An Oddball World In A Loopy Orbit

Ever since the first exoplanets were discovered over a generation ago, it has been clear to planet-hunting astronomers that many other planetary systems circling distant stars do not resemble our own. Indeed, a treasure trove of planetary oddballs have demonstrated that there are myriad brave new worlds that travel to the beat of a different drummer than the familiar planets of our Sun’s own family. Such an oddball world is HR 5183 b, whose very unusual orbit resembles a slingshot. The loopy world is three times the mass of our Solar System’s banded behemoth, Jupiter, and it travels on a long, egg-shaped path around its parent star, HR 5183. While other exoplanets with highly elliptical orbits have been discovered circling distant stars, none of those strange worlds were located at the outer limits of their star systems like this one. If this planet were somehow placed into our own familiar Solar System, it would take a breathtaking swing around our Sun, that would take it from within the Main Asteroid Belt between Mars and Jupiter to beyond the outermost major planet Neptune.

“This planet is unlike the planets in our Solar System, but more than that, it is unlike any other exoplanet we have discovered so far,” commented Ms. Sarah Blunt in an August 27, 2019 California Institute of Technology (Caltech) Press Release. Blunt is a Caltech graduate student and first author on the study published in The Astronomical Journal.

“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 a high eccentricity speaks to some difference in the way that it either formed or evolved relative to other planets,” Blunt added.

This loopy giant world was discovered by astronomers using the radial velocity method, which is the method originally used to discover the first exoplanets to be discovered more than twenty years ago. The radial velocity method detects new and distant worlds by observing how their parent stars “wobble” as a reaction to gravitational tugs from those very planets. However, this method–though highly successful–requires analyses of data that usually demand observations taken over a planet’s entire orbital period. For those alien planets orbiting far from their stellar parents, this method of discovery can present a problem. This is because an entire orbit can take tens of hundreds of years for planets that orbit far from their stars.

There are many methods, in addition to radial velocity, that astronomers use to detect alien planets circling distant stars. For example, transit photometry has also been used successfully, but it–like the radial velocity method–suffers from an observational bias in favor of the discovery of distant worlds that orbit close to their parent-stars. For this reason, 85% of the exoplanets found so far circle their stars so closely that they are tidally locked to their glaring stellar parents. In several cases, multiple planets have been detected circling a star.

Almost 1 in 5 stars that are similar to our Sun are known to sport an Earth-sized planet in the habitable zone. The habitable zone surrounding a star is that “Goldilocks” region where the temperature is not too hot, not too cold, but just right for water to exist in its life sustaining liquid state. Life as we know it depends on the presence of liquid water.

Assuming that there are about 200 billion stars in our barred-spiral Milky Way Galaxy, it can be hypothesized that there are 11 billion potentially habitable Earth-sized exoplanets in our Galaxy alone. This number skyrockets to 40 billion if planets orbiting the numerous red dwarf stars in our Milky Way are included in the calculations. Red dwarf stars are both the most abundant, as well as the longest-lived, true nuclear-fusing stars in our Galaxy. Indeed, astronomers think no red dwarf star in the Universe has had enough time to run out of its nuclear-fusing fuel and perish. Red dwarf stars require trillions of years to burn up their hydrogen fuel, and the Universe is a mere 14 billion years old. Small stars live longer than giant stars because they are cooler. Searing-hot, roiling, broiling giant stars, live fast and die young, generally living for “only” millions of years–a blink of the eye on cosmological time scales.

Almost all of the planets discovered so far dwell within our own Milky Way. Nevertheless, there is strong evidence that extragalactic planets exist. Extragalactic planets reside much farther away from Earth, inhabiting host-galaxies located far beyond our own.

In addition to exoplanets, there are also rogue planets. These unfortunate worlds do not orbit any star at all, and are usually considered as a separate category, especially if they are gas-giants like Jupiter and Saturn–in which case they are classified as a type of substellar object called a brown dwarf (failed star). There are possibly billions (or more) rogue planets in our Milky Way, and these tragic worlds probably had parent-stars of their own at one time, but were hurled out of their planetary systems by gravitational perturbations originating from either sister planets or closely passing stars.

As of August 1, 2019, there are 4,103 confirmed exoplanets in 3,056 systems, with 665 systems sporting more than one planet.

The Chase

The first possible detection of an exoplanet was noted back in 1917–but it was not recognized as such. The first confirmed exoplanet detection didn’t come until 1992, with the discovery of a batch of several strange terrestrial-mass worlds in orbit around a dense city-sized stellar corpse termed a pulsar. Pulsars are newborn neutron stars. This means that pulsars are the remnants of massive stars that perished in the noisy, fiery fury of a supernova explosion, after having used up their necessary supply of nuclear-fusing fuel. These small stellar remnants spin rapidly, and they hurl out very regular beacons of light that have been compared to those of a lighthouse on Earth. Pulsars are about the same size as Dallas, but are so amazingly dense that a teaspoon full of neutron star material can weigh as much as all of the houses on a suburban street put together. The planets in orbit around neutron stars are likely hostile worlds that are showered with a perpetual downpour of deadly radiation emanating from their bizarre, ghostly parent-star–that is essentially one big atomic nucleus.

The first validated discovery of a planet in orbit around a Sun-like, hydrogen-burning star, came three years later. The historic discovery was made by astronomers using the radial velocity method, and the planet itself was a surprise. Dimidium, as the planet is now called, is an enormous gas-giant world that hugs its parent-star fast and close in a broiling-hot orbit. Dimidium circles its star, 51 Pegasi, at the roasting distance of a mere 4,300,000 miles, which amounts to only a small fraction of the distance between our Sun and the innermost planet Mercury. Dimidium was the first exoplanet to be classified as a hot Jupiter, although others of its kind were discovered later. Hot Jupiters orbit their stars at very close distances, and have indisputably earned their nickname “roasters” for this reason. Because they are so close to their stars, hot Jupiters are tidally locked to their stellar parents. This means that a “roaster” always shows only one face to its star, while the other face is always turned away.

Until Dimidium’s discovery, astronomers generally thought that gas-giant worlds like Jupiter could only exist much farther away from their stars–inhabiting colder and more remote regions of their planetary systems, where our Solar System’s own quartet of giant gaseous planets–Jupiter, Saturn, Uranus, and Neptune–orbit our Sun.

Dimidium circles 51 Pegasi every 4.2 days!

Loopy Planet

The California Planet Search, headed by Caltech Professor of Astronomy, Dr. Andrew W. Howard, is one of only a few groups that observe stars over decades-long periods of time. These lengthy timescales are necessary in order for astronomers to discover long-period exoplanets using the radial velocity method. The necessary data collected to make the important discovery of this loopy planet came from two observatories used by the California Planet Search–the Lick Observatory in Northern California and the W.M. Keck Observatory in Hawaii. Data obtained from the McDonald Observatory in Texas was also used.

The astronomers began observing the oddball planet’s parent-star, HR 5183, back in the 1990s. However, they did not manage to obtain data corresponding to one full orbit of the planet, HR 5183 b, because it circles its star approximately every 45 to 100 years. The astronomers detected the oddball planet because of its loopy orbit.

“This planet spends most of its time loitering in the outer part of its star’s planetary system in this eccentric orbit, then it starts to accelerate in and does a slingshot around its star… (We) detected this slingshot motion. We saw the planet come in and now it’s on its way out. That creates a distinctive signature that we can be sure that this is a real planet, even though we haven’t seen a complete orbit,” Dr. Howard explained in the August 27, 2019 Caltech Press Release.

The study’s new findings clearly show that it is possible to use the radial velocity method to discover planets with far-flung orbits around their stars–without having to wait for decades. Furthermore, the scientists suggest that, by hunting for more planets like HR 183 b, the role that giant planets have played in shaping their planetary systems could be revealed.

Planets are born within disks of gas and dust left over after a baby star is born. This means that planets start off sporting flat, circular orbits. For the oddball HR 183 b to travel on such an eccentric orbit, it must have received a gravitational kick from a different object. The astronomers propose that the most plausible model for this “kick” would include a neighboring planet of about the same size as HR 183 b. According to this scenario, when the duo of neighboring planets traveled close enough to each other, one booted the other out of the planetary system–thus forcing HR 5183 b into a highly eccentric orbit.

“The newfound planet basically would have come in like a wrecking ball, knocking anything in its way out of the system,” Dr. Howard continued to explain in the Caltech Press Release.

This discovery of an oddball planet, in a loopy orbit, shows that astronomers’ undrstanding of the strange worlds that circle stars beyond our Sun is still incomplete. Scientists continue to discover distant worlds that are unlike anything in our Sun’s own familiar family of planets–or, for that matter, in other planetary systems that have already been discovered.

Dr. Howrd explained in the August 27, 2019 Caltech Press Release that “Copernicus taught us that Earth is not the center of the Solar System, and as we have expanded into discovering other solar systems of exoplanets, we expected them to be carbon copies of our own system. But it’s been one surprise after another in the 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 diversity.”

This new study is published under the title: Radial Velocity of an Eccentric Jovian World Orbiting at 18AU.

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