The Week in Space and Physics
On the future of our Solar System, dust clouds, trojan asteroids and bizarre radio signals.
Over the years astronomers have come up with all sorts of clever ways to spot distant planets. The most obvious way – to look for them through telescopes – is almost impossible. Planets, even ones as big as Jupiter, are, cosmically speaking at least, tiny. They barely shine, reflecting only what little light they get from their stars; a feeble glow that is anyway drowned out by the fierce brightness of a nearby star.
That means astronomers hunt instead for the influence planets have on things around them. Some stars dim periodically and predictably, a pattern caused by a planet regularly passing between it and Earth. Others wobble, tugged around by giants like Jupiter. Careful observations of distant stars can, then, reveal the presence of planets.
Astronomers, though, have long wondered if planets might be found around white dwarfs: objects that are not quite a stars, but rather the fading remains of dead ones. Almost every star in our galaxy will, once it burns through its supply of nuclear fuel, end up as a white dwarf. As such, the sky is probably littered with them – though most glow far too faintly to be spotted, let alone to find planets around.
Spotting planets around one would, however, be interesting: a hint that a solar system can survive the death of its star. Indeed, the discovery would be relevant to our own system of planets. Five billion years from now, the Sun will enter a final, destructive, phase of existence. It will expand, its outer layers stretching beyond the orbit of Venus. The inner planets – Mercury, Venus and perhaps Earth – will fall inwards, and be consumed.
Eventually those outer layers will dissipate, dispersed into the coldness of space. The rest of the Sun – its core, mostly – will collapse into a dense white dwarf. For millennia it will shine on, its residual heat gradually fading away.
What then happens to the outer planets, to Jupiter, Saturn and the rest? They should survive the heat of the Sun’s final surge. But would they stick around for aeons longer? Or would they drift away, condemned to an eternity wandering the heavens alone?
To find out, astronomers turned to a third method of spotting exoplanets, known as microlensing. Einstein’s theory of relativity tells us that big things like stars warp space, bending light rays. If two stars happen to line up in the sky, the closer star may – if the alignment is just right – warp the light coming from the further star. Should that closer star also have a planet circling around it, then its presence will be revealed by slight changes in the pattern of warping.
This approach was recently used to reveal the existence of a Jupiter sized planet orbiting a small white dwarf star a few thousand light years away. That, together with a similar discovery last year, shows that planets can, at least sometimes, survive the death of a star.
They do, however, seem to move inwards; a disturbance that astronomers predict should sometimes shatter planets, smearing them out into a disk of debris. Jupiter may need to be lucky, then, if it wants to survive for an eternity more.
Dust Clouds Make for a Bizarre Solar System
Sometimes the search for distant solar systems produces a result so bizarre it almost seems to defy natural explanation. One example came in 2015, when astronomers found a star, later nicknamed Tabby’s Star, that seemed to darken suddenly and unpredictably; a phenomenon that some attributed to alien engineering.
In the end researchers concluded that a moon had been ripped to shreds around the star, creating a cloud of dust and debris. They found other stars, too, that showed similar patterns of brightening and darkening, strengthening the idea that a natural, rather than intelligent, process lies behind the observations.
Now, however, astronomers have found another star that darkens in mysterious ways. Roughly once every three weeks the star dims somewhat: a regular pattern that would normally be explained by an orbiting planet. Astronomers would expect to see the same level of dimming on each occasion, as a planet should always block out the same amount of light.
In this system, though, the darkening was radically different each time. On some occasions up to three quarters of the light of the star was blocked, while other events were so weak they could hardly be detected. The duration of each event was variable: sometimes lasting two days, sometimes much less.
This, the researchers conclude, is likely caused by a large cloud of dust and debris. But – and this is the interesting part – that cloud seems to vary in size, creating an odd pattern of dimming. That means something must be regularly creating large amounts of debris in that solar system. Something like regular, catastrophic collisions between large asteroids or small planets.
One possible scenario, the researchers write, is a large asteroid frequently running into a cloud of smaller asteroids. The result is a series of collisions, enough to create clouds of dust, but not enough to fully destroy either the large asteroid or the smaller cloud.
For now, though, that idea is only a guess. Astronomers plan to spend more time watching the star to glean more hints about its strange solar system. They will also examine historical records – hoping to find observations showing when the dust first appeared.
Lucy in the Sky with Diamonds
Early on Saturday morning a rocket blasted skywards from Cape Canaveral. Onboard was Lucy, a probe that will now head out as far as Jupiter, visiting two groups of asteroids known as the Trojans. Over the next decade Lucy will visit at least eight of those asteroids, casting light, scientists hope, on the early days of the Solar System.
The asteroids in question, the Trojans, share Jupiter’s orbit around the Sun. They are not moons, but instead form two separate groups, one lying some way ahead of Jupiter and the other the same distance behind. The two asteroid clouds move together with Jupiter, maintaining their position around the giant planet.
Astronomers believe the Trojans are extremely old, forming just a few hundred million years after the planets did. They should, therefore, offer an opportunity to study relics from that turbulent time. Since different models of how the Solar System formed predict different properties for the two asteroid clouds, Lucy may even be able to rule out or endorse one of those models.
A second NASA mission, the James Webb Space Telescope, moved closer to launch. After spending just over two weeks sailing across the Caribbean, its transport ship arrived at Kourou in French Guiana. Despite nervous astronomers worrying about the dangers of the voyage – even going so far as to slightly panic over pirates – the telescope arrived without incident. The James Webb will now be prepared for launch. That should take place on December 18th.
Odd Radio Signals from Deep Space
When astronomers pick up a radio signal from deep space flashing on and off, they usually mark it down as a pulsar. These objects are ultra dense stars rotating rapidly while giving out powerful jets of radio energy. The result is somewhat akin to a lighthouse: two bright beams of radio waves sweeping regularly across the cosmos.
That, then, was the first thought of Ziteng Wang, a PhD student at Sydney University, when he examined a signal picked up by the ASKAP radio telescope. Further observations, however, showed something odd: the signal was indeed flashing on and off, but in a strangely irregular way. At times it would light up for weeks, disappear, and then return and vanish again within a day.
A pulsar would be far more regular – flashing on and off in a precise pattern. This signal must, Wang concluded, be something else, though quite what that might be is hard to say. A search for stars or supernova in the direction of the signal found nothing, ruling out any easy explanations. Instead some astronomers speculate that heavy objects – black holes or neutron stars, perhaps – are distorting the radio waves coming from some small and magnetically intense object, creating the strange patterns we see on Earth.
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