Gotta See It! Four Planets Directly Imaged In Motion Around The Star HR 8799

Four planets orbit the star HR 8799 in Pegasus. Seven images taken over seven years were combined to create the time lapse. Three of the planets orbit at right and one at upper left. “20 au” is 20 times the Earth’s distance from the sun. Credits: Video making & motion interpolation: Jason Wang Data analysis: Christian Marois Orbit determination: Quinn Konopacky Data Taking: Bruce Macintosh, Travis Barman, Ben Zuckerman

It all happened so fast. The first planet discovered orbiting another star was confirmed in 1991. It was found indirectly by measuring changes in its parent star’s speed toward and away from Earth caused by the planet’s gravity tugging on the star. By the early 2000s, we’d taken the first direct photos of extrasolar planets, which despite their sometimes large size, all appear no larger than stars in images. Today, witness this brief but stunning animation of four super-Jupiter planets orbiting the star HR 8799 with periods ranging from decades to centuries.

I’ve labeled the planets around the star HR 8799. HR 8799e orbits once every ~45 years; D every ~100 years; C every ~190 years and B every ~460 years. The distances from their home star range from 27 to 68 times Earth’s distance from the sun. Credit: Jason Wang and Christian Marois

It took over 7 years using one of the 10-meter (394-inch) Keck telescopes to Wang and team make this all-too-brief time lapse video. The exoplanet system is located 129 light years from Earth in the constellation Pegasus.

HR 8799 is only 30 million years old, very young by stellar standards, and about 1.5 times the sun’s mass and 4.9 times its brilliance. Because it’s so bright and would otherwise overwhelm the light of the much fainter planets, the star has been digitally subtracted from the photos, so we can see the planets clearly.

They’re being dubbed super-Jupiters because each is about 5-7 times more massive that our own solar system’s biggest planet. You’d think being that much more massive, they’d be much bigger than Jupiter, but that’s not how gravity works. Planets up to 80 times Jupiter’s mass remain near its size because gravity’s grip pulls all that mass into a tighter ball. As a result, all four are just 1.2 to 1.3 times the size of Jupiter.

The presence of sodium in the atmosphere of another Jupiter-sized exoplanet around the star HD 209458 is measured by studying its spectrum. Light from the host star filters through the planet’s atmosphere. The element sodium in the atmosphere absorbs the starlight in a predictable way, leaving a narrow gap of “fingerprint” of itself in the rainbow spectrum of the star. Credit: A. Field, STScI

Astronomers have used spectrographs, instruments that spread a planet’s light out into a rainbow of colors called a spectrum, to look for fingerprints of atoms and molecules in each of the four planets’ atmospheres.


This animation, also created by Jason Wang, shows the exoplanet Beta Pictoris b, another super Jupiter, orbiting its parent star.

Planet b shows water in its spectrum along with ammonia, carbon dioxide and smidge of methane. Planet c contains ammonia, possibly some acetylene (a hydrocarbon and used as a fuel) but neither carbon dioxide nor substantial methane; Planet d contains acetylene, methane, and carbon dioxide but no ammonia; Planet e contains methane and acetylene but no ammonia or carbon dioxide. Interestingly, planet e’s spectrum resembles that of our planet Saturn.

At this rate, we’ll soon be looking at the first direct images of the disks of exoplanets and eventually features on those disks. Am I jumping too far ahead? Too optimistic? I’d like to think not!

** Want to know which bright naked eye stars are circled by exoplanets? Pick up a copy of my recently published book, Night Sky with the Naked Eye, at Amazon or Barnes and Noble.  It covers all the great things you can see at night with just your eyeballs. No equipment required!