Stars are born encircled by a swirling disk composed of gas and dust, which is called a protoplanetary accretion disk, and these whirling, gaseous rings surrounding baby stars harbor the necessary ingredients from which a family of planets form. Astronomers have observed a number of such protoplanetary accretion disks circling distant, bright, and fiery baby stars, and these disks take shape at approximately the same time that the baby star–termed a protostar–is born within its obscuring, veiling natal cloud. In May 2015, an international team of astronomers announced their discovery of a very young, distant planetary system which may help astronomers to gain an understanding of how our own Solar System was born and evolved about 4.56 billion years ago. A ring of planetary debris, surrounding the youthful parent-star inhabiting this system, reveals a haunting and remarkable resemblance to our Solar System’s own Kuiper Belt that is situated beyond the outermost major planet Neptune.
The astronomers who discovered the distant disk used the Gemini Planet Imager (GPI) at the Gemini South telescope in Chile, to identify a disk-shaped bright ring of dust surrounding a distant star that is only a bit more massive than our own Star, the Sun. The dazzling disk is located between 37 and 55 Astronomical Units (AU)–or 3.4 to 5.1 billion miles–from its parent-star. This corresponds to approximately the same distance that separates our Solar System’s Kuiper Belt from our Sun. One AU is equal to the average distance between Earth and Sun, which is about 93,000,000 miles. The fascinating brilliance of the alluring disk is the result of glittering starlight being reflected by it–and it is also consistent with a wide range of dust compositions including ice and silicates inhabiting our Kuiper Belt.
Our Kuiper Belt is located in the outer limits of our Sun’s family just beyond Neptune, and it harbors thousands of small icy objects that are relics left over from the formation of our Solar System more than four billion years ago. These icy objects range in size from specks of debris dust, all the way up to moon-sized, icy bodies like the dwarf planet Pluto.
Protoplanetary accretion disks contain large quantities of nutritious gas and dust that feed hungry, growing protoplanets. Our own Solar System, as well as other planetary systems, form when a relatively small, very dense blob embedded within the undulating billows of a giant cold, dark, molecular cloud, collapses gravitationally under its own impressive weight. Haunting our Galaxy in huge numbers, these ghostly, frigid clouds–the strange, dark cradles of the shining stars–are primarily composed of gas, but also contain smaller amounts of dust. Most of the collapsing gaseous and dusty blob gathers at the center, and eventually catches furious fire as a result of the process of nuclear fusion–giving birth to a fabulous new stellar baby (the protostar). What is left of the gas and dust, that went into the formation of the protostar, eventually evolves into the protoplanetary accretion disk from which planets, moons, asteroids, and comets emerge. In their earliest stages, accretion disks are both very hot and very massive, and they can encircle their youthful stars for as long as ten million years.
By the time a fiery Sun-like stellar baby has reached what is termed the T Tauri stage in its development, the searing hot, massive encircling disk has become much thinner and cooler. A T Tauri is a stellar toddler–a young, variable star like our Sun that is very active at the tender age of only 10 million years. These starry tots possess large diameters that are several times greater than that of our Sun at present–but they are still in the process of shrinking, because young Sun-like stars shrink as they grow up. By the time the fiery toddler has reached this stage of its development, less volatile materials have begun to condense near the center of the surrounding disk, forming very fine and extremely sticky dust particles. The fragile, delicate dust motes carry crystalline silicates.
The sticky, tiny dust grains bump into one another and then merge within the dense environment of the protoplanetary accretion disk. In this way, ever larger and larger objects continually grow–from pebble size, to boulder size, to mountain size, to moon size, to planet size. These growing objects evolve into what are termed planetesimals–the primordial planetary building blocks. Planetesimals can reach sizes of 1 kilometer across, or even larger, and they are a very abundant population within a young accretion disk surrounding the fiery stellar tot. They can also hang around long enough for some of them to still be present billions of years after a mature planetary system has formed. In our Solar System, the asteroids are the remnant rocky and metallic planetesimals that were the fundamental building blocks of the four rocky, inner planets: Mercury, Venus, Earth, and Mars. On the other hand, comets are the icy leftovers of the planetesimals that went into the construction of the quartet of outer gaseous planets: Jupiter, Saturn, Uranus, and Neptune.
The Kuiper Belt
The Kuiper Belt was named in honor of the Dutch-American astronomer Gerard Kuiper (1905-1973), who is commonly credited with being the first to predict its existence. It is a region of our Solar System beyond the mysterious, dark, and frigid realm of the majestic outer planets, extending from the orbit of Neptune (at 30 AU) to approximately 50 AU from our Star. In many ways it is similar to the Main Asteroid Belt between Mars and Jupiter, but it is about 20 to 200 times as massive. The distant Kuiper Belt, like the Main Asteroid Belt, is composed of small bodies–planetesimals–that are remnants from our Solar System’s formation. A large number of asteroids are composed mainly of rock and metal, but most Kuiper Belt Objects (KBOs) are made of volatiles (called “ices”), such as water, methane, and ammonia. The Kuiper Belt is the frigid home of a trio of officially recognized dwarf planets: Pluto, Haumea, and Makemake. A handful of our Solar System’s moons, such as Triton of Neptune and Phoebe of Saturn, are also frequently thought to have originated in this distant region.
Since the Kuiper Belt was discovered in 1992, the number of known KBOs has skyrocketed to over a thousand, and more than 100,000 KBOs over 62 miles in diameter are believed to exist in our Solar System’s distant deep freeze. At first, astronomers believed that the Kuiper Belt was the primary domain of periodic comets–those with orbits lasting less than 200 years. However, more recent studies since the mid-1990s have demonstrated that the Kuiper Belt is dynamically stable, and that the true place of the comets’ origin is the scattered disk. The scattered disk is a dynamically active region formed by the outward migration of Neptune 4.5 billion years ago, when our Solar System was still in its infancy. Scattered disk objects such as Eris sport extremely eccentric (out-of-round) orbits that carry them as far as 100 AU from our glaring Star.
The objects that tumble around within the Kuiper Belt, along with the frozen denizens of the scattered disk and Oort Cloud, are all collectively termed trans-Neptunian objects. The very remote Oort Cloud is a thousand times more distant than the Kuiper Belt and not as flat. It is also the repository of long-period comets (those with orbits lasting longer than 200 years), and is a gigantic shell of icy objects around our entire Solar System–extending halfway to the nearest star beyond our Sun!
Poor Pluto is the largest denizen of the Kuiper Belt, as well as the second-largest known trans-Neptunian object–the largest being Eris that dances around within the scattered disk. Although originally classified as a major planet after its discovery in 1930, Pluto’s status as a member of the Kuiper Belt caused it to be unceremoniously evicted from the pantheon of major planets, and re-classfied as a dwarf planet in 2006. Poor Pluto is compositionally akin to numerous other KBOs, and its orbital period is characteristic of a class of KBOs termed plutinos. Plutinos share the same 2:3 resonance with Neptune.
In A Solar System Far, Far Away!
The star of the new study, conducted by the team of international astronomers, is a sparkling member of the very massive 10 to 20 million year-old Scorpius-Centaurs OB association, which is a region that is very much like the one in which our Sun was born. The primary distinguishing characteristic of the members of a stellar association is that most of them share similar characteristics. An OB association hosts a large number of searing-hot, roiling, fiery blue giant stars, of spectral classes O and B.
The disk-shaped bright ring of dust, being observed in this study, is not perfectly centered on the star. This is a powerful indication that the ring was likely sculpted by one or more unseen distant alien planets. By using models illustrating how planets carve debris disks, the astronomers found that “eccentric” versions of the giant planets in the outer realm of the solar system could explain the observed properties of the glowing ring.
“It’s almost like looking at the outer Solar System when it was a toddler,” commented Dr. Thayne Currie in a May 2015 University of Cambridge Press Release. Dr. Currie is principal investigator of this research, and an astronomer at the Subaru Observatory in Hawaii. The University of Cambridge is in the UK.
The Kuiper Belt is commonly thought to be composed of the remnants of our Solar System’s ancient formation, and so there is a possibility that once the new system develops, it may bear a striking resemblance to the way our own Solar System looks today.
“To be able to directly image planetary birth environments around other stars at orbital distances comparable to the Solar System is a major advancement. Our discovery of a near-twin of the Kuiper Belt provides direct evidence that the planetary birth environment of the Solar System may not be uncommon,” explained Dr. Nikku Madhusudhan in the May 2015 University of Cambridge Press Release. Dr. Madhusudhan is of Cambridge’s Institute of Astronomy, and one of the paper’s co-authors.
The parent-star of this system, known as HD 115600, was the first object the research team observed. “Over the next few years, I’m optimistic that the GPI will reveal many more debris disks and young planets. Who knows what strange, new worlds we will find,” Dr. Currie told the press.
The new research is to be published in The Astrophysical Journal Letters.