Shortlisting Stars With Planetary Systems
Shortlisting Stars With Planets
Markus Landgraf of the European Space Agency and colleagues have found the first direct evidence that a bright disc of dust surrounds our Solar System.
Remarkably, their discovery gives astronomers a way to determine which other stars in the Galaxy are most likely to harbour planets and allows mission planners to draw up a ‘short-list’ of stars to be observed by future planet-search missions, such as Eddington and Darwin.
Mature Stars Thought Not to be Dusty
The discovery of the Solar System’s dust ring strengthens the idea that such features around mature stars are signposts to planetary systems. The reason for this is that planetary systems are thought to condense from a cloud of gas and dust. Planets form near the central star, where the material is densest. However, at great distances from the star, the gas and dust is sparse and can coalesce only into a vast band of small, icy bodies. In our Solar System, they form the so-called Edgeworth-Kuiper belt that extends out beyond the orbit of Neptune. Any remaining dust is lost to deep space.
"After the first circumstellar dust disk were discovered around very young stars," says Landgraf, "it was concluded from numerical modeling that mature stars will not have such a disk." Ordinarily, dust is either incorporated into larger celestial bodies or ejected from the Solar System. "The reason is that dust disks erode very quickly (within 100 million years) by mutual collisions and evaporation of the dust grains," Landgraf says. "It was only after a dust disk was observed around the still young, but already mature star Vega, that people thought about dust disks around mature stars. Recently there was the finding of a circustellar disk around the 1 billion-year old star Epsilon Eridani, which confirmed this hypothesis. Whether or not our Sun has such a disk was discussed after the discovery of the Edgeworth-Kuiper belt. It was only now, after our analysis, that the dispute over its existence is settled, since the Pioneer 10 data give direct evidence for the disk."
But for such dust still to be present today around a mature star like our Sun, means a very large source must be found since something is replenishing it. "In order to sustain such a ring, 50 tons of dust have to be generated every second," says Landgraf. He and his colleagues believe that collisions between the icy remnants of the Edgeworth-Kuiper belt create the Solar System’s dust ring. If the same is going on in other planetary systems, then those stars will also have dusty rings around them.
"If you have a dust disc around a star that’s not particularly young, then it’s extremely interesting because the dust has to come from somewhere. The only explanation is that the star has planets, comets, asteroids or other bodies that collide and generate the dust," says Malcolm Fridlund, ESA’s study scientist for Darwin, the mission under development to search for life-supporting planets around other stars.
Celestial Dust Detectives
To trace the collisions in the Edgeworth-Kuiper Belt, Landgraf and colleagues had to do some celestial detective work. They began by sifting through data from the 1970s and early 1980s, when NASA space probes Pioneer 10 and 11 first found dust particles of unknown origin beyond Saturn’s orbit. "The motivation for this study," says Landgraf, "was actually to understand the data obtained with the Pioneer probes in comparison with the data we have received from the dust instruments on the ESA spacecraft Ulysses. So, we weren’t actually looking for the dust ring, but we found it to be the only plausible explanation for the many dust particles detected by Pioneer 10 outside Saturn."
Artist’s impression of Pioneer’s jovial fly-by
The hypothesis of dust coming from comets was discarded: in fact near the Earth, comets give off dust; beyond Saturn, however, they freeze and shed little material. So, no one knew whether the Pioneer dust grains were coming from inside the Solar System – from a source other than comets – or beyond it from the interstellar space. Now, using data from ESA’s Ulysses spacecraft, which has been orbiting the poles of the Sun for more than 10 years, Landgraf and colleagues have been able to rule out an origin beyond the Solar System. The Ulysses data shows that dust grains of interstellar origin are considerably smaller than interplanetary dust grains, which originate in the Solar System.
Planet Hunters Look for Tell-tale Dust Rings
The interstellar grains detected by Ulysses are typically ten to a hundred times smaller than the smallest grain that could be detected by Pioneer. Thus, the Pioneer grains have to be made somewhere within our Solar System. So, by a process of elimination and computer simulations, the scientists came to the conclusion that the only possible source of the dust is the collisions between the small, icy objects in the Edgeworth-Kuiper belt. Since these are the remnants of planet formation, the team believes that planetary systems around other stars will also produce constantly replenishing dust rings.
From the number of dust particles detected by the Pioneers, Landgraf and colleagues were able to calculate the density of dust in the ring. "There’s only one dust particle every 50 cubic kilometres but it’s enough for a bright dust ring like those we see around other stars," says Landgraf. Indeed, a number of such features have been observed shining brightly at infrared wavelengths around stars such as Vega and Epsilon Eridani.
So in addition to solving the puzzle of how the dust is created, scientists hope to use such models to narrow down a list of stars likely to harbor planets. "If we see a similar dust ring around a main sequence star (a mature star, like the Sun), we’ll know it must have asteroids or comets. If we see gaps in the dust ring, it will probably have planets which are sweeping away the dust as they orbit," says Landgraf.
"There is no other explanation for a structure in the ring than perturbation of the dust grain’s orbits by a planet," says Landgraf. "It is not only the sweeping effect of the planet, but also its gravity, which tends to catch the dust in resonant orbits, that is in orbits which have a period with a integer ratio compared to the planet’s orbit period. e.g. many of the dust particles from the Edgeworth-Kuiper belt are on orbits that take them around the Sun once every time the planet Neptune goes around the Sun twice. This is caused by Neptune’s gravity."
The result slots into place another piece of the puzzle for those scientists working on missions that will search for extrasolar planets, as it will allow them to draw up a well motivated list of target stars based upon whether they are surrounded by dust rings. "This finding has exciting implications for both missions (Eddington and Darwin)," confirms Fridlund.
After solving the main mystery of dust replenishment, Landgraf and his colleagues would like to know more about the dust size and composition. "From the Pioneer data there is not much we can say about the size, only that the inner edge of the disk is at Saturn’s orbit. The outer reaches are unknown and so is the total mass. There are theoretical calculations of the mass, but no direct measurement."
ESA’s Herschel observatory will help in the search for extrasolar planets.
Computer modelling is likely their only redress however as the experimental scale of collisions to produce these rings is so enormous. "Mutual collisions between Edgeworth-Kuiper belt objects as well as erosion by interstellar dust bombardment create grains of all sizes," says Landgraf. "It is not known exactly how many fragments of what size are created, because such a big collision like one between two Edgeworth-Kuiper belt objects can not be recreated in the lab. According to a model calculation both mechanisms (mutual collisions as well as interstellar erosion) cause the same amount of dust (in terms of mass). The interstellar dust grains are typically much smaller than the grains observed with the Pioneer instruments, they have about 100 times lower masses. Nonetheless they can release larger grains, because the hit the objects of the Edgeworth-Kuiper belt with high speed (more than 20 kilometre per second)."
To fulfill another goal of the researchers–detecting an equivalent to the Edgeworth-Kuiper belt around another star– presents some opportunities for future large telescopes. "With today’s technology we can only detect the dust ring, not the Edgworth-Kuiper belt itself," explains Landgraf. "The reason is that all the tiny dust grains together have a much bigger surface area. You can compare it with a comet. You can see the comet’s tail easily with the naked eye, but for the core you have to have extremely sensitive telescopes. This is despite the fact that the core is many million times more massive. Another example is the smoke of a fire. It is very visible without weighing much. The small ash particles and water droplets have just such a big surface area, which makes them visible. The big objects of the Edgeworth-Kuiper belt equivalent are in the order of 100 kilometres in diameter and are thus very compact and have little surface area for their mass."
Future missions, such as ESA’s Herschel mission will search for many more and take detailed pictures of stars that might harbor dust rings. As these images become available, astronomers will be able to predict the sizes and orbits of giant planets within the alien solar system.