How the solar system’s tail disappeared

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.