A Giant “Oddball” Planet That’s Too Big For Its Star
By Judith E Braffman-Miller
For centuries we have stared up at the mysterious sky and wondered how our Sun and its retinue of planets came into being. As ground-based telescopes became increasingly sophisticated over time, and the technology was developed to dispatch probes into the vast wilderness of unexplored Space, we have learned more and more about our own Solar System–and this new knowledge has provided astronomers with precious information about how our Star and its family of planets and smaller objects were born about 4.56 billion years ago. And, so, we are really searching for the secret of our own origins when we look into the bewitching Universe that exists beyond our own small Earth–and, in so doing, we discover worlds unlike any that exist in our Solar System; worlds that we previously never dreamed could exist, residing in orbits around alien stars beyond our Sun. In October 2017, a team of astronomers announced their new discovery of another “oddball” alien world–a giant planet, the existence of which was previously thought to be extremely unlikely. This discovery of a giant world circling a small star challenges existing formation theories.
NGTS-1b is the largest planet compared to the size of its parent-star ever discovered in the Universe, and its existence contradicts theories proposing that a planet of this enormous size could not be born to such a small star.
This “oddball” system was discovered by astronomers using the state-of-the-art Next-Generation-Transit Survey observing facility, designed to hunt for transiting exoplanets floating in front of the glaring, roiling face of their brilliant parent-stars.
NGTS-1b is a “mere” 600 light-years away from Earth. It is a gas-giant planet, about the same size as our own Solar System’s banded behemoth, Jupiter, and it orbits a star that is only 50% the size of our Sun. The discovery was made by an international team of astronomers, with participants from the University of Warwick in the UK playing a major role. The new research was headed by Dr. Daniel Bayliss and Professor Peter Wheatley from the University of Warwick’s Astronomy and Astrophysics Group.
The enormous NGTS-1b orbits a small star with a radius and mass about half that of our Sun. This type of system is not supposed to exist because a planet of NGTS-1b’s gigantic size should not be able to dwell in the family of such a comparatively tiny star. This is because, according to current theories, small stars can easily form rocky planets–but they are unable to gravitationally grab up enough material to give birth to Jupiter-sized planets.
NGTS-1b is a hot-Jupiter. Hot-Jupiters are exoplanets that are at least as big as our own Solar System’s Jupiter, but sport about 20% less mass. NGTS-1b circles its parent-star fast and close, in a very tight orbit that amounts to only 3% of the distance between Earth and Sun. It also orbits its star every 2.6 days, which means that a year on this exotic exoplanet lasts two and a half days.
The temperature on this jumbo, gassy world is about 800 kelvin.
“The discovery of NGTS-1b was a complete surprise to us–such massive planets were not thought to exist around such small stars. This is the first exoplanet we have found with our new NGTS facility and we are already challenging the received wisdom of how planets form,” explained study lead author, Dr. Daniel Bayless, in an October 31, 2017 Warwick University Press Release.
“Our challenge is to now find out how common these types of planets are in the Galaxy, and with the new NGTS facility we are well-placed to do just that,” Dr. Bayless added.
Solar System Formation
Until recently, astronomers could only observe the end result of planet formation, and not the formation process itself. This is because astronomers were only able to study our own Solar System. Even with the great amount of knowledge acquired about our own Star, as well as its family of planets and smaller objects, many questions remained unanswered, and scientists were left to wonder about this tantalizing mystery. Are there other solar systems out there in the space between stars? If so, did these remote and alien systems form the same way as our own? Recent technologies, such as NASA’s Hubble Space Telescope (HST), have helped astronomers to put together important pieces of the puzzle about the birth of planets.
According to current scientific understanding, a star and its planets are born from an especially dense blob, embedded within the undulating folds of one of the many giant, dark, and cold molecular clouds that swirl around within our Milky Way Galaxy. When the dense blob composed of gas and dust collapses, and gravity relentlessly pulls material in this cloud together, the center of this blob becomes increasingly more and more compressed–and, as it does so, it becomes hotter and hotter. This dense, searing-hot core evolves into a new and bright baby protostar–its stellar fires finally ignited by the process of nuclear fusion.
In the meantime, swirling movements within the collapsing cloud cause it to churn. The rotating cloud eventually undergoes a sea-change into a flattened disk that grows thinner as it spins. This has been compared to the way a spinning blob of pizza dough flattens out into the shape of the pizza-to-be. These “circumstellar” or “protoplanetary accretion disks” become the strange nurseries of newborn planets.
As the protoplanetary accretion disk spins, its material travels around the baby star in the same direction, and eventually the material in the disk starts to stick together. Protoplanetary accretion disks have been observed encircling many fiery baby stars within young stellar clusters. The swirling, flat disks that are composed mostly of gas, with a smaller amount of dust, form at about the same time as the protostar, and these nurturing accretion disks feed the hungry little newborn star a nourishing formula of gas and dust. At this stage of development, the protoplanetary accretion disk is both extremely hot and very massive. These swirling disks can linger around a young star for as long as 10 million years.
By the time a stellar toddler–that is similar to the way our now middle aged Sun was billions of years ago–has reached what is called the T Tauri stage of its development, this nutritious protoplanetary accretion disk has thinned out and cooled off considerably. A T Tauri is a very young, variable star that is less than 10 million years old. T Tauris show large diameters that are several times greater than that of our Sun at present. However, T Tauris shrink as they grow up. By the time the young star has reached this stage, less volatile materials have started to condense near the center of the disk. As a result, very “sticky”, and very fine motes of dust form–and these tiny grains of dust contain crystalline silicates.
Within the swirling disk, flying motes of very “sticky” dust collide with one another, and then merge to form ever larger and larger objects within the crowded environment of the disk. The very fine dust particles eventually merge together to form objects up to several centimenters in size. These larger objects then collide with each other, sticking together, and thus creating what are termed planetesimals. Planetesimals can grow to become 1 kilometer across–or even larger. The planetesimals are an abundant population of objects, and they can travel throughout the entire crowded accretion disk. Some of these very ancient objects can survive long enough to tell the story of a long-vanished era when our Star and its family of planets, moons, and smaller objects were first beginning to form.
Within the inner region of the disk, most of the material at this stage is rocky. This is because much of the original gas has been devoured and cleared out by the voracious, fiery, and growing baby star. The upshot is the formation of rocky, smaller planetesimals floating around close to their young star. Conversely, in the outer portion of the disk, more gas lingers, along with an abundance of ices that haven’t yet been vaporized by the growing, roiling, searing-hot young star. This additional material enables the planetesimals inhabiting the cooler outer regions of the accretion disk to collect more material and evolve into gaseous giants composed of gas and ice. In our own Solar System, the solid inner planets–Mercury, Venus, Earth, and Mars–formed from the rocky planetesimals inhabiting the inner region of the primordial accretion disk. The quartet of enormous gaseous denizens of our Solar System’s outer limits–Jupiter, Saturn, Uranus, and Neptune–formed from the lingering gas and ices of the ancient disk.
As each planetesimal grows in size, it starts to clear out the material in its path, sweeping up slow-moving, nearby rubble and gas, while gravitationally hurling other material out of its way. Ultimately, the debris in the forming planet’s path thins out and the planetesimal gains a relatively clear orbit around its hot young parent-star.
Hundreds of planetesimals form at the same time in the dense disk. As a result, they inevitably bump into each other. If their paths cross at exactly the right time, and if they are traveling fast enough relative to each other, a horrendous head-on smash-up occurs. This violent collision hurls debris everywhere. However, something else happens if the doomed planetesimals dance slowly towards each other. If this occurs, gravity gently draws them together, and as they meet, they merge to form one single, larger planetesimal. If the dancing duo are farther apart, they might not physically interact. However, their gravitational attraction can pull each object off of its original path. This gravitationally induced detour may cause the wayward objects to cross into other lanes of planetesimal traffic–sometimes with catastrophic results. That is because this type of detour sets the stage for additional collisions, as well as other meet-ups and mergers of the rocky, cataclysmic kind.
After millions of years have passed, innumerable encounters between these wandering, wayward planetesimals have managed to sweep out much of the debris. In addition, much larger–and many fewer–objects now dominate their regions of the vanishing protoplanetary accretion disk. A mature planetary system, similar to our own Solar System, is ready to emerge from the chaos, crash-ups, and meet-ups of the natal disk.
In our Solar System, the asteroids and comets are relic planetesimals. There is no reason to think that such lingering leftovers, of the bygone era of planet-building, do not also haunt other planetary systems beyond our own Star.
A Giant “Oddball” Planet That’s Too Big For Its Star
The team of astronomers that spotted the “oddball” NGTS-1b used the state-of-the-art Next Generation Transit Survey (NGTS) to make their discovery. The NGTS is a wide-field observing facility made up of a compact collection of telescopes that have been designed to hunt for transiting exoplanets in the process of floating in front of the fiery, glaring face of their parent-stars. The NGTS is run by the Universities of Warwick, Leicester, Cambridge, and Queen’s University Belfast in the UK, as well as the Observatoire de Geneve (Switzerland), the DLR Berlin (Germany), and the Universidad de Chile (Chile).
The jumbo, gassy world orbits a red M-dwarf, which is the most abundant type of star in the Universe, as well as the longest-lived. This discovery hints that there is a possibility that there could be still more of these planets waiting to be discovered by the NGTS survey.
NGTS-1b has the distinction of being the first planet beyond our Solar System to have been discovered by the NGTS facility, which is located at the European Southern Observatory’s (ESO’s) Paranal Observatory in Northern Chile.
“NGTS-1b was difficult to find, despite being a monster of a planet, because its parent star is small and faint. Small stars are actually the most common in the Universe, so it is possible that there are many of these giant planets waiting to be discovered,” commented Dr. Peter Wheatley in the October 31, 2017, University of Warwick Press Release.
“Having worked for almost a decade to develop the NGTS telescope array, it is thrilling to see it picking out new and unexpected types of planets. I’m looking forward to seeing what other kinds of exciting new planets we can turn up,” Dr. Wheatley added.
The astronomers made their discovery by monitoring patches of the night sky over the course of many months, and spotting red light from the little star with innovative red-sensitive cameras. The scientists noticed tattle-tale dips in the light emanating from the star every 2.6 days. This suggested that a hidden planet was orbiting and periodically blocking out a small amount of its parent-star’s red light.
Using these newly acquired data, the astronomers then went on to track the planet’s orbit around its star–and, from this, they were able to calculate the size, position and mass of NGTS-1b by measuring the radial velocity of its parent-star. This means that the astronomers were finding out many of the “oddball” planet’s well-kept secrets by observing how much its parent-star “wobbles” during its orbit–resulting from the gravitational pull of its planet, which changes depending on the planet’s size.
The research describing this “monster” planet, that is too big for its star, is scheduled for publication in the Monthly Notices Of the Royal Astronomical Society (UK) under the title: NGTS-1b: a hot Jupiter transiting an M-dwarf.
Judith E. Braffman-Miller is a writer and astronomer whose articles have been published since 1981 in various newspapers, journals, and magazines. Although she has written on a variety of topics, she particularly loves writing about astronomy because it gives her the opportunity to communicate to others the many wonders of her field. Her first book, “Wisps, Ashes, and Smoke,” will be published soon.
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