Hot Jupiters More Nuanced Than Expected
May 7, 2013

Hot Jupiters Are Tricky Little Exoplanets

April Flowers for - Your Universe Online

In addition to the eight beloved planets circling our Sun, our galaxy is literally teeming with an enormous variety of planets. More than 800 of the so-called exoplanets have been identified circling stars beyond our solar system. And several different “species” of exoplanets have been discovered as well, including so-called ℠roasters´ or ℠hot Jupiters,´ which are gas giants like our own Jupiter that orbit closely to their stars, blistering under the heat.

Researchers are using NASA´s Spitzer Space Telescope to begin the process of dissecting this exotic class of planets. The observations have revealed raging winds and other telltale signs of planetary turbulence — with one surprising twist. The hot Jupiters are not all alike and appear to actually have a wide range of climates. For example, some of the exoplanets are covered in a haze, while others are clear. Temperature profiles, chemistries and densities differ along a wide range as well.

"The hot Jupiters are beasts to handle. They are not fitting neatly into our models and are more diverse than we thought," said Nikole Lewis of the Massachusetts Institute of Technology, Cambridge. "We are just starting to put together the puzzle pieces of what's happening with these planets, and we still don't know what the final picture will be."

Lewis and colleagues have published an article in the Astrophysical Journal detailing the use of the Spitzer telescope to examine one such hot Jupiter, HAT-P-2b.

The very first exoplanet ever discovered was 51 Pegasi b — itself a hot Jupiter — detected in 1955 by Swiss astronomers using a technique called radial velocity, which measures the wobble of a star caused by the tug of a planet. Hot Jupiters tend to be heavy and whip around their stars quickly, making this the easiest technique for identifying them. After 51 Pegasi b, dozens of hot Jupiter discoveries followed, leading researchers to think they might represent a fairly common configuration for other planetary systems beyond our own solar system. New research, however, including data from NASA´s Kepler space telescope, has shown that hot Jupiters are actually relatively rare.

When Spitzer became the first telescope to detect light emitted by an exoplanet in 2005, scientists were excited. As the planet — a hot Jupiter — disappeared behind its star in an event known as a secondary eclipse, Spitzer monitored the infrared light emitted by both the planet and the star. Like the radial velocity technique, monitoring the infrared light works best for hot Jupiters because they are the biggest and hottest of the exoplanets.

Researchers have also used Spitzer to monitor the planets as they complete their orbits around the star, allowing them to create global climate maps. These maps reveal how the planets´ atmospheres vary from their hot, sun-facing sides to their cooler, night sides. This variance is due in part to fierce winds. Many hot Jupiters are tidally locked, with one side always facing the star, much as our Moon is locked to Earth.

Spitzer has probed the atmospheres of dozens of hot Jupiters since that first observation, along with even smaller planets, uncovering clues about their composition and climate.

"When Spitzer launched in 2003, we had no idea it would prove to be a giant in the field of exoplanet science," said Michael Werner, the Spitzer project scientist at NASA's Jet Propulsion Laboratory (JPL). "Now, we're moving farther into the field of comparative planetary science, where we can look at these objects as a class, and not just as individuals."

Lewis and colleagues made the longest-ever Spitzer observation of a hot Jupiter, training the infrared telescope on the HAT-P-2 system for six full days. The exoplanet crossed in front of its star, slipped behind it, and then reappeared on the other side, making one full orbit. The exoplanet has a comet-like eccentric orbit, which carries it as close as 2.8 million miles to the star and out to as far as 9.3 million miles.“¯To give a sense of scale, Mercury is approximately 28.5 million miles from our sun.

"It's as if nature has given us a perfect lab experiment with this system," said Heather Knutson, from the California Institute of Technology. "Because the planet's distance to the sun changes, we can watch how fast it takes to heat up and cool down. It's as though we're turning the heat knob up on our planet and watching what happens." In 2007, Knutson led the first team to create a global "weather" map of a hot Jupiter, called HD 189733 b.

Enabling the scientists to peer down into different layers of the planet, the HAT-P-2b study is one of the first to use multiple wavelengths of infrared light while watching a full hot Jupiter orbit.

The study demonstrates that HAT-P-2b takes about one Earth day to heat up as it approaches the hottest part of its orbit. Another four to five days of cooling are required as it swings away. When it is closest to its star, the planet also exhibits a heat inversion — a hotter, upper layer of gas. Astronomers are still trying to understand the data on the carbon chemistry of the planet, which seems to behave in unexpected ways.

"These planets are much hotter and more dynamic than our own Jupiter, which is sluggish by comparison. Strong winds are churning material up from below, and the chemistry is always changing," said Lewis.

Another challenge facing the scientists studying hot Jupiters lies in sorting through the data. The six-day observation of HAT-P-2b left the team with 2 million data points to map out while they try to remove instrument noise.

"Theories are being shot down right and left," said Nick Cowan of Northwestern University. "Right now, it's like the wild, wild west."