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.