Like lovely star-splattered ghosts haunting interstellar space, giant and frigid molecular clouds are the strange, secretive cradles of brilliant baby stars. These enormous, dark, and rippling clouds float through interstellar space in huge numbers, and they hide newborn stars as if they were gleaming pearls tucked within an oyster shell. When an especially dense blob within the whirling folds of one of these dark clouds reaches a critical size, mass, or density, it begins to collapse under the intense pull of its own strong gravity–giving birth to a bright new stellar baby. In July 2018, scientists from the Max Planck Institute for Astronomy (MPIA) in Heidelberg, Germany, and the SPHERE instrument consortium of the Very Large Telescope (VLT) of the European Southern Observatory (ESO) in Chile, announced that they have discovered an extremely young gas-giant exoplanet still forming within the protoplanetary accretion disk that swirls and whirls around its youthful parent-star. This newborn gas-giant, dubbed PDS 70 b sports a mass equal to several Jupiters, and it was spotted in orbit around its star PDS 70 within a tattle-tale gap of its natal protoplanetary accretion disk.
This indicates that PDS 70 b is still located near its place of birth, and that it is still probably accumulating material from its surrounding disk of gas and dust. The observations provide a one-of-a-kind chance for scientists to test models of planet-birth, and also learn more about the early history of planetary systems, including that of our own Solar System.
The hunt for exoplanets, which are planets that belong to the families of alien stars beyond our Sun, has so far revealed about 3800 distant worlds of various masses, sizes, and distances from their stellar parents. Alas, astronomers still do not know exactly how these planets are born, and actually observing the birth of a baby protoplanet has proven to be a difficult quest.
However, the team of astronomers at the MPIA and the VLT have now managed to accomplish this very difficult feat. Indeed, the protoplanet PDS 70 b was spotted at a distance of 22 astronomical units (AU) from its parent star. One AU is the average distance between our Sun and the Earth, which is about 93,000,000 miles. “For our study, we selected PDS 70, a star that was already suspected of having a young planet circling around it,” explained Miriam Keppler in a July 2, 2018 MPIA Press Release. Ms. Keppler is a doctoral student at the MPIA and lead author of the paper that highlights this important discovery.
PDS 70 is a 5.4 million year old T Tauri star that is still surrounded by a protoplanetary accretion disk of gas and dust that is approximately 130 AU wide. T Tauri stars are sun-like stellar toddlers that have formed at the center of the especially dense blob embedded within its natal molecular cloud. Most of the material belonging to this blob goes into the formation of the newborn star, while the rest creates the protoplanetary accretion disk from which planets, moons, and smaller objects eventually emerge. In their earliest stages, protoplanetary accretion disks are both very massive and searing-hot, and they can hang around their young stellar hosts for as long as ten million years before they finally completely disappear–possibly blown away by the especially powerful, fierce wind that T Tauris are famous for creating. Alternatively, the disappearing protoplanetary accretion disk may merely stop emitting radiation after accretion has come to a halt. The most ancient protoplanetary accretion disk observed so far is approximately 25 million years old.
Astronomers have observed protoplanetary accretion disks surrounding youthful stars in our own Milky Way Galaxy. Observations conducted by scientists using the Hubble Space Telescope (HST) have spotted proplyds and planetary disks forming within the Orion Nebula. The name proplyd is a syllabic abbreviation of Ionized Protoplanetary Disk, and these disks are externally illuminated photo-evaporating disks swirling around youthful stars. One hundred and eighty proplyds have been discovered within the Orion Nebula alone.
Protoplanetary accretion disks are primarily composed of gas, and they are very thin structures with a typical vertical height that is much smaller than the radius. Also, the typical mass of these accretion disks is considerably less than the mass of the central baby star.
Even though a typical protoplanetary accretion disk is mostly made up of gas, dust particles also play an important role in planet formation. Dust motes protect the mid-plane of the disk from intense, energetic radiation arriving from interstellar space. This energetic radiation creates what is called a “dead zone” in which the magnetorotational instability (MRI) no longer functions.
According to scientists, protoplanetary accretion disks are composed of a churning plasma envelope, called the “active zone”. The “active zone” contains an extensive area of quiescent gas (“dead zone”), which is located at the mid-plane. The “dead zone” can slow down the speed of matter traveling through the disk, and this effectively prevents achieving a “steady state.”
T Tauri tots display large diameters that are usually several times larger than that of our Sun. However, T Tauri’s develop in a way that may seem counterintuitive. This is because they shrink as they grow into full stellar adulthood.. By the time the hot stellar tot has reached this stage of its childhood, less volatile materials have begun to condense near the center of the surrounding protoplanetary accretion disk. This results in the formation of sticky dust motes that harbor crystalline silicates. These little grains of dust bump into one another and then stick together within the crowded environment of the disk. As a result increasingly larger objects grow, eventually becoming planetesimals. Planetesimals are the “building blocks” of planets–the “seeds” from which major planets grow.
In our Solar System, the asteroids–that primarily inhabit the Main Asteroid Belt between Mars and Jupiter–are what is left of the rocky and metallic planetesimals that served as the “seeds” of the four solid planets inhabiting our Solar System’s inner domain: Mercury, Venus, Earth, and Mars. The comets that inhabit the distant, frigid, and murky regions of our Solar System, far from the Sun, represent the relic population of dirty, frozen, and icy planetesimals from which the quartet of gas-laden behemoths of our Star’s family–Jupiter, Saturn, Uranus, and Neptune–ultimately were born.