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Illustration of an exoplanet. Credit: Ricardo Ramirez
Three times the mass of Jupiter, a unique planet sways around its star on a strange path.
Astronomers have discovered a planet three times denser than Jupiter, which follows a long, egg-shaped path around its star. If this planet was somehow placed in our own solar system, it would rock from inside our asteroid belt to the outside of Neptune.
Other giant planets with very elliptical orbits were discovered around other stars, but none of these worlds were at the edge of their star system, like this one.
"This planet does not look like the planets of our solar system, but even more so, it does not look like any other exoplanet we've discovered so far," says Sarah Blunt, a Caltech graduate student and first author of the new study published in The astronomical journal. "The other planets detected far from their stars usually have very weak eccentricities, which means that their orbits are more circular. The fact that this planet has such a high eccentricity suggests a difference in the way it has formed or evolved compared to other planets. "
The planet was discovered using the radial velocity method, an exoplanet discovery bomb that detects new worlds by tracking how their parent stars "flicker" in response to the gravitational tugs of these planets.
However, the analysis of these data generally requires observations made over the entire orbital period of a planet. For planets in orbit away from their stars, this can be difficult: a complete orbit can take tens, even hundreds of years.
This illustration compares the eccentric orbit of HR 5183B with the more circular orbits of the planets of our own solar system. Animation credit: Observatory W. M. Keck / Adam Makarenko
California Planet Search, led by Andrew W. Howard, professor of astronomy at Caltech, is one of the few groups to watch the stars in the decades needed to detect long-lived exoplanets at radial velocity.
The data needed to discover the new planet were provided for the first time by the W. M. Keck Observatory in Hawaii. In 1997, the team began using the Keck Observatory's High Resolution Scale (HIRES) spectrometer to take measurements of the planet's star, called HR 5183.
"The key was persistence," Howard said. "Our team has been following this star with the Keck Observatory for over two decades and has only seen evidence for the planet in the last two years! Without this long – term effort, we would never have found this planet. "
In addition to the Keck Observatory, California Planet Search also used the Lick Observatory in Northern California and the McDonald Observatory in Texas.
Astronomers have been observing HR 5183 since the 1990s, but do not have data for a complete orbit of the planet, called HR 5183b, because it surrounds its star every 45 to 100 years or so. The team instead found the planet because of its strange orbit.
This illustration compares the eccentric orbit of HR 5183B with the more circular orbits of the planets of our own solar system. Credit: Observatory W. M. Keck / Adam Makarenko
"This planet spends most of its time strolling in the outer part of the planetary system of its star in this very eccentric orbit, then begins to accelerate and throw a sling around its star," Howard Howard explains. "We detected this slingshot movement. We have seen the planet come in and now it is endangered. This creates a signature so distinctive that we can be sure that it is a real planet, even though we have not yet seen a complete orbit. "
New discoveries show that it is possible to use the radial velocity method to detect distant planets without waiting for decades. And, suggest the researchers, to search for more planets like this one could enlighten the role of the giant planets in the formation of their solar systems.
Planets take shape from discs of matter left behind after star formation. This means that the planets should start in circular and flat orbits. For the newly detected planet to be in such an eccentric orbit, it must have received a gravitational kick from another object.
The researchers suggest that the most plausible scenario is that the planet has already had a neighbor of similar size. When the two planets approached each other, one pushed the other towards the outside of the solar system, forcing HR 5183B into an extremely eccentric orbit. .
"This new planet would have basically come as a wrecking ball," says Howard, "eliminating anything that might escape the system."
This discovery demonstrates that our understanding of planets beyond our solar system is constantly evolving. Researchers continue to find worlds unlike anything in our solar system or solar systems that we have already discovered.
"Copernicus taught us that the Earth is not the center of the solar system, and as we discover other solar systems of exoplanets, we expected them to be true copies of our own solar system. Howard explains. surprise after each other in this area. This newfound planet is another example of a system that is not the image of our solar system but has some remarkable features that make our universe incredibly rich in diversity. "
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The study, titled "Radial Velocity of an Eccentric Jovian World Orbiting around 18 AU", was funded by the National Science Foundation, NASA, Tennessee State University and the State of Tennessee. , the Beatrice Watson Parrent Scholarship, the Trottier Family Foundation and Caltech. . Other Caltech authors include: BJ Fulton, IPAC researcher; Sean Mills, former postdoctoral fellow (BS 12); Erik Petigura, former postdoctoral fellow at UCLA; and Arpita Roy, R.A. & G. B. Millikan Postdoctoral Scholar in Astronomy.
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