Red giant stars have always held a fascination for me, doubtless spurred by an early reading of H.G. Wells’ The Time Machine. Who can forget the time traveler’s journey far into the future after his desperate escape from the Morlocks, millions of days passing in seconds as he flees:

So I travelled, stopping ever and again, in great strides of a thousand years or more, drawn on by the mystery of the earth’s fate, watching with a strange fascination the sun grow larger and duller in the westward sky, and the life of the old earth ebb away. At last, more than thirty million years hence, the huge red-hot dome of the sun had come to obscure nearly a tenth part of the darkling heavens.

We can forgive Wells the mistaken timing — thirty million years won’t account for this! — but still revel in the beauty of the concept. How it must have resonated at the end of the 19th Century. Today, red giants seem a bit more familiar as we’ve learned more about how they happen. And we do know that in perhaps two billion years, the Earth will become uninhabitable as our Sun swells toward the red giant status that awaits it in perhaps five billion years. At that point, the inner planets — Earth included — will probably be swallowed into the furnace.

Would that be the end of subsequent life? We’d like to know more, and the discovery of the tenth exoplanet found around a red giant may play a role in helping us answer such questions. For the more we learn about red giant planets, the better we’ll understand what happens to solar systems after the stellar scenario has changed. Habitable zones move outward as stars move to red giant status, and Alex Wolszczan, a key player on the discovery team of the new planet, thinks this can lead to interesting results even in our own distant future:

“In our solar system, places like Europa — a satellite of Jupiter that now is covered by a thick layer of water ice — might warm up enough to support life for more than a billion years or so, over the time when our Sun begins to evolve into a red giant, making life on Earth impossible.”

Perhaps life gets a second chance. But the planet Wolszczan and colleagues have found using data from the Hobby-Eberly Telescope doesn’t seem to be much of a candidate for habitability. It orbits the K0-giant HD 17092, 300 light years from Earth in the constellation Perseus. The star, roughly twice as massive as the Sun and ten times its size, is circled by its planet every 360 days. The discovery paper estimates the planet’s minimum mass at 4.6 times that of Jupiter.

What we need to do, of course, is expand the sample, and on that score, Wolszczan’s team have compiled a catalog of nearly a thousand giant stars that may host planetary systems. Radial-velocity methods take time to accumulate their data, particularly when dealing with red giants whose planets can take years to complete a single orbit. But three years of work are complete on over 300 stars, making it likely that the new planet is just the first of many we’ll be discussing in subsequent days.

All of which is useful as we try to understand planet formation. Note this interesting passage from the discovery paper (internal references deleted for brevity). It looks at the two major planet formation models — core accretion and disk instability — in light of red giant studies:

Current constraints on a stellar mass dependence of the disk mass and the timescale of depletion of its gas and dust components come from studies of disks around young stars. For example, in addition to the previous work, recent Spitzer observations appear to con?rm that disks around intermediate and high-mass stars have lifetimes signi?cantly shorter than 5 Myr. These results have direct consequences for the competing theories of giant planet formation, because the core accumulation scenarios require at least a few million years for a core to form, whereas planet formation from a disk instability can be very short. Clearly, the searches for planets around giant stars have a unique capability to provide the statistics which are needed to decisively constrain the efficiency of planet formation as a function of stellar mass and chemical composition.

These studies thus complement work on other stellar classes and begin to give us an idea of how planetary systems change over time. Their glimpse of our own Solar System’s future may not be as exotic as Wells’, but they should help us piece together a few more parts of the puzzle of planet formation. The paper is Niedzielski et al., “A Planetary Mass Companion to the K0 Giant HD 17092,” to be published in the Astrophysical Journal (preprint available).