With so many exoplanets being discovered, we have enough that we can start looking at the statistics of their various properties. Usually these deal with things such as their mass or habitability, but we’re also starting to look at the composition of their atmospheres. What we’re finding is that they share some common properties, but also hold a few mysteries. 

Only a few exoplanets can be observed directly, so we usually have to rely on indirect methods. This typically involves observing the spectra of starlight as an exoplanet passes in front of it. While the planet blocks a small amount of the star’s light, making the star appear dimmer, any atmosphere of the planet absorbs only some of the starlight. By studying which wavelengths of light the atmosphere blocks, we can determine what wavelengths might be scattered or absorbed by the atmosphere. From this we can determine certain properties of the atmosphere.

For example, if shorter blue wavelengths are blocked more than longer red wavelengths, it could indicate Rayleigh scattering in the atmosphere, meaning that it could have a blue sky like Earth (or Pluto). If wavelengths are more uniformly blocked, it could indicate the presence of clouds. But one of the more interesting things to look at is just how different wavelengths are absorbed as the exoplanet just begins to pass in front of the star, or just as its ending its transit. From this we can determine how a planet’s atmosphere absorbs light at varying depths (what’s known as the depth function). Previous work has used an exoplanet’s depth function to determine its mass, for example. Depth functions are important because different types of atmospheres have different depth functions.

Recently a team made transit observations of 15 exoplanets at visible and near ultraviolet wavelengths. They found 10 of the planets had fairly uniform absorption, implying the presence of clouds, and 2 had evidence of Rayleigh scattering. When they looked at the depth functions of these exoplanets, they found that two of the worlds had depth functions that were…odd.

Light curves of WASP-1b. Credit: Jake D. Turner, et al.

Typically an atmosphere absorbs more ultraviolet light than visible light. Our own atmosphere, for example, does this, which helps protect us against harmful ultraviolet rays. It’s generally what we’d expect, since ultraviolet light tends to scatter more when interacting with molecules. But in the case of these two exoplanets, they found that visible light was blocked or absorbed more than ultraviolet. What makes it more strange is that both of these exoplanets (WASP-1b and WASP-36b) are “hot Jupiters.” They have more mass than Jupiter and orbit their respective stars in a matter of days. It’s thought that such massive worlds should have atmospheres dominated by hydrogen and helium. If that were the case, they should block more ultraviolet than visible, since that’s what hydrogen/helium atmospheres do. So we aren’t sure what’s going on here.

Hot Jupiters were a bit of a surprise when they were first discovered. Now we know even their atmospheres can hold a few surprises.

Paper: Jake D. Turner, et al. Ground-based near-UV observations of 15 transiting exoplanets: Constraints on their atmospheres and no evidence for asymmetrical transits. MNRAS
doi: 10.1093/mnras/stw574 (2016)

The post The Wonder Of Exoplanet Skies appeared first on One Universe at a Time.

The transit method of exoplanet observation looks at the light from a star to observe dips in brightness. When a planet passes in front of a star (transits), it blocks some of the starlight, making the star appear slightly dimmer. Normally when we are looking for exoplanets we simply look for an overall dip in brightness, from which we can determine things such as the size of the planet relative to the star.

But once an exoplanet is known, you can begin to make more subtle observations, such as observing the dip at different wavelengths. This was done by a team recently to study the atmosphere of an exoplanet known as Gliese 1214 b. Their results were recently published in the Astrophysical Journal.

This particular planet is a “super earth”, with a mass about 6.5 earths, and a diameter about 2.5 times larger than Earth. It orbits a small red dwarf star. Because of the planet’s size and mass, its density is only about twice that of water, which has led some to speculate that it is a watery world with a relatively thick atmosphere. Now in this latest paper there is some evidence to support that idea.

If we could observe the planet directly, it would look like a disk, where the outer edge of the disk is the planets atmosphere. If you’ve seen a picture of Earth from space, you get the idea. As the planet passes in front of the star, the atmosphere starts to block the star before the disk of the planet does. Likewise, as the planet finishes transiting, the disk of the planet unblocks the star before the atmosphere does. So the beginning and ending portions of the transit are more strongly due to the atmosphere of the planet.

The team made observations of planetary transits at different wavelengths, and compared the different dips at those wavelengths to find the amount of light absorbed or scattered by the planet’s atmosphere. They then compared their results to three atmospheric models: one mostly of hydrogen; a water-rich atmosphere (similar to Earth’s); and a highly cloudy atmosphere (such as Venus or Titan). These three different types of atmospheres interact with light in different ways, as seen in the figure below.

Credit: NAOJ

In a hydrogen dominated atmosphere, Rayleigh scattering scatters most of the blue light, leaving red wavelengths to dominate. A similar effect occurs in our atmosphere, but to a less extent. You might remember from a previous post that the amount of Rayleigh scattering depends upon the size of atmospheric molecules. Since hydrogen is much much smaller than the nitrogen of our atmosphere, so we would expect more scattering. A water rich atmosphere would have less depth than a hydrogen one, so the absorption and scattering of light would be less severe and more uniform. Finally, a cloudy atmosphere would block much of the light at various wavelengths.

The model that best fit observation was one of a water rich atmosphere similar to our own. This seems to support the idea that Gliese 1214 b is a watery world. Because of this, news of the results have hit the popular press. But the authors note that hydrogen-rich atmosphere with heavy clouds is also a possibility. So while it seems likely that this is a watery world, more observations are needed.

It’s pretty amazing that we are now able to start analyzing the atmospheres of planets around other stars, but it’s even more amazing when you realize these observations were made with a ground based telescope, the Subaru telescope at Mauna Kea.

The post Water in the Sky appeared first on One Universe at a Time.

A neutron star is the remnant of a large supernova. When a large star explodes, a remnant of its core is compressed so tightly that the electrons are squeezed into protons, resulting in a mass of neutrons. A neutron star typically has a mass of about 2 solar masses, but it is only about 12 kilometers in diameter. Imagine taking two suns and squeeze it into the size of a small city, and you get the idea of how incredibly dense these objects are.

Neutron stars are often represented as a simple mass of neutrons, but we know that they actually have a complex structure. Just as the Earth has a crust, mantle and core, a neutron star has an iron crust, neutron mantle and core. We know from variations the rotation rate of pulsars that neutron stars undergo “starquakes”, that they likely have mountains (assuming you can call a rift a couple meters high a mountain range). They are geologically active. We also know they have atmospheres.

Just how we observe the atmosphere of a 12 kilometer wide neutron star light years away is pretty interesting. The surface temperature of a young neutron star is about a million Kelvin. This means it radiates a significant amount of light in the x-ray spectrum. If a neutron star had no atmosphere, then light should follow a distribution known as the blackbody curve. In other words, with no atmosphere the x-ray light should depend only on the surface temperature of the neutron star. But if it has an atmosphere, then the atmosphere will absorb some of the light and emit light at a different wavelength. So the x-ray spectrum would measurably differ from a blackbody spectrum. Just how it differs would depend upon the makeup and thickness of the atmosphere.

In 2009, Wynn Ho and Craig Heinke published a paper analyzing the x-ray spectrum of a neutron star known as Cassiopeia A. (Actually, Cassiopeia A is the name of the supernova remnant where the neutron star resides.) This is a young neutron star about 11,000 light years away. They found that the x-ray emission did not match a blackbody spectrum well, so they compared the spectrum to various model atmospheres such as pure hydrogen, helium, carbon, nitrogen, and oxygen. They found the best match to be a carbon atmosphere.

This carbon atmosphere isn’t like anything we’ve experienced. It’s only about 4 centimeters thick, and while it is “gaseous”, its density is about the same as diamond (3.5 grams/cc). Over time this atmosphere is expected to change. The gravity is so intense that carbon would eventually settle out of the atmosphere while lighter elements such as hydrogen and helium accumulate. Eventually even the lighter elements would settle to the surface.

But Cassiopeia A is still young, so for now it can enjoy its diamond sky.

The post Diamond Sky appeared first on One Universe at a Time.

When we view stars from the surface of our planet, they appear to twinkle. This is due to turbulence in the air, which creates air fluctuations that cause the starlight to deflect slightly. Since stars appear point-like due to their distance, the small deflections are enough to cause the star to twinkle.

Usually we just notice the variation in brightness, but air also acts like a prism, bending different colors of light by different amounts. So not only do stars appear to vary in brightness, they can also appear to vary in color.

Recently +Roshaan Bukhari revealed this rainbow effect with the star Sirius. As he photographed the star, he shifted the telescope slightly to create a kind of time-lapse trail. You can see the result below.

If you like this one, check out his page for more pictures.

The rainbow of Sirius. Credit: Roshaan Bukhari

The post Rainbow Star appeared first on One Universe at a Time.

With the New Horizons spacecraft on its way to Pluto, and scheduled to flyby the dwarf planet in 2015, there is a growing interest in this distant world.  Much of this focuses on aspects of the planet that might be important for the flyby. A while back I talked about how computer simulations of Pluto and its largest moon Charon showed that they likely formed through a large collision, and as a result New Horizons should find additional plutonian moons. 

Pluto and its largest moon Charon are sometimes referred to as a double planet.  Charon’s mass is almost a tenth that of Pluto, and their barycenter (center of mass) is actually outside the volume of Pluto.  In contrast, our quite large Moon has only 1% the mass of Earth.  Pluto and Charon are also so close to each other that they are tidally locked with each other. Now a new paper in Icarus has looked at the atmosphere of Pluto, and found that it may share its atmosphere with Charon.

We actually know quite a bit about Pluto’s atmosphere.  It was first discovered in 1988 when Pluto transited a dim star.  Since then, spectroscopic analysis has shown it contains methane  and ethane. We’ve also found that it is thicker than anticipated, although “thicker” in this case means a pressure of about 0.3 pascal, as compared to Earth’s 100,000 pascal. By Earth standards it is basically a vacuum.

It is also apparently thick enough and warm enough that some of it is exchanged to Charon. Using the known characteristics of Pluto’s atmosphere, the team showed that this exchange is large enough to consider that Pluto and Charon have a common atmosphere.  The timing of this discovery is perfect, because detectors onboard New Horizons will be able to detect the atmospheres of Pluto and Charon and determine if they have the same chemical signature.  If so, it is one more reason to consider them to be a double system.

And who knows, it might also be enough reason for astronomers to re-label Pluto as a planet, which is what everyone really wants in the end.

Paper: O.J. Tucker, et al. Gas transfer in the Pluto–Charon system: A Charon atmosphere. Icarus. DOI: 10.1016/j.icarus.2014.05.002 (2014).

The post Sharing is Caring appeared first on One Universe at a Time.

The first image below tells us something about the atmospheric makeup of four planets orbiting a star 130 light years away.  Think on that for a bit.  We’re now able to study the atmospheres of extra-solar planets.  What’s even more amazing is it was done from a ground-based telescope.

Credit: B. R. Oppenheimer, et al.

The figure comes from a recent arxiv preprint in which a team of astronomers measured the absorption spectra of four planets planets of the star HR 8799.  The second image shows these planets labeled b – e (HR 8799a is the star itself).    As you might imagine, simply observing exoplanets directly is a challenge since the light from planets is overwhelmed by the light of the star.  It helps that these are large Jupiter-like worlds, so they are relatively bright in the infrared, but the star is as well so it doesn’t help that much.  So how was the team able to image not only the planets, but the spectra of those planets?

To pull this off, the team had to take long exposures of the star using a coronagraph.  This is a device that blocks the direct light of the star, so you can measure light from other nearby sources such as the planets.  Unfortunately you can’t block all the light from the star.  Turbulence in the  Earth’s atmosphere distorts the starlight we see, so you get a random pattern of scattered light in your image called speckle.  With a long exposure and adaptive optics you can average out much of the speckle, but there is some speckle that occurs simply due to tiny flaws in your optics, and you can’t get rid of them.  Since you can’t eliminate the defect speckle, you can’t be entirely sure what you are observing is a real planet or a flaw in your system.

Credit: B. R. Oppenheimer, et al.

But the position of the “defect speckle” depends on the wavelength of the light you are observing.  So the team took long exposures at a range of wavelengths in the infrared.  At each wavelength the speckle pattern is different, but the position of planet light stays the same.  By combining the different wavelength images you can eliminate the speckle, leaving only the light from the planets, which is seen in the second image here.  Once you know where the planets are, you can look at the light from those planets across the range of wavelengths you’ve observed, and thus get a spectrum for each planet.

In this infrared range, much of the light comes from the heat of the planets themselves.  This means the spectra we observe is light that has passed from the planet through the planet’s atmosphere to reach us.  Different gas molecules absorb light at different wavelengths, so by looking at a spectrum we can see at what wavelengths light is absorbed, which tells us what type of molecules are in the atmosphere.

The spectra gathered from these planets are pretty low resolution, so we can’t determine an exact makeup of the planetary atmospheres, but we can determine some broad characteristics.  One interesting thing that was found is that these planets have very different atmospheres.  For example, the atmosphere of planet b contains ammonia and carbon dioxide, but little methane.  Planet e contains methane but not ammonia or carbon dioxide.  This is rather surprising, since presumably large planets would tend to form early and therefore have similar makeups.  How these planets can be radically different from each other is a bit of a puzzle.

But this is just the first of many planetary systems to be observed.  The project, known as Project 1640 plans to observe about 200 nearby stars over a three year period using this method. So we’re likely to know about the atmospheres of a lot more extra-solar planets in the near future.

The post All These Worlds appeared first on One Universe at a Time.