Unravelling a rotational riddle

Credit: NASA/NOAO/ESA/the Hubble Helix Nebula Team/M. Meixner (STScI and T.A. Rector (NRAO)

Hot gas ejected from the Helix Nebula as imaged by Hubble in 2003

Discoveries of new worlds orbiting other stars in our Galaxy (exoplanets) have been pouring in over the past 16 years. Many of the earliest detections were massive gas giants, so-called ‘Hot Jupiters’, orbiting scorchingly closely to their stars. This is known to be simply an observational bias – large, tightly-orbiting planets are easiest to spot – and over time smaller planets in wider orbits have been found. However, one mystery has remained. A substantial percentage of the Hot Jupiters seen transiting across their star have a misaligned orbit, ie they orbit around their star in the opposite direction to the star’s rotational spin. This is contrary to what you’d expect from the standard model of planet formation: worlds condensing out of a disc of gas and dust swirling around the nascent star.

In a new paper, Prof Shlomo Dado and his colleagues in Israel, have proposed a new theory that may explain these puzzling planets as well as another long-running galactic mystery. They argue that these misaligned planets are interstellar migrants – worlds that have formed elsewhere and been captured into reverse orbit by the star’s gravity. Dado draws attention to the clumps of gas that are seen being ejected at high speeds from supernovae or planetary nebulae. These gas blobs, he says, cool and decelerate by gathering interstellar gas and dust, then gravitationally contract into giant planets. These may then be captured by a host star to yield the misaligned planets. Or, gravitational interaction of a passing gas blob may perturb a young planetary system and kick native planets into misaligned orbits.

Dado calculates that the number of these gas blobs ejected by, for example, the Cassiopeia A supernova remnant is over a thousand, and more than 40,000 launched from a planetary nebula like the Helix Nebula. If these are representative of supernovae and planetary nebulae in general, then the number of free-floating, Jupiter-mass objects may be a hundred times higher than the number of stars in the Galaxy. If this is indeed true, then Dado’s gas blobs might not only explain the observations of misaligned alien planets, but also account for a large proportion of the missing baryons, or ‘dark matter’, in galaxies like our own.

This bold hypothesis will certainly cause controversy. A single theory that explains both unusual extrasolar planetary systems and the
missing mass of the Galaxy seems too good to be true. But the history of science is punctuated by radical ideas. Following proper scientific methodology, Dado and his colleagues have proposed a novel hypothesis as well as two different means by which it can be tested observationally. Free-floating gas giants ought to be detectable in interstellar space by microlensing surveys and perhaps even infrared telescopes if they are still hot from their formation.

This article first appeared in the June 2011 issue of Sky at Night Magazine.

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