The blast from a supernova should be enough to clear almost all the dust from the solar system, and this may have last happened 3 million years ago. But like the dust on furniture, these fine particles will gradually be replenished.
Dust we see drifting through space consists of small grains, resulting from events such as asteroid collisions. The grains are typically less than a millimetre in size, going down to just nanometres across. About 70 per cent of the solar system’s dust is concentrated in the Kuiper belt, a region of icy asteroids and comets beyond Neptune, where there are an estimated tiny grains.
at Boston University and his colleagues have modelled what would happen to the dust in the Kuiper belt if a star exploded within 160 light years of Earth, or if our solar system passed through a dense star-forming interstellar cloud of material.
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One of these events is thought to have occurred about 3 million years ago based on increases in levels of a radioactive isotope of iron – iron-60 – seen in ice on Earth dating to that period. The isotopes would have been delivered when the blast also removed the sun’s heliosphere, which protects planets in our solar system from the galaxy’s radiation.
Miller and his colleagues have found that either event would sweep all dust smaller than a millimetre from the Kuiper belt. “It’s like turning on a giant fan,” says Miller. “There’s a lot of hydrogen atoms hitting these dust grains and changing their orbits”, with the dust either being pushed into the sun or ejected from the solar system entirely.
at Cardiff University, UK, says the wind produced by a supernova can travel at “a few thousand to 10,000 kilometres per second”, which could have “enough kinetic energy to blow off material in the solar system”.
It would have taken tens of thousands of years for the solar system to pass through the supernova’s blast wave, but up to a million years to traverse an interstellar cloud.
The former’s low density, “in the ballpark of 0.01 atoms per cubic centimetre”, says Miller, would mean it would have been halted at about Saturn’s orbit by the solar wind of charged particles heading outwards from the sun. An interstellar cloud – being much denser, at 1000 atoms per cubic centimetre, but much slower, at about 20 kilometres per second – could have reached the orbit of Mercury, reducing the amount of interplanetary dust near Earth, although “we don’t have a lot of dust near Earth” compared with in the Kuiper belt, says Miller.
Some of the dust in the Kuiper belt could also have been moved into reservoirs in a different orbit out of the flat plane of the solar system. “That would mean it’s not interacting with the planets as much,” says Miller. This could explain why NASA’s New Horizons spacecraft, currently travelling through the Kuiper belt region, has recently detected an increase in dust, as it is possibly moving through such an area.
The researchers calculate that it should take about 11 million years for dust in our solar system to return to an equilibrium state, where it is being produced as fast as it is falling into the sun or being ejected by more usual processes. If a dust-sweeping event happened 3 million years ago, that “means we’re still in the building-up phase”, says Miller.
arXiv
