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Stars' tilted disks clarify odd orbits: mismatched planet nurseries may shape trajectories.

Exoplanet orbits come in all shapes and sizes, and a pair of nearby stars is helping astronomers figure out why. Cockeyed disks of gas and dust that give rise to planets circle the two stars, new observations show. The disks' misalignment might nudge future planets into off-kilter orbits. Researchers hope that the discovery is the first step toward understanding the origin of the galaxy's diverse planetary arrangements.

Compared with some planetary systems, the solar system is pretty dull. Earth and its siblings travel along nearly circular orbits in almost the same plane. A handful of planets around other stars are a bit more free-spirited. Some orbit at angles that are highly inclined relative to their star's equator; some plunge in close to their sun before flying out to the backwaters of their system; still others carry the mass of Jupiter yet whip around their stars in mere days.

One leading explanation of the odd orbits lays the blame on a second, nearby star. This hypothesis works only if the orbit of the second star is tilted relative to the disk in which planets form. Until now astronomers weren't certain if this arrangement was possible.

But the disks around the stars in a binary system located 525 light-years away in the constellation Taurus are misaligned, Eric Jensen of Swarthmore College in Pennsylvania and Rachel Akeson of Caltech report in the July 31 Nature.

The researchers used the Atacama Large Millimeter/submillimeter Array in Chile, or ALMA, to measure light emitted from carbon monoxide gas and microscopic dust grains, which revealed the orientations of the two rotating disks. The discovery implies that disks can form at odd angles relative to a stellar duo, creating conditions that are perfect for shaping the strange orbits of some planets.

Imagine a planet orbiting one of the stars in a pair, Akeson says. That planet senses the gravity of its own star, but it will also feel a nudge from the second star. If the companion star lies above or below the plane of the planet's orbit, gravity will finesse the planet into a path that more closely aligns with the two stars. The planet's orbit will also stretch. If the orbit becomes highly stretched, the host star can drag the planet into a much smaller orbit, which could explain the origin of Jupiter-mass planets with tiny orbits.

The observations demonstrate that the binary-nudging mechanism can work, "but this can't be the whole story," Jensen says. There are many planets with eccentric, inclined orbits and many may have unseen binary companions--but probably not all of them. Interactions between planets might also scatter orbits.

Previously, researchers could get only limited information about disk orientation, which prevented them from testing ideas about the formation of both planets and binary stars, notes astronomer Andrew Skemer of the University of Arizona in Tucson. Obtaining the orientation of both disks is "completely new," he says. It was possible because ALMA is a sensitive telescope that can probe long wavelengths of light in which the disks appear much brighter.

Caption: If two nearby stars have misaligned disks of gas and dust, as seen in this artist's illustration, then any planets that form in those disks may have unusual orbits. New observations confirm that slanted star pairs do exist.


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Title Annotation:ATOM & COSMOS
Author:Crockett, Christopher
Publication:Science News
Date:Sep 6, 2014
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