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.
Ring Around A Baby Star
PDS 70, a T Tauri tot, is a mere 54 million years old. It is also encircled by a protoplanetary disk (circumstellar disk) composed of gas and dust that is about 130 AU wide. In order to envision the extent of this disk, the Kuiper Belt–that orbits our Sun beyond Neptune–extends only up to approximately 50 AU. These vast surrounding accretion disks are made up of the material left over from the parent-star’s birth within a frigid dark molecular cloud.
But the accretion disk surrounding PDS 70 is particularly interesting because it shows a large gap. Such gaps are believed to be tattle-tale clues that planet formation is occurring. That is because these gaps result from a young giant protoplanet gathering up more and more disk material as it travels around its young star. By interacting with the disk, the protoplanet gradually changes its distance to its parent-star. As time goes by, the growing protoplanet excavates a broad circular swath through the disk.
In a subsequent study, led by Dr. Andre Muller of the MPIA, the team of astronomers obtained a truly spectacular image of the developing PDS 70 system, in which a baby protoplanet can be readily observed at the inner rim of the gap of the surrounding disk. The distant protoplanet takes approximately 120 years to orbit its parent-star. A spectrum of PDS 70 b enabled the astronomers to determine the alien planet’s atmospheric and physical attributes.
“This discovery provides us with an unprecedented opportunity to test theoretical models of planet formation,” Dr. Muller commented in the July 2, 2018 MPIA Press Release.
A Giant, Distant Protoplanet
The new research demonstrates that PDS 70 b is a gas-giant planet, sporting a mass several times that of our own Solar System’s banded behemoth Jupiter. The exoplanet’s surface has a temperature of approximately 1200 Kelvin, which makes it considerably hotter than any planet in our own Solar System. Because a protoplanet must be younger than its stellar parent, PDS 70 b is likely still in the process of growing. The data gathered by the astronomers indicate that the planet is surrounded by clouds that change the radiation emitted by the planetary core and its atmosphere. “We corrected our calculations to take into account the new data published by the Gaia satellite for stellar distances. According to Gaia, PDS 70 is at a distance of 370 light-years,” Ms Keppler explained in the July 2, 2018 MPIA Press Release.
The astronomers must still apply sophisticated observational and analytical techniques in order to acquire an image of a protoplanetary accretion disk. On conventional images, all of the objects in the vicinity of the parent-star will be be lost in the glare of brilliant starlight. However, with the SPHERE instrument the bright star’s light can be removed. In order to accomplish this, the camera must use a property of the light known as polarization. Linearly polarized light can oscillate in only one plane. But the light emanating from a star is mostly non-polarized. However, the light reflected by the disk will become linearly polarized when scattered by the accretion disk’s dust motes.
When used with the proper polarization filter–which would transmit light waves in only one plane of oscillation–the light traveling from different regions of the disk would be either detected or cancelled out, as a result of the filter’s orientation. Photographers use a similar technique to suppress reflections emanating from a smooth surface.
In contrast, starlight can be observed no matter how the filter is oriented. By making use of the difference between light reflected by the accretion disk and the light streaming out directly from the star, astronomers can eliminate the direct starlight. In order to support their measurements, the observers also block the star with a mask. All that remains is an image of the protoplanetary accretion disk.
Dr. Thomas Henning, director at MPIA, senior author of the two studies and the German co-I of the SPHERE instrument, commented to the press that “After ten years of developing new powerful astronomical instruments such as SPHERE, this discovery shows us that we are finally able to find and study planets at the time of their formation. That is the fulfilment of a long-cherished dream.”
The results of this research appear as Keppler et al., “Discovery of a substellar companion within the gap of the transition disk around PDS 70”, and as Muller et al., “Orbital and atmospheric characterization of the planet within the gap of the PDS 70 transition disk. Both papers are published in the July 2, 2018 issue of the journal Astronomy & Astrophysics.
Spectro-Polarimetric High-Contrast Exoplanet Reseach (SPHERE) is the Extreme Adaptive Optics System and Coronagraphic Facility (SPHERE) of the VLT. SPHERE was built by an international consortium led by MPIA and the Institut de Planetologie et d’Astrophysique de Grenoble (IPAG).
