Great Balls of Fire

It's a brand new year. That's one year less from the billions of years—about seven billion, in fact—before the inevitable happens: our Sun will evolve onto the asymptotic giant branch (AGB) region of the Hertzsprung-Russel Diagram, the final stage of its red gianthood ere the ultimate dissolution into a planetary nebula and lapse into a white dwarf star-cinder.

Currently, our Sun is a placid yellow dwarf whose gentle radiations allow life to exist on this little planet of ours. But what happens when it evolves?

"[Six] billion years from now, the sun will have grown into a red giant star, more than a hundred times larger than its current size." This is the rosy outlook for the Solar System's future, according to author Leen Decin from the KU Leuven Institute of Astronomy. Their study of the system L2 Puppis—considered to be a close analogue for what the Solar System will look like in the distant future—was published in the journal Astronomy & Astrophysics.

"It will also experience an intense mass loss through a very strong stellar wind. The end product of its evolution, 7 billion years from now, will be a tiny white dwarf star. This will be about the size of the Earth, but much heavier: one tea spoon of white dwarf material weighs about 5 tons," he added.

The magnificent Sun—witness to the countless aeons of Life's struggle on the Earth—will be reduced to an incredibly bizarre stellar cinder supported only by electron degeneracy pressure; examples of such exotic objects abound in the Solar Neighborhood, from Sirius B to van Maanen's Star.

Credits: P. Kervella et al. (CNRS / U. de Chile / Observatoire de Paris / LESIA / ESO / ALMA)

And it was this spectacular transitional phase that researchers wanted to view using the world's largest radio telescope. Of course, we can't wait around for six billion years—but the science of astronomy has always been capable of achieving a kind of time travel, and this is just what the researchers did. Using the ALMA radio telescope, an international team of astronomers decided to study an older, neighboring star whose properties and age make it a likely far-future solar analogue.

What they discovered gives us an idea of what our Solar System will look like when the Sun begins its terminal evolution.

Chances for Survival?

The team observed an evolved star called L2 Puppis, which is about 208 light-years from the Earth. About five billion years ago, this star was closely identical to our own Sun, with about the same mass and physical characteristics.

"We discovered that L2 Puppis is about 10 billion years old," explained co-author Ward Homan. "One third of this mass was lost during the evolution of the star. The same will happen with our sun in the very distant future."

Without question, Mercury and Venus are bound to be swallowed when the Sun begins its expansion. "But the fate of the Earth is still uncertain," Decin said. "We already know that our sun will be bigger and brighter, so that it will probably destroy any form of life on our planet. But will the Earth's rocky core survive the red giant phase and continue orbiting the white dwarf?"

Credits: P. Kervella et al. (CNRS / U. de Chile / Observatoire de Paris / LESIA / ESO / ALMA)

The answer to that all-important question may lie in an object the researchers detected orbiting L2 Puppis at a distance of about 300 million km—about twice the Earth's distance from the Sun. In all likelihood, this object is a planet or low-mass brown dwarf which has managed to remain intact even after the star's metamorphosis; and it may exhibit a phenomenon that astronomers have noted in the few planetary objects found around red giant stars—a kind of "gigantism" that might be caused by the planet's accretion of the outflowing solar wind. Furthermore, the planet may once have orbited closer to the star, since orbits have a tendency to loosen up as the red giant's enter their mass-loss phase.

However the case, there is much to learn about this planet's interactions with L2 Puppis before we can have substantial information as to how our Sun's transformation will affect the Earth and the rest of the Solar System.

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