The Strange Case Of The Planet That Shouldn’t Be There

Historically, the quest to find the holy grail of planets in orbit around stars beyond our Sun, proved to be a difficult endeavor. The discovery of the first exoplanet, circling a Sun-like star, happened a generation ago, and it certainly stands as one of humanity’s greatest accomplishments. Discovering a giant exoplanet can be likened to observing light bouncing off the wings of a flying moth fluttering near the 1,000-watt light-bulb of a glowing street lamp–when the observer is 10 miles away. One half of the Nobel Prize in Physics 2019 was awarded jointly to Michel Mayor and Didier Queloz “for the discovery of an exoplanet orbiting a solar-type star.”

Some of the exoplanets discovered since then resemble the planets inhabiting our own Solar System, while others have been proven to be genuine oddballs–so different from our Sun’s family of planets that they defied the wildest dreams of planet-hunting astronomers before their discovery. In September 2019, Spanish and German astronomers of the Calar Alto high-Resolution search for M dwarfs with Exoearths with Near-Infrared and optical Echelle Spectrographs (CARMENES) consortium announced their discovery of yet another exoplanet oddball that should not exist according to current knowledge. The group of scientists who discovered the planet that shouldn’t be there detected a behemoth of a gaseous planet whose mass is unusually hefty compared to its puny parent-star GJ 3512. The astronomers conclude that this weird world likely originated from a gravitationally unstable protoplanetary accretion disk composed of gas and dust that circled around its then still-youthful dwarf parent-star. This contradicts the currently most widely accepted model of planet formation, which requires a solid protoplanetary core to gather surrounding gas.

Indeed, planet-hunting astronomers are certain that baby planets are born as a by-product of the process of star formation. According to this viewpoint, planets are born in what is left of the accretion disk from which their parent-star emerged. The most widely accepted model for the formation of baby planets is based on the idea that an object first emerges from the aggregation of naturally sticky solid dust particles within the accretion disk. The gravitational tugs of these planetary embryos (protoplanets) cause an atmosphere to form from the ambient gas. The scientists of the CARMENES consortium, led by Dr. Juan Carlos Morales, a researcher from the Institute of Space Sciences (ICE, CSIC) in Spain, with contributions from Dr. Diana Kossakowski and Dr. Hubert Klahr from the Max Planck Institute for Astronomy (MPIA) in Heidelberg, Germany, discovered this gas-giant planet, that is similar to our own Solar System’s banded behemoth Jupiter. The distant world contradicts the most widely accepted model of planet formation because it seems to have formed directly out of the accretion disk, without a solid nucleus of condensation to collect the surrounding gas.

The distant world, dubbed GJ 3512 b, along with its dwarf parent-star GJ 3512, are only approximately 30 light-years from our Sun. The planet sports a mass of at least 50% that of Jupiter, and it takes 204 days to complete a single orbit around its star.

On its own, GJ3512 b isn’t an oddball. However, because it is in orbit around a small red dwarf star it joins the ranks of the most unusual and special exoplanets. Tiny red dwarf stars are the smallest, as well as the most abundant, of all true nuclear-fusing stars in our Milky Way Galaxy. GJ 3512 has only about 12% of our Sun’s mass. This means that the maximum gas ratio between the parent-star and its planet is 270. In comparison, our Sun is approximately 1050 times heftier than Jupiter. This observation of a planet that has no right to be where it is–if the most widely accepted model is true–gives theoretical physicists a headache. The gas and dust protoplanetary accretion disks from which low-mass stars, like the red dwarf GJ 3512 emerge, contain comparatively little material. Indeed, too little, as the models show, to be able to even form planetary embryos that could be born and then grow up to become gas giants like GJ 3512.

“One way out would be a very massive disk that has the necessary building blocks in sufficient quantity,” commented Dr. Klahr in a September 26, 2019 MPIA Press Release. Dr. Klahr leads a working group on the theory of planet formation at the MPIA. However, if the protoplanetary accretion disk swirling around a star has more than approximately 1/10 of the stellar mass, the gravitational effect of the parent-star is no longer enough to keep the disk stable. The gravity of the disk material itself becomes a major performer in the play, and its influence grows both noticeable and significant. The upshot is a gravitational collapse similar to what occurs during the birth of a baby star. However, this has not yet been observed to occur around young dwarf stars.

The Quest

On January 9, 1992, the radio astronomers Dr. Aleksander Wolszczan and Dr. Dale Frail announced their important discovery of a duo of indisputably bizarre planets orbiting a form of stellar corpse called a pulsar. Pulsars are rapidly and regularly spinning newborn neutron stars, and they are the relics left by a doomed massive star that has collapsed in a Type II (core-collapse) supernova event. These objects are very dense. A teaspoon full of neutron star material can weigh as much as a fleet of limos.

The discovery of the exotic pulsar planets, that orbit the wildly spinning dead and dense star, dubbed PSR 1257+12, is generally considered to be the first validated detection of exoplanets. However, the pulsar planets are extremely odd beasts inhabiting the planetary zoo. This batch of alien exoplanets likely formed from the unusual leftovers of the supernova that gave birth to their parent-pulsar, in a second round of planet-birth–or, alternatively, to be the lingering rocky cores of gas giants that managed to survive the supernova explosion, and then decayed into their present orbits.

On October 6, 1995, Nobel laureates Drs. Michel Mayor and Didier Queloz of the University of Geneva in Switzerland announced the first confirmed discovery of an exoplanet in orbit around a main-sequence (hydrogen-burning) still-“living” star like our own Sun. The enormous planet circled the nearby G-type star 51 Pegasi. The historic discovery, made at the Observatoire de Haute-Provence, ushered in the modern era of exoplanetary discovery. Technological advances since then, most importantly in high-resolution spectroscopy, resulted in the rapid discovery of a treasure trove of brave new worlds around stars beyond our Sun. Astronomers could now detect the existence of exoplanets indirectly by measuring their gravitational tugs on the motion of their glaring parent-stars. More exoplanets were later discovered by astronomers using the variation in a star’s apparent luminosity, in order to determine if an orbiting planet floated in front of its fiery face (transit method).

At the start of this new era of exoplanet discovery, most of the validated exoplanets were massive gas-giant planets that hugged their parent-stars in close, roasting orbits. The discovery of these alien hot Jupiters surprised astronomers. This is because theories of planetary formation suggested that giant planets should only be able to form at greater distances from their stars. Eventually, however, more planets of other types were detected, and it has now become clear that hot Jupiters account for a minority of exoplanets. In 1999, Upsilon Andromedae became the first main-sequence star recognized to host multiple planets. Kepler-16 hosts the first discovered alien planet circling around a binary main-sequence star system.

On February 26, 2014, NASA announced the discovery of 715 newly verified alien planets, circling 305 stars, that had been detected by the Kepler Space Telescope. These exoplanets were checked by astronomers using a statistical method called verification by multiplicity. Before these new results were obtained, most verified exoplanets were gas giants similar in size to Jupiter–or larger. This is because they are more easily spotted. In contrast, the Kepler worlds are mostly somewhere between the smaller size of Neptune and Earth.

On July 23, 2015, NASA announced the discovery of Kepler452 b , a near-Earth-size planet circling within the habitable zone of a G2-type star. The habitable zone surrounding a star is that “Goldilocks” region where it is not too hot, not too cold, but just right for liquid water to exist. The presence of liquid water is essential for life as we know it to emerge, and so planets within the habitable zone of their stellar parents suggest the possibility–though not the promise–of hosting life.

Astronomers who are on the hunt for exoplanets have discovered thousands of these worlds in our Milky Way Galaxy. As of October 1, 2019, there are 4,118 confirmed exoplanets in 3,063 systems, with 669 systems hosting more than one planet.

The Planet That Shouldn’t Be There

The mysterious case of the planet that shouldn’t be there becomes still more complicated because there is evidence of a second actor in this unusual drama. There are important clues indicating the existence of yet another planet traveling in a long-term orbit around the star GJ 3512. Furthermore, in addition to these two planets, the strongly elliptical orbit of GJ 3512 b suggests that it was once under the gravitational influence of a possible third planet of similar mass. However, this possible third planet has to have been hurled screaming out of the planetary system. This is because that, in addition to the disk mass required to produce GJ 3512 b, there must have once been significantly more matter to create the conditions for the formation of one or even two more planets. This is well outside the boundaries of current planetary and stellar formation models.

For this reason, the scientists at MPIA, the University of Lund in Sweden and the University of Bern in Switzerland, who devise simulations of the formation of planets, concluded that the core accretion model fails to explain the existence of the oddball GJ 3512 b. Hence, the scientists have investigated the conditions under which the scenario of gravitational disk collapse could result in the formation of that weird planet.This particular model has been neglected up until now.

By using different approaches, the team arrived at the same conclusion that GJ 3512 b could have been born as a result of this process. The regions in the disk beyond 10 astronomical units (AU) of the central star are extremely cold with temperatures of approximately -263 degrees C. One AU is equal to the averaage Earth-Sun separation of 93,000,000 miles. Within this very frigid region of the disk, the thermal pressure cannot compensate for the gravitational effect of the material–so it collapses under its own weight. Afterwards, the youthful planet must have migrated over great distances to reach its current location–a distance that is considerably below 1 AU from its stellar parent. This is compatible with current models for the formation of planetary systems.

“Red dwarf stars like GJ 3512 show very active behavior and generate signals similar to those of planets. The infrared spectra were then important to confirm that what we found is indeed a planet,” commented Dr. Diana Kossakowski in the September 26, 2019 MPIA Press Release.

“Until now, the only planets whose formation was compatible with disk instabilities were a handful of young, hot and very massive planets far away from their host stars. With GJ 3512 b, we now have an extraordinary candidate for a planet that could have emerged from the instability of a disk around a star with very little mass. The find prompts us to review our models,” Dr. Hubert Klahr explained to the press.