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 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.

Figure 2 from McQuillan+ (2013) showing rotation periods of host stars (P_rot) vs. orbital periods of planetary candidates (P_orb). The size of the circle indicates the size of the planetary candidate, and the colors indicate stellar temperature.

In journal club today, we first discussed the recent tenth release of data from the third Sloan Digital Sky Survey (SDSS III). The survey basically takes a mammoth picture of the sky every night and provides a wealth of astronomical data that can be used to map the Milky Way, search for extrasolar planets, and solve the mystery of dark energy. According to the wikipedia, SDSS collects about 200 GB of data every night, so, over the last year, the survey has collected tens of terabytes of data.

We also talked about a recent paper by McQuillan, Mazeh, and Aigrain. These authors analyze the brightness variations of many stars observed by the Kepler mission that are orbited by planetary candidates.

Like the Sun, many stars have star spots, cooler and darker regions on their faces, and these spots rotate in and out of view as the stars rotate, just like those on the Sun. When the spots rotate into view, the star appears to darken slightly, and so you can use  brightness variations to determine the star’s rotation rate. (Watching the motion of sun spots is how the Sun’s rotation rate was first determined.)

The authors suggest a correlation between the rotation periods of these host stars and the orbital periods of their planetary candidate companions: there is a dearth of planetary candidates with short orbital periods around stars with short rotation periods.

Our journal club group raised several questions about these results. Among them, we wondered whether there are any biases in the sample of studied stars that could account for the correlation. Also, could it be harder to find planetary candidates around faster rotating stars (faster rotating stars could exhibit brightness variations rapid enough that they confuse the Kepler transit search for planets) — could this idea account for the correlation?

However, if no underlying biases or sampling issues account for the correlation, then the results say something interesting about the connection between planets and stars. Why do planets close to their stars (i.e., with short periods) seem to prefer orbiting stars that rotate slowly?

The false-color, infrared image of the GJ 504 system. The star (center) has been blocked out, and the planet appears a white blob at the upper right.

At today’s journal club, we discussed the recent paper Kuzuhara et al. (2013), which reports the discovery of an extrasolar planet with a mass of about 4 times Jupiter’s mass but in an orbit more distant from its sun than Neptune is from ours.

The image at left shows one of the discovery images, taken at infrared wavelengths. At these wavelengths, usually a planetary host star is shining much less brightly than at visible wavelengths, which makes it easier to see radiation from the planet. However, for the kind coronagraphic imaging here, the star still has to be blocked out to prevent the glare from masking the planet (and blocking out the star completely is hard to do, as evidenced by the orange and blue speckles in the image).

This planet is particularly interesting because it is the oldest planet (it’s at least 160 million years old) to be observed with this kind of direct imaging technique. Younger planets are usually hotter and emit more radiation, and so it’s easier to see them when they’re younger.

Although this system is one of the oldest directly imaged, it’s still very young compared to our solar system, which is about 4.5672 billion years old. 160 million years after our Sun formed (and the solar system got its start), the Earth and other planets had already formed, but the Earth was very different from the way it is today. For example, water may have already collected in the oceans, but our atmosphere was probably composed of carbon dioxide and water, with essentially no oxygen. Oxygen came later when life got started.

So studies like this one can allow us to peer back in time to figure out, among other things, what planets look like when they are very young and piece together to story of our solar system.

In journal club this morning, we discussed the recent paper by Deming et al. (2013), just accepted for publication by the Astrophysical Journal. In this study, the authors observed the planetary transits of HD 209458 b and XO-1 b using the Hubble Space Telescope in the near-infrared (about 1-1.6 microns in wavelength) to look for spectral features of water and other molecules. These features tell us whether water and other molecules are present in a planet’s atmosphere, how abundant those molecules are, and even where the atmosphere they are located. In this case, the authors saw spectral absorption from water molecules (see the left portion of the figure at left), but the amount of absorption (the depth of the absorption band) isn’t as large as it would be if the atmosphere were clear. Instead, the relatively shallow absorption feature suggests that there are clouds or haze suspended high in the planet’s atmosphere.

The cartoon in the right portion of the figure shows how this works: Panel (a) illustrates the passage of light emitted by a planet-hosting star through a clear planetary atmosphere, on the way to the telescope. As the light passes through, the water molecules (blue circles) imprint their spectral absorption feature. By contrast, panel (c) illustrates the passage for a cloudy atmosphere. The clouds block a lot of the light before it reaches the telescope, and so only the photons that pass high above the clouds, where there’s not much water to absorb light, get to the telescope. As a result, the water spectral feature is less deep.

The new techniques developed in Deming et al. (2013) may open the door to totally new astronomical observations. For example, the authors suggest that the significantly improved sensitivity of their technique may allow astronomers to directly observe meteorological variability (weather) on these distant exoplanets.

Figure 10 from Cleeves et al. (2013) showing the stellar wind driving cosmic rays away from the disk

In journal club this morning, we discussed a new paper by Cleeves, Adams, & Bergin that pointed out the important (and apparently previously unrecognized) role of stellar wind in the chemistry and dynamics of protoplanetary disks. Astronomers have long believed that cosmic rays induce significant ionization in protoplanetary gas disks around young stars. This ionization is important because (1) it drives chemical reactions, helping to set the initial compositions of planets, and (2) it causes the disk to interact with magnetic fields, producing turbulence in the disk, which affects where/how/when planets form. If, as suggested by the authors, the stellar wind can exclude cosmic rays from the inner reaches of the disk, then sufficient ionization to power these two effects requires some additional energy source. Cleeves and co. suggest decay of radio nuclides, which produces ionizing gamma radiation, may be able to take up the slack, but in our discussion, it was suggested that gamma rays might drive different chemistry from what was expected of cosmic rays.