HL Tau is a young star, only about a million years old. Despite its young age, the star is already busy at making a family. 

The star is probably most famous for an early image from the Atacama Large Millimeter/submillimeter Array (ALMA), which showed a disk of gas and dust surrounding the star. What was most striking about the image was the clear gaps in the dusty disk, which seemed to be due to young protoplanets as they began the process of formation. ALMA’s strength is the ability to observe light at millimeter wavelengths, which is the type of light emitted by cold gas and dust. Unfortunately the resolution of ALMA isn’t high enough to observe individual protoplanets, which left some doubt as to whether the gaps were caused by some other process.

Combined ALMA/VLA image of HL Tau.
Credit: Carrasco-Gonzalez, et al.; Bill Saxton, NRAO/AUI/NSF.

But new evidence from the Very Large Array (VLA) confirms the existence of at least one protoplanet. The VLA observes at longer radio wavelengths, but the VLA antennas can be spread across 36 kilometers vs ALMA’s maximum spread of 16 kilometers. This larger spread means that the VLA can make higher resolution images (though only at radio wavelengths). Recent observations of the central region of HL Tau clearly shows a clump in the inner ring of material, indicating a protoplanet in the early stages of formation.

These results show the power of combining observations from different facilities and at different wavelengths. They also demonstrate how astronomy can still surprise us. With an age of only a million years HR Tau is still in the earliest stage of its life, and yet it is already on the path toward becoming a solar system.

Paper: Carlos Carrasco-Gonzalez, et al. The VLA view of the HL Tau Disk – Disk Mass, Grain Evolution, and Early Planet Formation. arXiv:1603.03731 [astro-ph.SR] (2016)

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Every now and then in astronomy we’ll get an image that lets us actually see phenomena we have previously just deduced from other observations. The image above is one of them. It was taken by the Atacama Large Millimeter/submillimeter Array (ALMA), and shows an exoplanetary system in the process of forming. This isn’t an artistic rendering, it’s an actual image.

We’ve known for a while that planetary systems form along with a star. Known as the nebular model, the basic idea is that as a star forms within a large nebula (known as a stellar nursery) the surrounding gas and dust form an accretion disk around the star. Over time, protoplanets form within these disks, eventually clearing the system and becoming a planetary system such as our own solar system. We have a lot of evidence to support this model. We’ve observed stars forming within nebulae, and we have computer simulations showing how stars form an are cast out of a stellar nursery.  We’ve observed protoplanetary disks around young stars, and we have computer simulations showing how protoplanets begin forming within these disks. We’ve also found lots and lots of exoplanets. So we’ve known that out in the universe there are young stars where protoplanets are actively forming, we just haven’t observed them directly. Until now.

This particular image is of a star known as  HL Tauri. It is a young T-tauri type star about 450 light years away. The image was taken at wavelengths on the order of a millimeter, which is particularly good at penetrating the nebular material surrounding the young star. Because ALMA is an array of telescopes spread across 15 kilometers, it can capture images with a higher resolution than Hubble.

The ALMA telescope array. Credit: ALMA (ESO/NAOJ/NRAO), C. Padilla

It’s the detail of this image that is astounding. Not only can you clearly see the disk, you can actually see gaps in the disk. These gaps are due to protoplanets either clearing the region of their orbit, or creating resonances within the disk to produce gaps, similar to the way Jupiter produces Kirkwood gaps in the asteroid belt. This image is crystal clear evidence of protoplanet formation, just as the nebular model predicts.

ALMA is still in its early stages. In a way, it is one of ALMA’s baby pictures as it gets up to speed. It also happens to be a baby picture of a whole new solar system.

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Observations from the ALMA telescope array has made some interesting observations of carbon monoxide in the vicinity of the star Beta Pictoris.  The results have been published recently in Science, and it tells an interesting story about comets and planetary formation.

Beta Pictoris is a young star with a protoplanetary disk.  It is only about 63 light years away, so it provides us with a good opportunity to study the formation of a solar system.  In this new paper, large quantities of carbon monoxide have been observed within the protoplanetary disk.  The reason this is interesting is that carbon monoxide is unstable when exposed to sunlight, so any carbon monoxide around Beta Pictoris would typically break down within a century.  Since the disk contains large quantities of CO, there must be a replenishing source, and that is likely cometary fragments.  Given the quantity of carbon monoxide, it is estimated that you’d need a cometary collision about once every five minutes.  That would imply a dense cluster of cometary bodies.

An extrapolated top-down view of CO around Beta Pictoris.
Credit: ALMA (ESO/NAOJ/NRAO)/W. Dent et al.

Further support of this idea comes from the distribution of the carbon monoxide around the star.  From our vantage point, the protoplanetary disk of Beta Pictoris is edge on.  This can make it difficult to determine the distribution of material within the disk.  But since carbon monoxide has a clear line spectrum, the Doppler shift of that spectrum can be used to determine the motion of CO within the disk, and hence its distribution.  What we see is at least one dense clump, possibly two, within the disk, as you can see in the figure.

Such a concentration is evidence of a planetary body within the disk.  This planet could cause cometary fragments to cluster in the region of the planet, resulting in a high rate of collisions.

So it seems that the disk of Beta Pictoris is far more active than originally thought.

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IRAS 04368+2557 is a protostar about 450 light years from us.  It is a particularly young protostar, at about 300,000 years.  Because of its age and proximity, it provides an excellent opportunity to study the early stages of stellar and planetary formation.

Structure of a protoplanetary disk. Credit Sakai et al.

Like most protostars, this one has a protoplanetary disk out of which planets are expected to form.  Surrounding that is a larger protostellar envelope.  Between the two is a transition zone.  Because of interactions within the surrounding envelope, material can be slowed so that it gradually falls inward toward the protoplanetary disk.  This can enrich the chemical composition of the disk.  Now a new paper in Nature has found that the transition zone can have a centrifuge effect that chemically filters material entering the protoplanetary disk.

The team looked at line spectra of both the disk and the surrounding envelope.  The found that while the surrounding envelope was dominated by hydrocarbons, the region of the transition zone had high concentrations of sulphur monoxide.  In other words the chemical composition of the two regions are dramatically different.

This is likely due to a centrifugal effect where some molecules can more easily penetrate the transition zone, while others have a more difficult time.   So it seems that early solar systems are not simply the product of the material surrounding a young star, but that they are more dominated by materials that can penetrate the transition zone.

Paper:  Sakai N, Sakai T, Hirota T, et al. Change in the chemical composition of infalling gas forming a disk around a protostar. Nature. (2014)

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Above is an image of a protoplanetary disk in the Orion nebula, taken from the Hubble telescope.  The interesting thing about this particular image is that it’s edge on, and it’s silhouetted against the nebula itself.  This means we have a good view to the dust and rock of the disk.  This means we can actually observe the dust beginning to coalesce into larger bodies, which should eventually lead to planets.

The discovery of protoplanetary disks such as this one agree with the nebular hypothesis, which posits that a star and planetary system form together.  The star collapses out of a nebula, and it forms an accretion disk around itself, out of which the planets form.

While an image such as this is useful in observing the early stages of planetary formation, the very conditions that make it easy for us to observe also make it more difficult for planets to form.  The Orion nebula is dominated by four very bright stars known as the Trapezium cluster.  Because the surrounding nebula is brightly illuminated by these stars, the protoplanetary disk is easy to observe.  But this means the disk itself is also being irradiated by the cluster, so much of the gas and dust of the disk may be driven away from the star before planets have a chance to form.  It may be the case that some Jupiter-size planets can form quickly before too much gas and dust is driven away, but it isn’t likely that a solar system like ours will form in this case.

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