THE DAILY ASTRONOMER
Monday. April 25, 2016
Renegade Worlds
Even by astronomical standards, this is a wild concept. Imagine a
planet that was once part of a binary star system close to the galactic
nucleus. (Yes, we've determined that binary star systems can harbor
planets.* ) Next imagine a strange scenario in which said binary star
system ventures a wee bit too close to a black hole, not an uncommon
sight (ha ha ha ha ha ha) around the galactic nucleus. When in close
proximity to such a powerhouse stellar remnant, the binary star system
might experience a violent disruption. Not only will the disruption
sever the system, it might well propel the circumbinary planet at
extreme speed away from the wreckage. Or, so goes this freshly baked
scenario from the theoretical astrophysics oven.
The actually term describing these runaway worlds is "hypervelocity
planets." And, not only do such renegade worlds exist, they might be
currently roaring away from the galaxy is a veritable swarm at speeds
that only interstellar protons can surpass. Or, so asserts a paper
slated to appear in the Journal of the Royal Astronomical Society: the
same publication that's given us breathless page-turners such as
"Modeling the flaring activity of the high-x, hard X-ray selected blazar
IGR J22517+2217," and "The non-thermal emission of extended radio
galaxy lobes with curved electron spectra."
The notion of runaway planets arose from the discovery of hypervelocity
stars more than seven years ago.** These stars, like the hypothesized
planets mentioned in the introductory paragraph, were binary components
ejected following their close approach to a black hole.
Astronomers suspect that one star might be captured by the black hole
while the other flees. A planet might either be captured by the
black hole or expelled at high velocity.
At the moment, though, the hypervelocity planet is theoretical, as
none have yet been discovered. That astronomers haven't observed one
yet is hardly surprising. How can one find a planet rushing around
in the dark? At least hypervelocity stars still emit light and are
therefore visible. Hypervelocity planets are invisible. However,
perhaps astronomers might catch a glimpse of one as it passes along a
star's line of sight. For instance, if a star's brightness diminishes
slightly over a brief time period and doesn't experience the
diminishment again, an astronomer might conclude that the occulting
object was one of these breakneck worlds. They could determine the
speed by measuring the brightness diminishment duration. Of course,
this technique is the ultimate long shot.
What would life be like on such a world? Well, the answer is a quick
"impossible." Any planet hurtling through space without the warmth
and light of a parent star would be barren and lifeless. If it had an
atmosphere when it was orbiting a star, that atmosphere would rapidly
freeze out soon after being ejected from its parent stars. That air
freeze-out wouldn't bode well for the life forms dependent on that air,
of course. However, since the
DA is nothing
more than a pretend Universe, we'll assume we can flourish on a
space-cold rock. One advantage is the sky: without a Sun, it
would be perpetually dark. And, if our world is moving at speeds in
excess of 2 million miles per hour, we could actually observe the galaxy
growing smaller over time as our world barrels madly away from it. Of
course, as our distance from the home galaxy increases, we'd slowly lose
our view of its stars. Computer models suggest that these runway
worlds would continue out of the galaxy and beyond. Even if the
planet encounters an intergalactic star (a star between galaxies), it
would likely remain uncaptured. Traveling at such a speed would make
the world extremely difficult to snatch. Perhaps it would
continue its voyage through the Universe always dark , cold and speeding
along at a rapid clip as its exhilarated inhabitants shout "Righteous! Righteous!"
Wild...
*NASA's Kepler Probe discovered Kepler 16-b, the first known
circumbinary planet - the term applied to a planet in orbit around two
stars- in September 2011.
** SDSS J090745.0+024507 was the first hypervelocity star discovered.
Found in 2005 by astronomers at the Harvard-Smithsonian Center for
Astrophysics, SDSS J090745.0+024507 is escaping the Milky Way Galaxy at
2,400,000 km an hour (1,500,000 miles per hour.)