Giant exoplanet is ‘like a wrecking ball,’ astronomers say

An illustration shows what the orbit of exoplanet HR 5183 b would look like if it was dropped down in our solar system. Full credit: Adam Makarenko/W.M. Keck Observatory

Astronomers have discovered an unlikely exoplanet with the strangest orbit they’ve ever witnessed more than 100 light-years away, according to a new study.

The exoplanet, known as HR 5183 b, orbits the star HR 5183 in the Virgo constellation, which is 102 light-years from Earth. The star has been of interest to astronomers since the 1990s, but they didn’t find evidence of an exoplanet orbiting it until now.

The planet, which is three times the mass of Jupiter, only completes one orbit around the star between every 45 to 100 years, the astronomers said. It was its strange, egg-like orbit that finally gave it away.

The details of the planet’s odd orbit have been submitted for publication in The Astronomical Journal.

“This planet is unlike the planets in our solar system, but more than that, it is unlike any other exoplanets we have discovered so far,” said Sarah Blunt, study author and California Institute of Technology graduate student .

“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.”

Astronomers detected the planet using the radial velocity method. The radial velocity method is based on gravity and the Doppler effect, in which light increases or decreases in frequency as a source and observed object move toward or away from each other.

Stars don’t remain completely still when they are orbited by planets; they move in small circles as a response to the pull of gravity from the planets. These movements change the light wavelength of the star, going between red and blue depending on the location of the planet. Tracing the shifts can help astronomers find planets.

The California Planet Search, which is led by California Institute of Technology professor of astronomy Andrew Howard, watched the star for years to make the detection.

“For almost 20 years our data did not show any sign of a planetary companion” around this star,” said Michael Endl, study co-author and senior research scientist at the University of Texas at Austin’s McDonald Observatory. “And then we observed the ‘slingshot’ which only lasted about two years. If we would have stopped observing the star after 15 years, we would have missed it. It makes me wonder how many other stars have massive planets on these slingshot orbits and we usually miss them.”

The exoplanet’s very elliptical orbit is not unusual for giant planets around other stars, but this is the only exoplanet found so far that skirts along the very edge of its star system.

“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,” Howard said. “We detected this slingshot motion. We saw the planet come in and now it’s on its way out. 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.”

The researchers believe that searching for more planets like HR 5183 b could reveal how giant planets have helped shape star systems. Planets form out of the leftover materials from star formation. They normally begin with circular orbit. But something happened to this exoplanet. to change its orbit.

The researchers believe that the planet once had a similarly-sized neighbor and when they came too close to each other, one got knocked out of the solar system and HR 5183 b was pushed into a weird orbit.

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

For astronomers, it’s further proof that exoplanet science is a growing and diverse field of study that broadens with every discovery.

“Copernicus taught us that Earth is not the center of the solar system, and as we expanded into discovering other solar systems of exoplanets, we expected them to be carbon copies of our own solar system,” Howard said, “but it’s just been one surprise after another in this field. This newfound planet is another example of a system that is not the image of our solar system but has remarkable features that make our universe incredibly rich in its diversity.”

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