How the solar system’s tail disappeared
K. Dialynas et al/Nature Astronomy 2017
The solar system doesn’t have a long, twisted tail after all.
The sun and all its planets are surrounded by a bubble of particles. For more than 50 years, many scientists had thought the bubble was comet-shaped, says Tom Krimigis. By that he meant it would have a round head trailed by a long tail. But this space physicist at the Johns Hopkins Applied Physics Laboratory in Laurel, Md., is part of a team that now reports this bubble has no tail. It instead appears to be shaped like a sphere.
“You can’t really argue with the new result,” says Merav Opher. She is an astronomer at Boston University in Massachusetts. She was not involved in the new study. “The data," she says, "loudly say that there is no tail.”
That bubble is called the heliosphere (HE-lee-oh-sfear). It comes from helios, the Greek word for sun. This sphere is inflated by particles that shoot out from the sun and envelop the entire solar system. Its shape is important because it provides clues to how the solar system interacts with interstellar space. That’s the region between the stars, out well beyond our solar system.
Astronomers have puzzled about the heliosphere’s shape for a long time. In the 1960s, some of them proposed that it was shaped either like a comet — rounded on one side with a trailing tail — or a sphere. Magnetic fields surrounding the sun and the planets look sort of like comets. They have long tails extending behind them. Some scientists therefore speculated that the heliosphere would likely have a tail, too.
And in 2013, new data showed signs of a tail. A space probe called the Interstellar Boundary Explorer, or IBEX, counted fast-moving atoms near the edge of the solar system. These speedy atoms are thought to be kicked back into the solar system when they collide with the edge of the heliosphere. By mapping those atoms, IBEX suggested that the solar system had a long, twisted tail that looked like a four-leaf clover.
But it wasn’t clear from the data exactly how far the heliosphere’s tail might extend, Krimigis says. He and his colleagues sought a clearer picture. They studied more than a decade’s worth of data from two other spacecraft. The Voyager and Cassini probes have explored the planets and outer reaches of our solar system. Krimigis’ team tracked how many speedy atoms there were in different parts of the heliosphere. The intensity of charged particles streaming from the sun — what’s known as the solar wind — changed in different regions, too.
The researchers compared the number of fast atoms to the strength of the solar wind. Voyager probes sit at the front of the heliosphere. Here, when the intensity of the solar wind decreased, so did the abundance of speedy atoms. When the wind increased, the flux of passing atoms went up too. Data from the Cassini probe showed the same changes at the back of the heliosphere.
Krimigis says this means there is no tail. A long tail would cause the number of atoms to change differently in response to the solar wind. Atoms would have farther to travel in a tail, he says. So it would take longer for them to build up again. Krimigis and his colleagues reported their findings online April 24 in Nature Astronomy.
However, the matter isn’t completely settled, notes Opher at Boston University. The new observations support a spherical heliosphere. But recent computer models suggest that its shape might actually be more exotic. The bubble’s shape might in fact resemble a croissant, she says. These models add in data from Voyager 1. They show that the interaction of the magnetic fields from the sun and interstellar space could squish the solar wind into two jets. Those jets might look like two short tails.
No such thing has been observed yet. But if it is, Opher says, stubby tails could help astronomers understand other sets of jets seen in the universe. Such jets might shoot out from young stars or possibly from black holes.
(for more about Power Words, click here)
astronomy The area of science that deals with celestial objects, space and the physical universe. People who work in this field are called astronomers.
atom The basic unit of a chemical element. Atoms are made up of a dense nucleus that contains positively charged protons and uncharged neutrons. The nucleus is orbited by a cloud of negatively charged electrons.
black hole A region of space having a gravitational field so intense that no matter or radiation (including light) can escape.
Cassini A space probe sent by NASA to explore the planet Saturn. Cassini was launched from Earth in 1997. It reached Saturn in late 2004. The craft included a variety of instruments meant to study Saturn’s moons, rings, magnetic field and atmosphere.
colleague Someone who works with another; a co-worker or team member.
comet A celestial object consisting of a nucleus of ice and dust. When a comet passes near the sun, gas and dust vaporize off the comet’s surface, creating its trailing “tail.”
computer model A program that runs on a computer that creates a model, or simulation, of a real-world feature, phenomenon or event.
exotic An adjective to describe something that is highly unusual, strange or foreign (such as exotic plants).
field (in physics) A region in space where certain physical effects operate, such as magnetism (created by a magnetic field), gravity (by a gravitational field), mass (by a Higgs field) or electricity (by an electrical field).
heliopause A term for the outer edge of the solar system’s heliosphere.
heliosphere The region of space, encompassing the solar system, in which the solar wind has a significant influence.
interstellar Between stars.
magnetic field An area of influence created by certain materials, called magnets, or by the movement of electric charges.
particle A minute amount of something.
physics The scientific study of the nature and properties of matter and energy. Classical physics is an explanation of the nature and properties of matter and energy that relies on descriptions such as Newton’s laws of motion. A scientist who works in such areas is known as a physicist.
planet A celestial object that orbits a star, is big enough for gravity to have squashed it into a roundish ball and has cleared other objects out of the way in its orbital neighborhood. To accomplish the third feat, the object must be big enough to have pulled neighboring objects into the planet itself or to have slung them around the planet and off into outer space.
solar system The eight major planets and their moons in orbit around our sun, together with smaller bodies in the form of dwarf planets, asteroids, meteoroids and comets.
solar wind A flow of charged particles (including atomic nuclei) that have been ejected from the surface of the star, such as our sun. It can permeate the solar system. This is called a stellar wind, when from a star other than the sun.
star The basic building block from which galaxies are made. Stars develop when gravity compacts clouds of gas. When they become dense enough to sustain nuclear-fusion reactions, stars will emit light and sometimes other forms of electromagnetic radiation. The sun is our closest star.
sun The star at the center of Earth’s solar system. It’s an average size star about 26,000 light-years from the center of the Milky Way galaxy. Also a term for any sunlike star.
termination shock A term for that point at which the solar wind slows in speed.
universe The entire cosmos: All things that exist throughout space and time. It has been expanding since its formation during an event known as the Big Bang, some 13.8 billion years ago (give or take a few hundred million years).
Voyager spacecraft Two NASA missions to conduct close-up explorations of Jupiter, Saturn, Saturn's rings and the larger moons of the both large planetary gas giants. Despite their names, the Voyager 2 craft launched Aug. 20, 1977; Voyager 1 launched 16 days later. Both are near the edge of the solar system and still flying on into space.