We all know that our Solar System contains eight planets and their moons. Bu there is a lot of other stuff out there too of course, ranging in size from several hundred kilometres across to microscopic dust. Much of it orbits the Sun in wide belts between certain planets - the asteroid belt between Mars and Jupiter; the Kuiper Belt (source of many comets) well beyond Neptune, and the Zodiacal cloud (responsible for the Zodiacal Light) mainly between the Sun and Jupiter.
The potential asteroid threat to Earth from space is a hot topic these days. And it's not surprising – there are millions of space rocks of vastly different sizes orbiting the Sun, and every so often they can get kicked out of our orbit and come heading our way. Luckily, most that reach us are small enough to get burned up in Earth’s atmosphere but in the past every now and again a larger one has got through with catastrophic effects. Maybe Earth will dodge the threat for now, but collisions in space are very common – just look at the number of craters on the Moon and Mercury for example. Scientists consider that much of the orbiting rocky and dusty material is debris from such collisions.
What about stellar systems beyond our own? ‘Exoplanets’ appear regularly in science news these days – do their star systems also contain tell-tale remains of collisions? If so, this could potentially help us understand better the way that planets and planetary systems form. But given that exoplanets themselves are pretty difficult to detect, being so distant and small, what chance is there of being able to detect orbiting debris that’s even smaller?
Despite the apparent difficulties, astronomers have developed a very successful method for spotting so-called debris discs around other stars. Dust orbiting a star absorbs the star’s radiation and re-emits it at infra-red wavelengths. If telescopic observations show that a star has more infra-red emission than expected, the chances are that the culprit is a debris disc. The Spitzer and Herschel Space Telescopes and the ALMA earth-based telescope have detected hundreds of debris discs around stars using this method. The Hubble Space Telescope has also produced images of discs glowing faintly in visible light, like these ones here.
As well as simply finding debris discs, the results show that many are separated into several belts, just like in our Solar System. In many cases planets have also been detected orbiting stars with debris discs and may have a role to play in creating separate debris belts. And this also means that finding debris discs can be a valuable tool for spotting stars likely to host exoplanets.
Research has even spotlighted some discs changing over time. For example, debris discs around 5 stars observed over a period of several years have been shown to suddenly increase and decrease in brightness over short periods – evidence suggesting that they are indeed formed by sudden collisions.
Other recent research suggests that there is likely a close relationship between planet formation and disc formation. Giant, Jupiter-sized planets in particular appear to play an important role because of their strong gravitational influence. There are still many unresolved questions about the processes involved but this work on debris discs is one more way for scientists to develop an understanding of how our own solar system works.
https://solarsystem.nasa.gov/news/792/10-things-to-know-about-the-kuiper-belt/
https://solarsystem.nasa.gov/asteroids-comets-and-meteors/asteroids/in-depth/
https://www.ast.cam.ac.uk/research/star.formation.and.exoplanets/debris.disks
http://www.spitzer.caltech.edu/news/1989-ssc2017-12-Giant-Exoplanet-Hunters-Look-for-Debris-Disks
1. NASA/Caltech
2.NASA, ESA, G. Schneider (University of Arizona), and the HST/GO 12228 Team
http://imgsrc.hubblesite.org/hvi/uploads/image_file/image_attachment/26357/web.jpg
3. NASA/ESA/D.R. Ardila (JHU)
http://www.spitzer.caltech.edu/uploaded_files/images/0008/8153/ssc2004-22d2_Ti.jpg