Bizarre Exoplanets Hint at Disturbed Upbringing

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The exoplanets in Upsilon Andromedae are wonky.

For some reason, two of the massive exoplanets orbiting the primary star of the binary system (called Upsilon Andromedae A, a yellow-white dwarf, younger and more massive than our sun) have very tilted orbits, wildly offset from the “normal” orbital plane.

This discovery, announced today, comes after researchers from The University of Texas at Austin McDonald Observatory used a combination of precise observations from the Hubble Space Telescope and ground-based observatories.

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Usually, one can expect planets to orbit their parent star around the same approximate plane. In the case of our solar system, all of the 8 planets (from Mercury to Neptune) orbit in the same direction, in a flat imaginary disk known as the “plane of the ecliptic.” There is little deviation from this plane — except for oddball Mercury, with an orbit tilted by 7 degrees.

Beyond the orbit of Neptune, however, things start to get a little jumbled, where the dwarf planets and Kuiper Belt objects (KBOs) have orbits that are more inclined. Pluto, for example, has an orbital inclination of 17 degrees. These inclined outer solar system orbits are down to the smaller planetary bodies getting bullied by the massive gravity of Neptune.

However, the deviation in orbits around our sun is nothing when compared with the disturbed exoplanets orbiting Upsilon Andromedae A, some 44 light-years from Earth.

There are three known gas giants orbiting this star called Ups And b, c and d. The two outermost exoplanets (c and d) have a significant deviation from the system’s ecliptic plane and they are inclined by 30 degrees from one another.

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“The findings mean that future studies of exoplanetary systems will be more complicated. Astronomers can no longer assume all planets orbit their parent star in a single plane,” says Barbara McArthur, of The University of Texas at Austin McDonald Observatory, who is leading this study using observations from the Hubble Space Telescope and the giant Hobby-Eberly Telescope.

“Most probably Upsilon Andromedae had the same formation process as our own solar system, although there could have been differences in the late formation that seeded this divergent evolution,” McArthur said. “The premise of planetary evolution so far has been that planetary systems form in the disk and remain relatively co-planar, like our own system, but now we have measured a significant angle between these planets that indicates this isn’t always the case.”

In the case of Pluto, as it’s so tiny, the dwarf planet’s orbit has been modified by the presence of Neptune. But in the case of Ups And c and d, McArthur’s team has measured their masses to be huge, approximately 14 and 10 times the mass of Jupiter respectively. Therefore, something rather extreme must have knocked the Upsilon Andromedae A system off-kilter.

What could have disturbed these two exoplanets so much that they meandered off the straight and narrow?

In another study made by McArthur and co., using Hubble data they confirmed Upsilon Andromedae A does indeed have a binary partner (Upsilon Andromedae B), a small red dwarf star located approximately 750 times the sun-Earth distance from its larger sibling. Could this renegade red dwarf have something to do with the exoplanetary turmoil?

Although we know very little about the orbit of the two stars, it could be that Upsilon Andromedae B has a very eccentric orbit, causing it to zoom past the exoplanets every few thousand years, giving them a gravitational “punch” and destabilizing their orbits.

If this is the case, the exoplanets orbiting Upsilon Andromedae A will have pretty regular doomsday events as the red dwarf makes its closest approach, reconfiguring the gas giants’ orbits every time it does so. If this red dwarf can play orbital pinball with these massive gas giants, one has to hope there aren’t any inhabited Earth-like exoplanets in the vicinity… I wouldn’t want to bet on their odds at staying in orbit for very long.

Image credits: NASA, ESA, and A. Feild (STScI)

Source: Hubblesite.org