Massive World Slingshots Around Star

The Cosmic Companion
Aug 28 · 6 min read

A planet several times the mass of Jupiter is seen slingshotting around its parent star, revealing signs of a close encounter in the distant past.

A massive exoplanet, at least three times as massive as Jupiter, has been spotted slingshotting around its parent star. The bizarre, egg-shaped orbit of this world, dubbed HR 5183 b, brings this world as close to its star as the asteroid belt is to the Sun in our own system, and as far out as the orbit of Neptune.

This ancient planetary close encounter may not have have wrecked havoc for this planet, but instead, could have exiled this world far from its star, HR 5183. Planets with highly-elliptical orbits have been discovered in alien solar systems before, but this newly-discovered world is the first one found this far from its parent star.

An artist’s conception of a massive exoplanet orbiting far from its star. Image credit: flflflflfl/Pixabay

“Other planets detected far away from their stars tend to have very low eccentricities, meaning that their orbits are more circular. The fact that this planet has such a high eccentricity speaks to some difference in the way that it either formed or evolved relative to the other planets.” said Sarah Blunt, a graduate student at CalTech.

That was a close one!

The unusual orbit of HR 5183 b suggests the orbit of this world may have been perturbed by a close encounter with another planet with several times the mass of Jupiter, or a star passing through its solar neighborhood.

Close encounters between worlds, as well as collisions, helped to shape alien solar systems, as well as our own. Image credit: Comfreak/Pixabay

“We searched for stellar or planetary companions that could have excited the planet’s eccentricity, but found no candidates, potentially implying that the perturber was ejected from the system,” researchers wrote in The Astronomical Journal.

The most likely explanation for the unusual orbit could be that this world, long ago, had a neighbor with a similar mass. Due to orbital interactions, the neighbor closed in on HR 5183 b, pushing that world into its distinctive egg-shaped orbit before the intrusive neighbor flew off into space.

“This newfound planet basically would have come in like a wrecking ball, knocking anything in its way out of the system,” Howard describes.

Recent research suggests that in our own solar system, long ago, Jupiter may have migrated within the space between worlds.

The team on this new study did identify a nearby star, gravitationally bound to HR 5183. However, the distance between the stars — 15,000 astronomical units (roughly one-quarter of a light year) — makes it unlikely that this star altered the path of the planet.

“HR 5183 b is likely a gas giant planet around the same radius as Jupiter. I’m not sure if it has moons or not; that would definitely be an interesting subject for future study! It’s probably got a pretty wacky atmosphere, since its equilibrium temperature varies more than 120 K [120 C/240 F] as it swings through its orbit,” Blunt speculates for The Cosmic Companion.

Although HR 5183 b may travel in an orbit unlike that traced out by any other exoplanet, eccentric orbits are seen in our own solar system. The orbit of Pluto carries it between 30 and 48 times the distance from the Sun as the orbit of the Earth. And a lesser known dwarf planet, Eris, also follows a similar, egg-shaped path through the heavens.

This highly-elliptical orbit brings HR 5183 b far from its star, where temperatures are much too cold for liquid water to form. If this planet were made of rock, and did not have an atmosphere, temperatures would range from -223 degrees Celsius (-370 Fahrenheit) to -102 C (-151 F), depending on the position of that world, relative to its parent star. The presence of an atmosphere acts as a blanket, warming planets. Still, HR 5183 b is a frigid world, even when it is closest to its sun.

Seeing Stars like Lucy in Hollywood

See how the orbit of HR 5183 b compares in size to our own solar system. Video credit: Caltech

Astronomers have been studying the star HR 5183 since the 1990’s. The star, sitting 102 light years from Earth can be seen, using binoculars or a small telescope, in the constellation of Virgo.

This massive world was found through the radial velocity method, in which planets are detected through the gravitational tug they have over the star they orbit.

These studies usually require astronomers to watch stars as they wobble over the course of an entire orbit of a large planetary companion. For planets with orbits taking decades or centuries to complete, these cycles can prove to be too long-term for astronomers to make useful observations.

The 52 to 117 year-long orbit* of HR 5183 b is too long for most astronomers to easily track, but the California Planet Search is able to watch extra-solar planets over the timescales necessary for worlds like HR 5183 b.

“I’m gonna slingshot around your star — You’ll stretch my gravity too far
You’re gonna spin me off my mark — You’ll bend my molecules too far…”
On, Slingshot

It was this highly-unusual orbit which allowed astronomers to recognize HR 5183 b as a planet.

“This planet spends most of its time loitering in the outer part of its star’s planetary system in this highly eccentric orbit, then it starts to accelerate in and does a slingshot around its star… That creates such a distinctive signature that we can be sure that this is a real planet, even though we haven’t seen a complete orbit,” explains Andrew Howard, professor of astronomy at Caltech.

Astronomers currently know of more than 4,000 confirmed planets orbiting other stars, with thousands of additional signals awaiting confirmation.

The Kepler spacecraft found thousands of exoplanets, opening a Pandora’s box of discoveries of worlds around other stars. Image credit: Screenshot from 4000 Exoplanets video, credit: SYSTEM Sounds (M. Russo, A. Santaguida)

This new finding was made using the two telescopes utilized by the California Planet Search, the Lick Observatory in Northern California and the W. M. Keck Observatory in Hawaii, as well as the McDonald Observatory in Texas.

“Initially, HR 5183 was chosen for long-term monitoring because it is a bright, solar-type star. Bright stars are easy to observe because they don’t require lots of telescope time; we can get all the photons we need to make a radial velocity measurement in a few seconds, while for fainter stars each exposure might take half an hour or more,” Blunt tells The Cosmic Companion.

Bright stars are also easier to observe near dawn and dusk, when light conditions might impede the study of other targets.

As new telescopes grow larger, young astronomers are now able to use older, but still valuable, telescopes to examine the skies, revealing new discoveries. This study shows how great science can still come from classic observatories.

Techniques used to detect HR 5183 b could be used to detect other massive exoplanets in similar highly-elliptical orbits, perhaps in unexplored data from the GAIA spacecraft. We may also learn more about how the planets of our own solar system formed billions of years in the past.

  • This story was originally published 8/28/19, but was expanded two days later, adding original material from researcher Sarah Blunt.
  • * = Nearly every other outlet reporting this news, including the original press release from Caltech, states the orbit is between 45–100 years long. The correct length is 52–117 years.

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Exploring the wonders of the Cosmos, one mystery at a time

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James Maynard is the author of two books, and thousands of articles about space and science. E-mail:

The Cosmic Companion

Exploring the wonders of the Cosmos, one mystery at a time

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