Pick up a science textbook and chances are it will say the solar system includes everything that orbits the sun, from planets and their retinues of moons to tiny scraps of icy debris.
That much is taken for granted. Yet mention is unlikely to be made of the controversy over where our familiar neck of the celestial woods ends – and the dark and distant expanse of interstellar space begins.
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Searching for the 'Oort cloud'
For a time scientists believed the outermost planet’s orbit formed the limit of our solar system, but a growing number of experts now believe that it might extend much further out.
Astrophysicists say the solar system's size depends on where the boundary is set.
That limit has been pushed out ever since Italian astronomer Galileo Galilei's first telescope revealed a world beyond the world then imagined. In a sense, therefore, the solar system has been burgeoning in size – at least in terms of human awareness – as astronomical technologies, and analytical techniques, have improved.
For a time, it was considered that the outermost planet's orbit formed the limit. Since the demotion of Pluto to dwarf planet status, that has been the cool blue gas giant Neptune. By this definition, our solar system would be roughly 9 billion kilometres across.
Scientists today are not satisfied with this reasoning. Instead, many pundits reckon the boundary lies at the outer edge of the heliosphere, where the sun's sphere of influence, through the solar wind, peters out. That gives a diameter of more than 27 billion kilometres.
A growing number of experts now believe it might extend much further out – for more than two light years, to an unimaginably remote region where the sun's gravity is no longer felt. This is the outer edge of what's known as the Oort cloud, a vast spherical collection of icy debris believed to have been ejected by gravitational interactions with the giant planets.
"Gravity is a very long-range force and is responsible for the sun's most far-flung influence," says Greg Poole, a Canadian astrophysicist, now at Melbourne University, and a star of Australia's first IMAX 3D movie, Hidden Universe. "The outer edge of the Oort cloud represents the point where the sun's pull gives way to that of the other stars in its neighbourhood," Dr Poole explains.
The cloud, he says, supposedly encircles the sun and planets, though exactly where it ends is far from certain. If indeed the hypothetical cloud exists, and is considered part of the solar system, that would yield a diameter of perhaps more than four light years or roughly 40 trillion kilometres.
"There is disagreement about the outer edge of the Oort cloud, but most estimates suggest it is something like 50,000 to 100,000 astronomical units away – in other words, one or two light years from the sun," says CSIRO astrophysicist Kurt Liffman.
Melbourne University astrophysicist Alan Duffy agrees. The outer edge of the Oort cloud, he says, is probably the true boundary. "This is a dark and frozen zone filled with the leftover debris of early processes that formed the planets," Dr Duffy says.
Probing the limit
For those believing that the heliosphere represents the outer limit, NASA's Voyager 1 spacecraft recently became the first man-made object to have left the solar system.
At 120 times further from the sun than Earth is, the lonesome probe, travelling at a tad under 18 kilometres a second, is at a distance that would take the average highway motorist driving at breakneck speed a staggering 20,000 years to reach.
With limited nuclear battery power left, Voyager 1 will most likely shut down within the next decade or so. "All the same, it will sail on, finally approaching our neighbouring stars in 40,000 years," Dr Duffy says.
Whether or not it's deemed to have left the solar system, most scientists are in awe of the intrepid probe.
"Voyager 1 is now truly on a par with Earth's greatest human explorers," says Swinburne University astronomer Chris Fluke. "The hard-working scientists and engineers who designed this spacecraft deserve to be congratulated for this amazing achievement – our first step beyond the solar system and towards the stars."
The US space agency's Pioneer 10 and 11 probes, which set off in 1972 and 1973 respectively, were also the first man-made objects to start the long-haul mission of leaving the solar system.
In 1977, NASA launched its better-equipped Voyager probes, both of which flew by Jupiter and Saturn; Voyager 2 also explored Uranus and Neptune.
The Pioneers and Voyagers are all on one-way journeys out of the solar system.
They will eventually by joined by another robot brainchild of NASA's Jet Propulsion Laboratory in California –
the New Horizons spacecraft, which launched in 2006 and is scheduled to farewell the solar system after paying a flying visit to one of the sentinels of the outer solar system, Pluto, and its five known mini-moons, Charon, Nix, Hydra, Kerberos and Styx.
Beyond our world
When Voyager 1 eventually makes it through the Oort cloud, it will enter what is called the interstellar medium. As the name suggests, this refers to the space between stars.
In popular imagination, this is considered to be empty space. But it is not.
"The interstellar medium consists mainly of hydrogen gas with a very low density of roughly one hydrogen molecule per cubic centimetre," Dr Liffman says.
"The medium’s temperature can vary dramatically," he says. "In low-density regions, temperatures can reach 10,000 degrees or higher. In the coldest areas, temperatures are of the order of minus 260 degrees on average. These colder regions usually have much higher gas densities, with molecules packed together much closer.”
In these super-hot zones, the gas is also ionised in the form of a highly conducting plasma. "In such a medium, magnetic fields play a major role in shaping the gas distribution and the motion of high-energy particles, such as cosmic rays," Dr Liffman says.
At midday today, or 1pm tomorrow, watch the latest IMAX Melbourne movie Hidden Universe 3D. Narrated by Golden Globe winner Miranda Richardson, the movie was produced in association with Film Victoria, Swinburne University and the European Southern Observatory. Details: imaxmelbourne.com.au/movie/hidden-universe-3d
Discover the intricacies of the heliosphere at: web.mit.edu/space/www/helio.review/axford.suess.html
Enter the mysterious realm of the ultra-distant Oort Cloud at: solarsystem.nasa.gov/planets/profile.cfm?Object=KBOs
Explore the solar system in all its glory at: http://solarsystem.nasa.gov/planets/profile.cfm?
Check out NASA's plans for the ambitious New Horizons probe at: http://pluto.jhuapl.edu/
Learn more about the solar system, space travel and gravity in the Australian curriculum textbook Oxford Big Ideas Science 7, chapter 5 (Oxford University Press, 2011)
VCE Astrophysics – Detailed study 3.2 for Units 1 and/or 2 Physics – accessed at VCE Physics website above, p21.
F-10 Physics (sub-strand "physical sciences" in AusVELS): ausvels.vcaa.vic.edu.au/Science/Curriculum/F-10
F-10 Astronomy ("Earth and space sciences" sub-strand in AusVELS): ausvels.vcaa.vic.edu.au/Science/Curriculum/F-10
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