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“Ping Pong” Planets

By David Reneke

News from the space and astronomy communities around the world.

A gravitational tug of war more than a million years old could be in play between nearby binary stars. It’s generally accepted that planets can be ejected from solar systems, especially during the dynamic and unstable period early in the solar system’s life where planets are jostling for the most stable orbits.

Astronomers have recently found exoplanets in double and even triple star systems, and in a new study, Nickolas Moeckel and Dimitri Veras of the Institute of Astronomy, Cambridge, found that a planet ejected from orbit around one star could find itself passed onto its binary companion.

It’s more than a good bet that the planet may then find itself “bounced” between the stars, providing one possible explanation for the eccentric orbits of some exoplanets. Taken to the extreme, planets could be kicked out of one star’s clutches only to be sent careening around, or into, its binary companion.

Moeckel feels there are two key prerequisites for this phenomenon. “First, the stars need to be on a wide enough orbit so that one is not interfering with planet formation around the other. Second, there needs to be multiple planets around one of the stars,” he said.

Researchers are in agreement on one thing: the orbital instability that leads to the bouncing phenomenon requires that two planets come in close proximity to each other. In fact, they say it speeds up the process.

Advanced computer modelling shows the bouncing effect typically occurs for stars separated by between six and 25 times the distance between the Sun and Pluto. Up to 85% of the ejected planets will make at least one journey through the companion star’s space and more than half will bounce repeatedly, depending on the initial orbital parameters of both systems.

As Moeckel and the team determined, a planet that begins bouncing will very rarely, if ever, settle back into its original planetary system. Once the bouncing begins, the planet will eventually end up leaving the binary system.

This planetary ping pong game lasts for up to one million years and could result in the stars swapping their planets. Planets with such wildly erratic orbits would be inhabitable, with searing temperatures as it passes by the host star and super-cold temperatures as it swings back out into space.

Is this typical stellar behaviour for binary star systems? We just don‘t know as there’s not enough data available yet, but with more intended research there soon will be.

Voyager One Nears Edge of Interstellar Space
When NASA first envisioned the Voyager space probes in 1972, Ed Stone was there to help design them, and he was there when Voyager One launched in 1977.

Thirty five years later, this emissary from Earth has reached the outermost edge of the solar system, and is still sending back signals. As the Voyager Program’s chief scientist, Stone is still listening.

Voyager One is now about 18 billion km from Earth. It is the most distant object ever sent out by the human race. It is fast closing in on true interstellar space but remarkably still inside the huge bubble that the Sun creates around itself called the “heliosphere”.

Outside of that bubble, the space probe will pass by material from stars that exploded 5–10 million years ago. “We just hope that happens while we still have enough electrical power for it to send the data back to Earth,” Stone said

For the past 34 years, the probe has transmitted data back to NASA 24 hours per day. Mission specialists like Stone believe that around 2025 there will no longer be enough power for the spacecraft to keep transmitting. Voyager One will become a silent sentinel in the blackness of interstellar space.

It takes about 16.5 hours for a radio message to reach us here. Voyager is currently sending back data on the stellar “superwind” – the 1.5 million km/h stream that blows out from the Sun.

If Stone had to pick one scientific discovery as the most significant of the many uncovered by the Voyager spacecraft, it would be the eight volcanoes on the moon Io identified during the first flyby of Jupiter. A chance discovery, it hinted that the solar system was alive and active, not cold and inert as presumed.

According to Stone, Voyager One will continue moving through our galaxy forever as it travels on a course to the constellation Ophiuchus. “It’ll take 40,000 years to pass another star,” he said. “Perhaps someday we’ll get a radio signal back saying: ‘Guess what I found?’”

David Reneke is an astronomy lecturer and teacher, a feature writer for major Australian newspapers and magazines, and a science correspondent for ABC and commercial radio. Subscribe to David’s free Astro-Space newsletter at