Extrasolar planets are a diverse collection of brave new worlds in distant orbits around stars beyond our own Sun. Out of this vast population of newly discovered worlds, some look very much like planets inhabiting our own familiar Solar System, while others are so exotic that they are unlike anything astronomers ever dreamed could exist–that is, until they were finally discovered. In December 2018, a team of astronomers from the Universities of Zurich in Switzerland and Cambridge in the UK, announced their own discovery of a treasure trove filled with just such exotic exoplanets–a new oddball class of distant worlds beyond our own Solar System, never before dreamed of. These super-Earths were born at high, roasting temperatures near their searing-hot parent-stars, and they contain large amounts of aluminum, calcium, and their oxides, including sapphire and ruby.
The story begins far away, in the constellation Cassiopeia, twenty-one light-years from Earth. In that remote location, there lurks a planet with the telephone book sounding name of HD219134 b, and this distant bewitching and bewildering world orbits its parent-star with a year that is only three days long. Because the oddball planet’s mass is almost five times that of Earth, it has been classified as a super-Earth. However, unlike our own planet, it probably does not contain a massive hidden core composed of iron. However, HD219134 b is richly endowed with both aluminum and calcium instead.
“Perhaps it shimmers red to blue like rubies and sapphires, because these gemstones are aluminum oxides, which are common on the exoplanet,” commented Dr. Caroline Dorn in a December 19, 2018 University of Zurich Press Release. Dr. Dorn is an astrophysicist at the Institute for Computational Science at the University of Zurich. HD219134 b is one of a trio of candidates that astronomers think could belong to this new and exotic class of exoplanets. Dr. Dorn and her colleagues at the Universities of Zurich and Cambridge report their findings in the British journal Monthly Notices of the Royal Astronomical Society (MNRAS). The Royal Astronomical Society (RAS) is in London.
Super-Earths!
A super-Earth is defined as an extrasolar planet that sports a mass higher than Earth’s, but is considerably less than those of the duo of our Solar System’s blue ice-giants, Uranus and Neptune. Of this frigid duo, that dance around our Sun in the outer dimly-lit regions of our Solar System, Uranus boasts a mass of 15 Earths, while Neptune’s mass is 17 times that of our planet’s.
The term super-Earth itself refers exclusively to the mass of a planet, and it does not suggest anything about the surface conditions or the potential habitability of the distant world itself. The alternative term gas dwarfs is sometimes used as a reference to these worlds, and may be a more accurate term than super-Earths for those at the higher end of the mass scale. This alternative designation was suggested by the planet-hunting astronomer Dr. Sara Seager of the Massachusetts Institute of Technology (MIT) in Cambridge, Massachusetts. However, the designation mini-Neptunes is also used by some astronomers.
Because super-Earths are defined only by their masses, the term does not refer to compositions, temperatures, orbital properties, potential habitability or environments. While astronomers are mostly in agreement on the upper boundary of 10 Earth-masses, the lower limit has not been as neatly determined and varies from about 1.9 to 5 times Earth’s mass. The term super-Earth is also used by astronomers to define those planets that are larger than Earth-like planets, but smaller than mini-Neptunes (from 0.8 to 1.25 Earth-radii). However, some researchers propose that the term super-Earth should only be used in reference to rocky planets without a significant atmosphere, or planets that have not only atmospheres but also solid surfaces or oceans with a definite boundary existing between liquid and atmosphere–a structure that the quartet of giant gaseous planets in our own Solar System (Jupiter, Saturn, Uranus and Neptune) do not possess. Planets with masses greater than 10 times that of Earth are usually referred to as massive solid planets, mega-Earths or gas-giant planets–depending on whether they are primarily composed of rock and ice or are mostly made up of gas. The two enormous gas-giants belonging to our Sun’s family are the banded behemoth Jupiter, and the beautiful ringed planet Saturn.
The first super-Earths were discovered by Dr. Alexander Wolszczan and Dr. Dale Frail back in 1992, and they were among the very first group of exoplanets (four in total) ever to be discovered. Of the quartet of distant worlds, Poltergeist and Phobetor sport masses approximately four times that of Earth–making them much too small to be gas-giants, but just right for them to be classified as super-Earths.
Dr. Wolszczan and Dr. Frail used the Arecibo Radio Telescopes in Puerto Rico to make their historic discovery of the first planets to be verified in orbit around a distant alien star. However, the star that these first-to-be-discovered exoplanets circle is a true stellar “oddball”, a millisecond pulsar dubbed PSR B1257+12. Pulsars are the extremely dense city-sized remnant cores of what were once massive stars that have gone supernova. These strange stellar relics are infant neutron stars that are born wildly spinning. Pulsars send brilliant beacons of dazzling light out into space with such regularity that they are frequently compared to the familiar lighthouse beacons on Earth. At one time, most astronomers thought that pulsars could not host planets–that is, until the bizarre quartet of pulsar-planets were discovered. After having carefully observed the radio emissions emanating from PSR1257+12, the two astronomers reached the conclusion that it was being orbited by several exotic worlds. The pulsar-planets themselves are inhospitable. This is because they are bathed in a constant shower of deadly radiation that their “dead” stellar-parent has sent screaming out into space.
Pulsars are bizarre inhabitants of the stellar zoo. Only 12 miles in diameter, they are really the burned-out, collapsed cinders of what were once doomed massive main-sequence (hydrogen-burning) stars on the Hertzsprung-Russell Diagram of Stellar Evolution. These stellar ghosts are all that remain of the former massive progenitor star that blasted itself to smithereens after having consumed its entire necessary supply of hydrogen fuel. Neutron stars contain 1,000,000,000 tons of material squeezed by gravity into a Chicago-sized volume. A teaspoon of neutron star stuff is as massive as the Empire State Building.
The first super-Earth to be discovered in orbit around a still-“living” hydrogen-burning main sequence star was found by a team of planet-hunting astronomers in 2005. The team was led by Dr. Eugenio Rivera of the University of California at Santa Cruz (UCSC). This super-Earth circles a star dubbed Gliese 876 and it is designated Gliese 876 d (a duo of gas-giants had previously been discovered in that system). Gliese 876 d has an estimated mass of about 7.5 Earth-masses and a very short orbit around its star–with a period of only about 2 days. Because Gliese 876 d hugs its red dwarf parent-star fast and close in a roasting orbit, it may have a surface temperature of a sizzling 430-650 Kelvins. Hence, it is much too hot to have liquid water. Liquid water is necessary for life as we know it to exist. Red dwarf stars are both the most abundant, as well as the smallest, true hydrogen-burning stars in our Milky Way Galaxy
A Treasure Trove Of Exotic Worlds
The team of astronomers from the University of Zurich and Cambridge University made use of theoretical models in order to study the formation of exoplanets. They then went on to compare their models with data obtained from observations. It was already known that newborn stars are surrounded by an accretion disk composed primarily of gas with a smaller quantity of dust–and from these accretion disks a family of planets are born in orbit around their star. Rocky planets, like our own, are born from the solid bodies left as relics when this protoplanetary disk falls apart. These planetesimals–the building blocks of planets–condense out of gas as the disk cools off.
“Normally these building blocks are formed in regions where rock-forming elements such as iron, magnesium and silicon have condensed,” Dr. Dorn explained in the December 19, 2018 University of Zurich Press Release. The planets that are born from rock-forming elements possess Earth-like compositions and contain an iron core. Most of the super-Earths that have been discovered so far have been formed in such regions of their accretion disk. However, there are regions of disks located closer to the fires of their stars where it is considerably hotter. “Many elements are still in the gas phase there and the planetary building blocks have a conspicuously different composition,” Dr. Dorn added.
With their new models, the team of astronomers went on to calculate what a planet being born in such a searing-hot region would look like. They discovered that calcium and aluminum should be the primary components of such planets, along with magnesium and silicon, and that there would hardly be any iron. “This is why such planets cannot have a magnetic field like Earth,” Dr. Dorn continued to explain. Because the inner structure of these planets would be so different, their cooling behavior and atmospheres should also be different from those of normal super-Earths. For this reason, the astronomers proposed the existence of the exotic, new class of super-Earths that are born close to the seething heat and glaring brilliance of their parent-stars.
“What’s exciting is that these objects are completely different from the majority of Earth-like planets, if they actually exist,” Dr. Dorn added.
“We looked at different scenarios to explain the observed densities,” Dr. Dorn noted. For example, a thick atmosphere could result in a lower overall density. But two of the exoplanets observed, 55 Cancri e and WASP-47 e, hug their parent-star so closely that their surface temperature skyrockets to almost 3,000 degrees and they should have lost this gas envelope very long ago.
“On HD219134 b it’s less hot and the situation is more complicated,” Dr. Dorn noted in the December 19, 2018 University of Zurich Press Release. At first, the lower density could also be attributed to deep oceans. However, further study indicates that a second planet orbiting the star at a slightly greater distance makes this scenario unlikely. A comparison of the two objects revealed that the inner planet cannot harbor more water or gas than the outer one. It’s still uncertain whether fiery magma oceans play a role in creating a lower density.
Dr. Dorn continued to note that “We’ve thus found three candidates that belong to a new class of super-Earths with this exotic composition.” The astronomers are also in the process of correcting an earlier image of super-Earth 55 Cancri e, which made headlines back in 2012 as the “diamond in the sky” alien planet. Scientists had assumed earlier that this distant world was made up primarily of carbon, but they had to discard that theory as the result of later observations.
“We’re turning the supposed diamond planet into a sapphire planet,” Dr. Dorn added jokingly.
The paper describing this research is published under the title A new class of Super-Earths formed from high-temperature condensates: HD219134 b, 55 Cnc e, and WASP-47 e. In addition to Dr. Dorn, J.H.D. Harrison, A Bonsor, and T. Hands, co-authored the paper.