Why Is Jupiter So Bright? At Its Brilliant Best This Week, The Icy Asteroid That Became A Gas Giant

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An image by citizen scientist Kevin Gill and JunoCam of Jupiter’s Great Red Spot.

NASA/JPL-Caltech/SwRI/MSSS/Kevin M. Gill © CC BY

Jupiter is a giant. It’s the giant. Hence its name. Jupiter, also called Jove, was the god of the sky and thunder, the king of the gods in Roman mythology.

The giant planet (it’s 11 times the size of Earth) that takes its name today reaches “opposition”, which is when Earth is precisely between it and the Sun. Consequently, it shines brightest tonight for the entire year, and for some weeks it will remain a marvelous object to put any small telescope or pair of binoculars on. Tonight it will rise in the east at sunset and set in the west at sunrise. For a few weeks, it will be visible virtually all night. And what a sight!

The paths of the eight major planets as they orbit the Sun, plus the asteroids and comets. The four inner planets are, from inner to outer, Mercury, Venus, Earth and Mars. The four outer planets are, inner to outer, Jupiter, Saturn, Uranus and Neptune.

Getty

It’s the fifth planet in the solar system, but was it always where it is now? How did it get so darn big? At the time of writing, Jupiter has 79 moons, by far the strongest gravitational force of any planet in the solar system, and it even has Saturn-like rings (albeit harder to see). However, research suggests that Jupiter actually started its life as an ice asteroid far from the Sun. Here’s the incredible story of where Jupiter came from, and how it got to where it is now.

This artist’s concept shows planet KELT-9b, an example of a “hot Jupiter,” or a gas giant planet orbiting very close to its parent star. KELT-9b is an extreme example of a hot Jupiter, with dayside temperatures reaching 7,800 degrees Fahrenheit (4,300 Celsius).

NASA/JPL-Caltech

Jupiter’s journey

So-called ”hot Jupiters” are everywhere but our solar system. As any exoplanet hunter will tell you, the cache behind finding a Jupiter-sized planet is minimal; everyone wants to find Earth-sized exoplanets. Hot Jupiters orbit close to their host star, whereas our own planet Jupiter is the fifth planet. “Jupiter was formed a long way from the Sun and then migrated to its current orbit,” says Simona Pirani, doctoral student in astronomy at Lund University, and the lead author of ‘Consequences of planetary migration on the minor bodies of the early solar system’. Advanced computer simulations show that Jupiter was formed four times further from the Sun than its current position indicates.

The Jovian Trojans, asteroids that lap the Sun in the same orbit as Jupiter, are uniformly dark with a hint of burgundy color, and have matte surfaces that reflect little sunlight.

NASA/JPL-Caltech

Jupiter and the Trojans

The scientists traced the migration of Jupiter by studying a group of asteroids thought to represent primordial material that formed the outer planets. Trojan asteroids are two large groups of thousands of asteroids that share Jupiter’s orbit around the Sun. These Trojans orbit the Sun in front of and behind Jupiter, respectively, but there are about 50 percent more in front of the gas giant than behind it. “This asymmetry has always been a mystery in the solar system”, says Anders Johansen, professor of astronomy at Lund University. By recreating Jupiter’s formation and how the planet gradually drew in its Trojan asteroids, the researchers calculated that the asymmetry could only have occurred if Jupiter was formed four times further out in the solar system before migrating to where it is now, drawing in more Trojans in front of it than behind it.

Artist’s concept of Lucy spacecraft at Trojan asteroid.

NASA

From icy asteroid to a gas giant

According to the calculations, Jupiter’s migration went on for around 700,000 years, in a period about two to three million years after Jupiter started its life as an ice asteroid far from the Sun. Pushed inwards by gravitational forces from gases around it, Jupiter’s journey started as a young planet with no gas atmosphere. As it moved inwards, it consumed the Trojan asteroids that probably make-up its core.

This diagram illustrates Lucy’s orbital path. The spacecraft’s path (green) is shown in a frame of reference where Jupiter remains stationary, giving the trajectory its pretzel-like shape. After launch in October 2021, Lucy has two close Earth flybys before encountering its Trojan targets. In the L4 cloud Lucy will fly by (3548) Eurybates (white), (15094) Polymele (pink), (11351) Leucus (red), and (21900) Orus (red) from 2027-2028. After diving past Earth again Lucy will visit the L5 cloud and encounter the (617) Patroclus-Menoetius binary (pink) in 2033. As a bonus, in 2025 on the way to the L4, Lucy flies by a small Main Belt asteroid, (52246) Donaldjohanson (white), named for the discoverer of the Lucy fossil. After flying by the Patroclus-Menoetius binary in 2033, Lucy will continue cycling between the two Trojan clouds every six years.

Southwest Research Institute

Has NASA ever visited the Jovian Trojans?

No, but plans are afoot. In October 2021, NASA’s space probe Lucy will begin a 12-year mission to fly by six Jupiter Trojans to learn more about planetary origins and the formation of the solar system. “We can learn a lot about Jupiter’s core and formation from studying the Trojans”, says Anders Johansen. The authors of the study also suggest that the gas giant Saturn and the ice giants Uranus and Neptune could have migrated in a similar way.

Io, Calisto, Ganymede and Europa, Jupiter’s four largest moons, can be seen with any pair of binoculars.

SkySafari

How to observe Jupiter at opposition

Jupiter is fairly low down in the south (as seen from the northern hemisphere) in the constellation of Ophiuchus, the “forgotten” thirteenth constellation. You can easily make-out its ammonia cloud bands (telescope) and its four largest “Galilean” moons (any pair of binoculars) Io, Calisto, Ganymede and Europa.

It’s frequently possible to see the shadow of Jupiter’s moons upon its clouds.

SkySafari

When to observe Jupiter at opposition

Although its opposition is on June 10, 2019, that represents the middle of the optimum viewing period. So anytime now is good, and throughout the rest of June and July.

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