Searching for life and planets
Life on other worlds? A fundamental issue in astronomy and biology and an important question for humankind. The SKA will be able to detect extremely weak extraterrestrial signals and may even spot other planets capable of supporting life. Astrobiologists will use the SKA to search for amino acids, the building blocks of life, by identifying spectral lines at specific frequencies.
The dusty disks that form around young stars are the sites where planets are made. The birth of a planet is thought to take a million years or more – much longer than a human lifetime – and it isn’t possible to watch an individual planet appear. Instead, the disks around many young stars must be observed in order to piece together the different parts of the formation process. Fortunately there are hundreds of these young stars within about 500 light years of the Sun, and many thousands more at greater distances. If placed at 500 light years distance, our own Solar System would be about 1 arcsecond across – roughly equivalent to a thumb-tack seen from a mile away – so observations at high angular resolution are very important. At radio wavelengths, such resolution is acheived by combining the signals from widely separated antennas. In the SKA, the antennas will be as much as 3000 km apart, enabling us to probe the “habitable zone” of Sun-like protostars, the region where Earth-like planets or the moons of gas giants are most likely to have environments favorable for the development of life.
Recent discoveries have shown that gas giant planets (similar to Jupiter) are common around other stars like the Sun, though there is no direct evidence yet for potentially habitable, small, rocky planets like Earth. However, they probably exist. Remote sensing of young stars shows they are surrounded by dusty disks that contain the materials needed to form Earth-like planets. By observing the process of planet building, the SKA will tell us how Earth-like planets are formed. In addition, the SKA offers the possibility of detecting radio transmissions that would provide evidence for intelligent life among the stars.
While the planets in the habitable zone of Sun-like protostars are far too small to be detected directly, the dust from which the planets form has a lot of surface area that intercepts starlight and converts the energy into heat that can be detected at short radio wavelengths. The SKA will image the thermal emission from dust in the habitable zone in unprecedented detail. In particular, the SKA will show where dust evolves from micron sized insterstellar particles to centimeter sized “pebbles”, the first step in assembling Earth-like planets.
Indirect observations show that gas giant planets are surprisingly common in the habitable zone around other stars, unlike in our Solar System. This surprising discovery raises many questions: What accounts for the diversity in planetary systems? Are terrestrial planets common in the habitable zone? Do gas giant planets form in the inner disk or do they migrate there? What are the implications for Earth-like planets? The SKA will image features in disks related to planet formation. The presence of giant protoplanets can open up nearly empty gaps in the disk material, revealing their presence, and they may also drive large scale spiral waves through the disk. Because orbital times in the inner disk are short, just a few years, observations made over time can track the evolution of these features. Giant planets may form by the slow growth of dust grains into large rocks that capture gas, or by rapid gravitational instabilities that disrupt the surrounding disk. The SKA will discern which mechanisms are active, and where in the disk they occur, which will reveal the impact of newborn giant planets on their Earth-like counterparts. When viewed from afar, the signatures of forming planets imprinted on circumstellar dust may be the most conspicuous evidence of their presence. The gaps in the dust clouds are much easier to detect than the planets themselves because of their much larger surface area. It’s akin to seeing the wake of a boat from an airplane when the boat itself is too small to be visible. The SKA may be the only instrument capable of imaging the inner regions of disks where Earth-like planets might be forming.
What about signals from an technologically advanced extraterrestrial civilisation? The SKA will be so sensitive that that it will be able to detect signals comparable in strength to television transmitters
operating on planets around the closest stars to the Sun. The SKA will be able to search for “leakage” signals from other civilisations for the first time. And it will expand the volume of the Galaxy that can be searched for intentional beacons by a factor of 1000, using a wider range of frequencies than attempted before. The detection of such extraterrestrial signals would forever change the perception of humanity in the Universe.
Find out more
Cradle of Life T. J. W. Lazio, J.C. Tarter, D.J. Wilner – in Science with the Square Kilometre Array, 2004.
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