The Week in Space and Physics: Cosmic Dust

On PAHs in young galaxies, the future of Arecibo, Kuiper Belts and evaporating planets

Cosmic dust glows around new forming stars in this image of the Tarantula Nebula, as seen by the James Webb Space Telescope. Image credit: NASA, ESA, CSA, STScI, Webb ERO Production Team

Astronomers often regard cosmic dust with annoyance. Rather like a faint mist, dust fills the cosmos, scattering light and rendering distant objects hard to see. Clouds of it can obscure distant objects entirely, blocking more interesting stars and galaxies from view. At times it can mess up sensitive observations - astronomers once, to much acclaim, declared they’d spotted a gravitational wave, only to later realise they had been looking at space dust all along.

Over time, however, they have come to appreciate the role dust plays in the cosmos. Since it forms in all kinds of processes, dust can reveal fine details about what, exactly, is happening around nearby stars or in far away galaxies. It is, too, surprisingly complex. Dust comes in many different forms, and is composed of many different chemicals. 

Of particular interest to researchers are types of dust known as polycyclic aromatic hydrocarbons, or PAHs. On Earth these are created when forests burn; appearing as a kind of soot. In space, they seem to somehow relate to the formation of stars. By tracing their presence - thanks to infrared telescopes like the James Webb - astronomers can thus find regions when new stars are being born.

They are commonly seen in galaxies close to our own. The James Webb has been actively searching for signs of them: last year it spotted PAHs swirling around black holes, a region where astronomers had thought they would be destroyed. Recently, however, researchers used the telescope to search for their presence in far away galaxies. In one - a galaxy seen as it was one and a half billion years after the Big Bang - astronomers found clear signs of PAHs.

This shows two things. First, the galaxy was producing stars at a rapid rate. In size this galaxy was already roughly the size of the Milky Way, even though we see it as it was twelve billion years ago. Yet it was also mature enough to have large amounts of carbon, an element that can only form through the life and death of previous generations of stars.

That suggests that younger galaxies, if we can see them, might lack PAHs altogether. There are some hints in these observations that this could be the case: though PAHs are present in this galaxy, they were not present everywhere. In some places, indeed, researchers saw stars forming without PAHs, though why this is happening is unclear.

Sadly, observing those earlier galaxies may prove rather hard. The instrument on the James Webb capable of picking out PAHs seems to be suffering from unexpected problems. Operators estimate that it should keep working well enough for another year - but after that, if nothing changes, spotting dust in distant galaxies will once again become impossible.

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Replacing Arecibo 

The collapse of the famous Arecibo Observatory in 2020 shocked and disheartened astronomers. While it still existed, it was the most powerful radio telescope in the Western hemisphere; outshone globally only by a five hundred meter wide observatory nestled into the mountains of China.

Despite the loss of its main telescope, the Arecibo site has remained open. Part of that is to allow for demolition work, of course; but the observatory is also keen to rebuild itself into a modern facility, one that builds on its past heritage. With this goal in mind, Arecibo’s staff have been working for some time on designs for a new radio telescope.

Instead of a single large antenna, however, the designs released in 2021 envisioned building over a thousand smaller dishes. By working together, these dishes should have a sensitivity twice that of the previous antenna. But they would also be more flexible, able to view more of the sky than before. That would allow the telescope to see the galactic centre - one of the brightest radio targets in the sky, but one out of reach of the original Arecibo.

Sadly, however, the National Science Foundation seems unwilling to fund these designs. Arecibo staff have thus been forced to scale down their plans. In a new report, submitted to Arxiv last month, they unveiled designs for a smaller, less ambitious replacement. 

Instead of a thousand small antennas, they now expect to build one hundred thirteen-meter dishes. These would work in tandem, as before, and would be able to view much of the sky, including the galactic centre. But the new plans limit the power of the proposed telescope. Altogether the one hundred antennas would have a collecting power equal to that of a one hundred and thirty meter wide dish, less than half that of the original Arecibo.

Arecibo, in truth, does not seem to be much of a priority for the National Science Foundation. Its collapse was partly due to limited maintenance budgets amid damage caused by humid conditions and hurricanes. Now, as astronomers consider a replacement, they are once again running into a lack of money and interest. Unless Arecibo can somehow capture extra funds, its future looks far from assured.

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A Second Kuiper Belt?

The asteroid belt, lying between Mars and Jupiter, is well known. Further out, the Kuiper Belt stretches beyond the orbit of Neptune. It, like the asteroid belt, is home to small and rocky worlds. Among them is Pluto, once considered the ninth planet, and others like Quaoar, Arrokoth and Makemake.

Since 2015 the New Horizons spacecraft has been flying through the Kuiper Belt. It has visited two worlds there - Pluto and Arrokoth - and mission planners have been searching for others that could be visited in the future. At the same time, however, NASA seems intent on reclassifying the mission, redirecting its focus from the Kuiper Belt towards the physical properties of the outer solar system.

In a recent presentation, however, a team of astronomers raised the possibility that another belt of asteroids could lie even further out. New Horizons, they say, could reach this belt by the end of the decade - and is also our only real prospect for exploring it. 

That said, the evidence that this belt really exists is still weak. Cynically one might even suspect researchers are trying to pull NASA back from their decision to reclassify the mission. Should its focus stay on the Kuiper Belt, New Horizons may be able to explore a few more worlds in the outer solar system. Should it switch instead to studying the heliosphere, as NASA wants, that is unlikely to happen. The sudden possibility of a second Kuiper Belt looks like an attempt to make that decision look unwise.

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A Planet With a Tail

Long ago Mars had a thick atmosphere. Over time that atmosphere was lost, its molecules stripped away by the pressure of the solar wind. The same process affects many planets - though in the case of Earth and Venus, far larger planets than Mars, gravity was strong enough to hold onto much of the atmosphere.

There are, however, more extreme possibilities. Around many stars we’ve spotted so-called “Hot Jupiters”. These giant gas planets orbit extremely closely to their stars, often far closer than Mercury is to our own Sun. For these worlds gravity is not enough to hold them together - under the pressure of their stars they gradually evaporate, shedding their outer layers.

The results can be dramatic. Recently astronomers spotted a long tail extending behind such a Hot Jupiter, circling a star some nine hundred light years away. This tail, researchers found, is made of helium stripped from the planet’s upper atmosphere. Other such tails are probably out there, but for now, at least, this is the most impressive example known.