When the Space Age began, astronomers knew of exactly zero planets outside the solar system. What a difference 50 years makes.
Modern, ground-based telescopes and NASA's Kepler spacecraft have
now confirmed more than 850 exoplanets, while thousands more await
confirmation. The pace of discovery suggests "there are at least 100 billion planets in our galaxy," says John Johnson of Caltech, who works with data from the Kepler mission. "That's mind-boggling."
When the hunt for exoplanets began, the focus was on Earth-like
worlds, planets like our own that might support alien life in distant
solar systems. Yet planets as small as Earth are difficult to detect
when they circle stars hundreds of light years away. Indeed, only a
handful have been found so far.
The real haul has been in gas giants, especially “hot Jupiters.”
These are behemoth worlds that orbit close to their parent stars,
blocking a fraction of the star’s light when it transits in front.
Observations of hot Jupiter “mini-eclipses” have yielded hundreds of
discoveries.
At first considered to be the "chaff" researchers would have to
wade through to get to the fainter Earth-like worlds, hot Jupiters are
now attracting their own attention.
Consider the case of "HD189733b," discovered in 2005 by a team
working at the Haute-Provence Observatory in France. Because it is
nearby, only 63 light years away, and because it blocks a whopping 3% of
the light from its orange-dwarf parent star, astronomers are rapidly
learning a great deal.
For one thing, it's blue. Data obtained by the Hubble Space
Telescope suggest that, seen from a distance, the azure disk of HD
189733b would look to the human eye much like Earth. Indeed, some
members of the media have taken to calling it "the other blue planet."
It is, however, anything but Earthlike.
In 2007, Heather Knutson of Caltech made a global temperature map
of HD189733b using NASA’s infrared Spitzer Space Telescope. She knew it
would be hot because HD189733b orbits its star 13 times closer than
Mercury. “Even so, we were impressed by the readings,” she recalls.
Temperatures ranged from 1200 F on the nightside to 1700 F on the
dayside. Thermal gradients drive winds as fast as 6000 mph, carrying
suffocating heat around the globe.
The blue color may be caused by silicate particles in the planet’s
atmosphere, which scatter blue wavelengths of light from the parent
star. The same physics plays out in Earth’s atmosphere, although the
chemicals are different. Silicates are a component of glass, so some
researchers have speculated that it is actually raining molten glass on
HD189733b.
The newest observations come from a pair of X-ray observatories.
NASA’s Chandra and the ESA’s XMM Newton watched HD189733b transit its
star and detected a drop in X-rays three times deeper than the
corresponding decrease in optical light. This means the outer atmosphere
is larger than anyone expected.
In fact, it is probably boiling away. Authors of the study
estimate HD189733b is losing 100 million to 600 million kilograms of
mass per second.
"The extended atmosphere of this planet makes it a bigger target
for high-energy radiation from its star, so more evaporation occurs,"
notes Scott Wolk of the Center for Astrophysics.
Blasts of stellar radiation hitting the planet at point-blank
range could have another effect: auroras that wrap around the planet
from pole to pole, orders of magnitude brighter than any Northern Lights
in our own solar system. This is speculative, though.
While the search for Earth-like planets proceeds, hot Jupiters are
a welcome albeit unexpected diversion. It makes you wonder, what will
we be looking for 50 years from now…?
Credits:
Author: Dr. Tony Phillips | Production editor: Dr. Tony Phillips | Credit: Science@NASA
More information: Big Weather on Hot Jupiters - ScienceCast video
