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.