They’re big, full of gas, and have a penchant for hanging out way too close to their parents. These “hot Jupiters” are among the most common extrasolar planets in the galaxy. Here’s what the latest science is telling us about these celestial wonders.

To date, astronomers have catalogued over 850 planets outside our solar system. This is incredible when you consider that just 20 years ago we had absolutely no conception of what other solar systems might look like. And in fact, prior to these discoveries, we wrongly assumed that our solar system was typical: small, rocky planets on the inside, large gas giants on the outside.

But nothing could be further from the truth. Solar systems come in all sorts of crazy shapes and sizes. If anything, we’re the oddballs.

Image: Artistic impression via Hubble

Scientists first got an inkling of this back in 1995 with the discovery of the very first exoplanet orbiting a sun-like star, 51 Pegasi b, also called Bellerophon. The astronomers were left baffled by the find: A Jupiter-like planet — but in a remarkably close orbit to its parent star. The discovery was so unexpected — and so weird — that astronomers brushed it off as a kind of celestial anomaly.


Selection Effect

But then they started to find others just like it. And another. And another.

The Milky Way, we have learned, is littered with these things, which have since been dubbed “hot Jupiters.”


As astronomers soon realized, however, these discoveries were marred by a kind of observational selection effect. Without question, they were finding so many hot Jupiters because of how they were finding them, namely the transit method of detection in which a planet is spotted when it passes directly over its parent star.

In other words, astronomers were discovering so many hot Jupiters because they’re the easiest type of exoplanet to detect. They’re big — which creates a much more noticeable eclipse effect — and they orbit close to their star — which gives them a short year, so the chances of detection are heightened.


They’re also easy to detect via the radial-velocity method. Because of their tremendous mass, hot Jupiters cause huge oscillations in their parent star’s motion — which scientists can detect from Earth. A shaky sun means a hot Jupiter is likely nearby.

And there’s another reason, really: The fact that there are so many of them.

An Exotic Class of Extrasolar Planet

Today, with nearly 20 years of observations under our belt, we’re starting to learn more about these remarkable extrasolar planets. It turns out they’re not the “chaff” of the galaxy, but an indelible component of many solar systems.


Hot Jupiters are a special class of exoplanet that are similar in size, mass, and composition to Jupiter. But they hang out at stellar distances typically between 0.015 to 0.15 AU (1 AU = average distance from Earth to the Sun).

Image: Artistic impression via Hubble

This is really, really close to the parent star. For comparison, Mercury orbits at 0.39 AU. And in fact, hot Jupiters are so close that their orbits are practically circular.


In terms of their origin and location, the going theory is that these planets formed like “regular” planets in the accretion disc, but migrated into the inner solar system soon afterward. This has to be the case, say scientists, because there simply isn’t enough material in the inner solar system for gas giants to form.

Early on, astronomers felt that hot Jupiters implied a dead, uninhabitable star system. But recent models show that Earth-like planets can form in the habitable zone after a hot Jupiter passes through the ~1 AU zone and that there’s enough material left over for terrestrial planets to form.

Extreme Weather

Hot Jupiters are tidally locked, meaning that they have permanent daysides and nightsides. They have a face that’s perpetually facing the sun — and at an uncomfortably close distance.


Consequently, hot Jupiters experience some of the most extreme weather documented by scientists.

Take WASP-33b, for example. It’s the hottest known planet. It orbits its parent star at a distance that’s only 7% of Mercury’s, giving it a 29 hour year. It’s surface temperature reaches nearly 6,000 degrees Fahrenheit, which is hotter than the surface of some red dwarf stars.


Then there’s HD189733b, a well studied hot Jupiter that was discovered in 2005 by a team working at the Haute-Provence Observatory in France. Located 63 light-years away, it has silicate particles in the atmosphere which gives it its brilliant blue appearance. But silicates are a component of glass, so some astrophysicsts speculate that it actually rains molten glass.

Back in 2007, Heather Knutson of Caltech conducted a study of the same hot Jupiter to determine its atmospheric conditions. Her team learned that HD189733b, which orbits its sun 13 times closer than Mercury, features a dayside temperature of 1,700 degrees F (927 degrees C) and a nightside temperature of 1,200 F (649 degrees C).


Scientists have also studied the weather on hot Jupiter HAT-P-2b — a planet in an eccentric orbit around its sun (it only takes 5 days or so to complete one orbit). According to Nikole Lewis of MIT, its star-facing side features a daytime temperature of 3,860 degrees F (2,127 degrees Celsius), while the night-facing side experiences a temperature of 1,700 degrees F (927 degrees Celsius). So even at night, this planet is ten times hotter than Jupiter. (Image credit: Nikole Lewis/MIT)

The ~1,000 degree difference between the two sides is extreme. It results in vicious winds that howl across the surface at thousands of miles per hour — possibly as fast as 6,000 miles per hour!


The Blow-Out Effect

All hot Jupiters are having their atmospheres blown out on account of the intense solar winds. NASA’s Chandra and the ESA’s XMM Newton recently chronicled HD189733b as it transited its star, detecting a drop in X-rays three times deeper than the corresponding decrease in optical light. This would imply an atmosphere that’s absolutely huge. But it’s boiling away — as much as 100 million to 600 million kilograms of mass per second.


Artistic impression. Credit: NASA

Sometimes, a hot Jupiter will get smashed by an intense stellar flare, which will skyrocket the planet’s rate of evaporation.

Last year, Astronomer Alain Lecavelier des Etangs and his team at the French National Centre for Scientific Research used the Hubble Space Telescope to describe a particularly dramatic blow-out effect, again on HD189733b.


Despite the fact that HD189733b has such a hot atmosphere to begin with, it's still not hot enough to cause the evaporation. Rather, the evaporation is driven by intense X-rays and extreme-ultraviolet radiation from its parent star. It’s getting an X-ray dose 3 million times higher than the Earth. When X-Rays are this intense, they heat the gases in the upper atmosphere to tens of thousands of degrees — hot enough to escape the gravitational pull of the gas giant.

As an aside, terrestrial planets like Mercury are not immune to similar effects. Last year, astronomers using Kepler confirmed the discovery of a short-period planet that requires a mere 15.7 hours to orbit around it — resulting in a dramatic comet-like tail that’s bursting outward from the planet, along with much of the planet’s surface.


Eventually, hot Jupiters will be stripped of their atmospheres, becoming chthonian planets — dead, burning cores in eccentric orbits.


An Aurora for the Ages

Another fascinating effect, this one a bit more theoretical, is the potential for auroras to form on hot Jupiters. According to models, stellar radiation blasts at close proximity should produce auroras that wrap around the planet from pole to pole — auroras that would be considerably brighter than our Northern Lights.

Here on Earth, auroras are caused when particles erupt from the sun and go skidding across the surface of the magnetosphere, creating vast glowing streaks in the sky. A hot jupiter would experience a stronger and more focused blast.

These stellar blasts would weaken the magnetic shield, allowing the particles to reach the atmosphere. The resulting ring would wrap around the equator and exhibit 100 to 1,000 times more energy than Earth’s auroras.


[Supplementary source: NASA]

Top image artistic impression via Hubble.