, combining several big concepts.

First, there is strong evidence that several white dwarfs (the ghostly remnants of Sun-like stars) are accreting rocky materials. The atmospheres of white dwarfs are very simple, hot hydrogen and helium cooling to space over billions of years. Any other, heavier elements quickly settle out of the atmospheres (on timescales of millions of years), and so if you find such, heavier elements in the atmospheres (via spectroscopy), those elements were probably recently dumped into the atmosphere — a process called pollution.

Such pollution has been observed for many white dwarfs, and the pollution typically consists of rocky elements, silicon, magnesium, etc. Consequently, the polluting materials probably come from rocky asteroids, falling into the white dwarfs. Jura and colleagues point out that some of the pollution may arise from the crusts of extrasolar rocky planets orbiting the white dwarfs, in the form of debris from large impacts with the planetary surfaces.

Second, on the Earth, the constant subduction and eruption of crustal materials from plate tectonics has the effect of sieving out certain elements and leaving them in the crust, producing a crustal composition distinct from that of other planets and asteroids.

Jura and colleagues went looking for such a chemical signature in the rocky pollution of white dwarfs but unfortunately don’t find it. However, this initial study may provide a novel to search for the signs of plate tectonics in other planetary systems. That’s important because plate tectonics is thought to be a key requirement for making a planet suitable for life, but observing it astronomically is almost impossible (there’s no strong evidence any solar system planets other than Earth experienced/s plate tectonics). Jura and colleagues may have provided us a new way to peer into the geophysical histories of extrasolar planets.

 

 

Predicted fatality counts. MFI indicates masculinity-femininity index (1 -> very masculine name, 11 -> very feminine name), and hurricanes with low MFI (vs. high MFI) are masculine-named (vs. feminine- named). Predicted counts of deaths were estimated separately for each value of MFI of hurricanes, holding minimum pressure at its mean (964.90 mb).

Predicted fatality counts. MFI indicates masculinity-femininity index (1 -> very masculine name, 11 -> very feminine name), and hurricanes with low MFI (vs. high MFI) are masculine-named (vs. feminine- named). Predicted counts of deaths were estimated separately for each value of MFI of hurricanes, holding minimum pressure at its mean (964.90 mb).

We discussed a very interesting paper today in Journal Club, Jung et al.’s (2014) study of correlations between the perceived masculinity-femininity of a hurricane’s name and its death toll. As reflected in the figure at left from the paper, the higher the masculine-feminine index for a hurricane (MFI, 1 -> very masculine name, 11 -> very feminine name), the larger the predicted fatality count. As a specific example, Jung et al. estimated that Hurricane Eloise (with a decidedly female MFI = 8.944) killed three times as many people as Hurricane Charley (MFI = 2.889).

Jung et al. also polled participants and found they consistently rated female storms as less likely to be severe and indicated they were less likely to evacuate in the wake of female storms, perhaps due to implicit gender biases. Ostensibly, their results suggest a lot of lives could be saved by simply not using female names for hurricanes.

But there are a lot of questions that were not addressed by the study, and others who have looked at the data have pointed out important unresolved issues.

A blog entry at Prooffreader.com pointed out that the authors should only have considered hurricanes after 1979 since there were no male hurricane before then. The blogger showed that, comparing total hurricane deaths since both male and female names have been used (1979), male hurricanes killed more people (413 total) until just two years ago, when Hurricane Sandy brought the total for females to 459. And this result used the authors’ own data.cum_alldeaths The plot at right is a recreation of the Prooffreader.com plot.

I spent several hours yesterday, poring over the paper and have to admit that I did not understand the statistical methods employed. The authors don’t give a lot of details, and the key references for the techniques seem to be two textbooks to which I don’t have access.

The paper talks about using a model to estimate the number of deaths for a storm of a given severity, but to the extent that I can compare their predicted death tolls to actual, the model seems pretty discrepant with the data. For example, their model estimates that Hurricane Eloise killed 41.45, but the actual number killed was 21. They also estimated Hurricane Charley killed 14.87, whereas Hurricane Charley from 1986 killed 5 and the one from 2004 killed 10 (they don’t say which Charley they meant).

So Jung et al. present a very interesting idea, but it’s not at all clear that their results hold up. I’m sure this paper will prompt a spat of sociological studies into hurricane statistics, though, which will probably lead to additional disaster preparedness and save lives.

 

Figures from Kreidberg et al. (2014). The top panel is an image of the Hubble Telescope CCD as it collected photons of many colors passing through GJ 1214 b's atmosphere. The bottom panel shows the infrared spectra that results from analysis of that image, showing no molecular features.

Figures from Kreidberg et al. (2014). The top panel is an image of the Hubble Telescope CCD as it collected photons of many colors passing through GJ 1214 b’s atmosphere. The bottom panel shows the infrared spectra that results from analysis of that image, showing no molecular features.

The year starts with a spectacular result from Laura Kreidberg and colleagues: the super-Earth exoplanet GJ 1214 b has high altitude clouds in its atmosphere.

The image at left shows observations from the Hubble Space Telescope. These data were collected as the planet GJ 1214 b passed in front of (transited) its host star. When that happens, light emitted by the host star passes through the planet’s atmosphere, and the atmosphere can imprint a spectral signature on that light, telling us what it’s made of.

But for GJ 1214 b, as shown by the bottom at left, there were NO spectral signatures — the spectrum is completely flat. The most likely explanation is that the planet has clouds high in its atmosphere that block the star light from passing through the part of the atmosphere where spectral signatures would be imprinted.

To make such a flat spectrum, GJ 1214 b’s clouds have to be very high in its atmosphere. Kreidberg and colleagues estimate the cloud deck can’t be lower than about 1 millibar in pressure. On the Earth, cirrus clouds, some of the highest clouds, live at pressures of about 300 millibars or 10 km in altitude. Earth’s atmospheric pressure doesn’t drop to 1 millibar until an altitude of about 70 km, above a height where meteors typically burn up. So GJ 1214 b’s clouds are very unusual.

P/2013 P5 as seen by Hubble on September 10, 2013. P/2013 P5 is about 790 feet (240 m) in diameter. It has six comet-like tails of dust radiating from it like spokes on a wheel. From http://www.sci-news.com/space/science-p2013p5-hubble-asteroid-six-tails-01530.html.

The comet P/2013 P5 as seen by Hubble on September 10, 2013. P/2013 P5 is about 790 feet (240 m) in diameter. It has six comet-like tails of dust radiating from it like spokes on a wheel. From http://www.sci-news.com/space/science-p2013p5-hubble-asteroid-six-tails-01530.html.

At journal club today, we talked about two interesting papers.

The first, “The Extraordinary Multi-tailed Main-belt Comet P/2013 P5” by Jewitt and colleagues, discussed observations using the Hubble Space Telescope of a comet in the asteroid belt that displayed five cometary tails (see image at left). The tails are made of particles shed by the comet, and using the particle trajectories inferred from the tails, the authors were able to figure out when, over the last few months, the particles were launched from the comet.

The second article, “Evidence for high salinity of Early Cretaceous sea water from the Chesapeake Bay crater” by Sanford and colleagues, presented chemical analyses of water collected from under the Chesapeake Bay, in the large impact crater at the southern end of the bay. (The crater is underwater and buried by sediment, so you can’t see it even if you’re just standing on it.) The water contain subtle chemical signatures that indicate it was originally part of the Early Cretaceous North Atlantic Ocean, an extinct ocean from more than 100 million years ago. Chemical analyses of this buried water will tell scientists what the ancient ocean was like.

Part of Figure 1 from Follette et al. (2013), comparing the Hubble (HST) image (top) to their image (bottom) of the protoplanetary disk.

Part of Figure 1 from Follette et al. (2013), comparing the Hubble (HST) image (top) to their image (bottom) of the protoplanetary disk.

Today in journal club, we discussed two papers.

The first one was Follette+ (2013), which presented the first images of a protoplanetary disk taken using a new adaptive optics instrument on the Magellan telescope.

This disk (shown at right) is a collection of gas and dust orbiting a young star, only a few million years old. Studying these kinds of young disks help astronomers understand what the early solar system was like, when our planets were still forming.

In addition to providing new images of this disk, this paper also presented evidence that, although the disk is young, the accumulation of dust particles that gives rise to planets may already be underway. This result means that, although astronomers have thought this particular disk may show us the very earliest conditions in a protoplanetary disk, instead this disk may be fairly far along in its evolution and the process of planet formation.

The second paper we discussed was Storch & Lai (2013), which studied the origin of hot Jupiters — gas giant planets (like Jupiter) but orbiting hundreds of times closer to their host stars than the Earth does the Sun.

The origins of these planets are still unclear, but they are so close to their stars that they undergo very strong tidal interactions. These tidal interactions distort the shapes of the planets, dissipating orbital energy within the planets’ interiors and causing the orbits to shrink over time.

Determining the rates and processes of tidal dissipation are key to understanding the origins and fates of these planets: Too much tidal dissipation will overheat the planets’ interiors and blow them up; too little, and the planets wouldn’t reside in the orbits in which we see them today.

Storch & Lai (2013) include the effects of dissipation within the planets’ icy and rocky cores, which they show can help explain the origins of the planets.

Figure 4 from Perez-Becker & Chiang (2013), showing the how the mass loss rate depends on the amount of dust in the atmosphere (x_dust).

Figure 4 from Perez-Becker & Chiang (2013), showing the how the mass loss rate depends on the amount of dust in the atmosphere (x_dust).

Today in Journal Club, we discussed a paper by Perez-Becker & Chiang (2013).

This paper looked at the mass lost by a rocky extrasolar planet so close to its host star that its surface is melted and a liquid rock lake has formed on the planet’s day side. This work was motivated by Rappaport et al. (2012), which claimed to have discovered a roughly Mercury-sized extrasolar planet candidate orbiting the star KIC 12557548 in data from the Kepler mission. The planet seems to be disintegrating and may disappear in the next few million years.

An atmosphere of rocky vapor likely forms over the liquid rock lake and can actually escape from the planet, owing to the planet’s very low surface gravity. As the gas escapes, it cools (through adiabatic expansion) and can potentially condense into little dust grains, which are then swept out into space by the escaping gas. This putative cloud of dust can help explain some of the observations from Rappaport et al. (2012).

As interesting as the paper is, though, it raises some big questions that we talked about in journal club. For example, the dust should be strongly heated by the starlight and should reach high temperatures (> 2000 K or 3140 degrees F). If the planet’s surface is hot enough that the rocky surface evaporates, why doesn’t the dust also evaporate?

Unfortunately, the star KIC 12557548 is very dim, so it’s hard to observe with other telescopes and learn more about the planet candidate. However, the upcoming TESS mission will probably find more planets like this one, and so we might be able to see other rocky planets that are disintegrating before our eyes.

We also discussed an older paper by Gaudi (2004), which predicts that the Kepler mission might have observed a handful of stellar occultations by Kuiper belt objects (KBOs). During such an occultation, a KBO will block out the light from a background star in a way that depends on its size and how far it is from the Sun. Since Kepler has been staring at about 150,000 stars over 3.5 years, there’s a good chance that a few of those stars were occulted by KBOs. Unfortunately, because Kepler wasn’t designed to look for such signals, it might be very hard to spot them in the data.

This figure shows chemical abundances for several stars relative to the Sun. The compositions for the unusually lead-rich stars are shown as a red circle, blue diamond, and green diamond.

This figure from Naslim N. et al. (2013) shows chemical abundances for several stars relative to the Sun. The compositions for the unusually lead-rich stars are shown as a red circle, blue diamond, and green diamond.

Today in journal club, we discussed two papers.

We started with Schlichting et al. (2013), which looked at the size distribution of Kuiper belt objects (KBOs) to figure out how they formed and evolved over the history of our solar system.

Since, as the paper says, the Kuiper belt “is a remnant from the early solar system”, its size distribution depends on the accretion processes that gave rise to planets and also on the collision processes that affect many aspects of planetary formation and evolution.

KBOs range in size from unobservabley small to about a thousand kilometers (km) in radius (Pluto-sized), but the numbers of objects of a given size depend on how the objects formed and on the nature of the collisions among the objects (which can break up the smallest objects).

Schlichting and colleagues argue that the size distribution of KBOs suggests that smaller objects (less than about 30 km in radius) have been dominated by collisions, while bigger objects are much less affected by collisions. So the bigger objects may provide a glimpse into the early history of our solar system.

The other paper was Naslim N. et al. (2013), which announced the discovery of the most lead-rich stars ever found. The stars discussed in the paper are sub-dwarf stars — essentially very small, retired stars — and have thousands of times more lead in their atmospheres than our Sun.

Not only is the amount of lead surprising, it’s surprising that we can see the lead at all. Since lead is so much heavier than the hydrogen and helium that make up most of the stars’ atmospheres, we would expect the lead to settle out of the atmospheres, deep enough in the star that we couldn’t see it.

However, something has caused the lead in these stars to remain suspended in their atmospheres, but not other heavy elements. The authors suggest preferential radiative levitation (essentially some kind of interaction between the atoms and the stellar light) keeps the lead suspended but not other heavy elements.