Artist's impression of a red giant star. If the star is in a binary pair, what happens to its sibling? Credit:NASA/ Walt Feimer
We’ve all heard this one: when you drink a glass of water, that water has already been through a bunch of other people’s digestive tracts. Maybe Attila the Hun’s or Vlad the Impaler’s; maybe even a Tyrannosaurus Rex’s.
Well, the same thing is true of stars and matter. All the matter we see around us here on Earth, even our own bodies, has gone through at least one cycle of stellar birth and death, maybe more. But which type of star?
That’s what a team of researchers at ETH Zurich (Ecole polytechnique federale de Zurich) wanted to know.
Collisions of neutron stars produce powerful gamma-ray bursts – and heavy elements like gold (Credit: Dana Berry, SkyWorks Digital, Inc.)
For about a century now, scientists have theorized that the metals in our Universe are the result of stellar nucleosynthesis. This theory states that after the first stars formed, heat and pressure in their interiors led to the creation of heavier elements like silicon and iron. These elements not only enriched future generations of stars (“metallicity”), but also provided the material from which the planets formed.
More recent work has suggested that some of the heaviest elements could actually be the result of binary stars merging. In fact, a recent study by two astrophysicists found that a collision which took place between two neutron stars billions of years ago produced a considerable amount of some of Earth’s heaviest elements. These include gold, platinum and uranium, which then became part of the material from which Earth formed.
A slice from the LaPaz 02342 meteorite which contains dust grains from an ancient comet. Image Credit: Carnegie Institution/Nittler et. al. 2019.
The early days of the Solar System are hard to piece together from our vantage point, billions of years after it happened. Now a team of scientists have found a tiny chunk of an ancient comet inside an ancient meteorite. They say it sheds light on the early days of the Solar System when planets were still forming.
This illustration depicts the spacecraft of NASA's Psyche mission orbiting the metal asteroid Psyche (pronounced SY-kee). Solar power with electric propulsion will be used to propel the spacecraft to Psyche. The asteroid's average distance from the sun is about three times the Earth's distance or 280 million miles. Credit: SSL/ASU/P. Rubin/NASA/JPL-Caltech
As NASA sets its sight on the next generation of space exploration, one area of focus is on missions that can teach us more about our Solar System. This was a major priority during the thirteenth round of NASA’s Discovery Program, which put out the call for proposals back in February of 2014. One of the proposals to make the cut was the Psyche mission, which will send an orbiter to the asteroid of the same name in 2o22.
This mission is unique in that it will entail visiting an asteroid that is entirely composed of metal, which scientists believe is the remnant core of an early planet. For the sake of the mission, NASA’s Glenn Research Center has been working hard to develop a cutting-edge, next-generation thruster that balances power with fuel efficiency. This thruster was recently subjected to tests designed to simulate its journey through space.
Originally discovered in 1852, the object known as 16 Psyche has been a source of fascination ever since astronomers were able to determine its composition. Unlike other asteroids that are largely carbonaceous (C-type), silicate (S-type), or composed of rock and metal (M-type), Psyche is the only asteroid to date that has been found to have an exposed nickel-iron surface.
Because of its unique nature, scientists have theorized that the metal asteroid is actually the core of a Mars-sized planet that formed during the early Solar System. This planet, they theorize, lost its outer layers after experiencing a massive collision, thus leaving an exposed core behind. The study of this asteroid is therefore expected to reveal a great deal about the interior of terrestrial planets and what powers their magnetic fields.
As David Oh, the mission’s lead project systems engineer, said in a recent NASA press release:
“Psyche is a unique body because it is, by far, the largest metal asteroid out there; it’s about the size of Massachusetts. By exploring Psyche, we’ll learn about the formation of the planets, how planetary cores are formed and, just as important, we’ll be exploring a new type of world. We’ve looked at worlds made of rock, ice and of gas, but we’ve never had an opportunity to look at a metal world, so this is brand new exploration in the classic style of NASA.”
The Psyche missions brings together researchers from Arizona State University and experts from NASA’s Jet Propulsion Laboratory. For the sake of designing the engine that would send their spacecraft to its destination, the joint Arizona-JPL team turned to NASA’s Glenn Research Center, which has been conducting research into Solar Electric Propulsion (SEP) for years.
SEP thrusters are essentially ion-engines that rely on electrically-charged inert gases (like xenon) to provide thrust. Like all Hall Effect ion-engines, this allows the thruster to provide a gentle, non-stop stream of thrust that gradually pushes a spacecraft up to greater and greater speeds. Such a system is ideal for deep-space missions where fuel-efficiency is a must.
As Carol Tolbert, the project manager for Psyche thruster testing at NASA Glenn, explained:
“For deep space missions, the type and amount of fuel required to propel a spacecraft is an important factor for mission planners. A SEP system, like the one used for this mission, operates more efficiently than a conventional chemical propulsion system, which would be impractical for this type of mission.”
The Psyche mission, which will be built jointly by JPL and Space Systems Loral (SSL), will use a SPT-140 Hall effect thruster that relies on solar power to provide electrical charges. The reduced fuel mass of this thruster will allow the mission to enter orbit around the metal asteroid while also providing additional space for the mission’s suite of scientific instruments.
These include a multispectral imager, a magnetometer, and a gamma-ray spectrometer, all of which will help the science team to obtain vital information on the asteroid’s origin, composition and history. The SEP also provides flexibility and robustness in the mission flight plan, since it will allow Psyche to get to its destination with greater speed and efficiency than conventional propulsion would allow for.
To test how the thruster performs during low-power operations, engineers at NASA Glenn placed the thruster into a space environment chamber designed to generate the low-pressures and temperatures it will encounter in space. As Carol explained:
“This mission will be the first to use a Hall effect thruster system beyond lunar orbit, so the tests here at Glenn, which had never been conducted before, were needed to ensure the thruster could perform and operate as expected in the deep space environment.”
Artist’s impression of the surface of 16 Psyche. Credit: Arizona State University / NASA
For decades, the Glenn center has used its compliment of chambers to simulate the conditions missions will encounter in space. However, this test is the first time that engineers have sought to determine how an SEP Hall-Effect thruster would fare. As Oh explained, this test is very important since it will simulate precisely how the spacecraft will fly, and the results have been encouraging so far:
“Glenn has a world-class facility that allowed us to go to very low pressures to simulate the environment the spacecraft will operate in and better understand how our thrusters will perform around Psyche. At first glance, the results confirm our predictions regarding how the thruster will perform, and it looks like everything is working as expected. But, we will continue to refine our models by doing more analysis.”
As the team works towards the mission’s proposed launch – which is scheduled for August 2022 – they will use the data collected at NASA Glenn to update their thruster modeling and incorporate it into mission trajectories. Once the spacecraft reaches its destination – the planned arrival will take place by 2026 – it is expected to reveal a great deal about this unique asteroid.
This data is also likely to teach us much about the history of the Asteroid Belt and the Solar System. If indeed 16 Psyche is the remnant of a Mars-sized planet that formed in the Main Belt, it could cause astronomers to rethink their notions of how the Solar System formed and evolved.
This artist’s impression shows exocomets orbiting the star Beta Pictoris. Credit:
ESO/L. Cacada
Between the years 2003 and 2011, the High Accuracy Radial velocity Planet Searcher – better known as HARPS – made more than a thousand observations of nearby star, Beta Pictoris. On board the ESO 3.6-metre telescope at the La Silla Observatory in Chile, the sensitive instrument normally combs the sky nightly in search of exoplanets, but lately it has contributed to another astounding discovery… exocomets!
Located about 63 light-years from the Sun, Beta Pictoris is a youthful star, estimated to be only around 20 million years old. Keeping it company in space is a vast disc of material. This swarm of gas and dust is the beginnings of an active planetary system and was likely created by the destruction of comets and collisions of rocky bodies like asteroids. Now a French team using HARPS has been able to create the most complete catalog of comets to date from this system. Researchers have found no less than five hundred comets belonging to Beta Pictoris and they divide in two unique branches of exocomets. Split into both old and new, these two active flows behave much like our own cometary groups… They have either made many trips around the parent star or are the product of a recent breakup of one or more objects.
Flavien Kiefer (IAP/CNRS/UPMC), lead author of the new study, sets the scene: “Beta Pictoris is a very exciting target! The detailed observations of its exocomets give us clues to help understand what processes occur in this kind of young planetary system.”
Beta Pictoris is located about 60 light-years away towards the constellation of Pictor (the Painter’s Easel) and is one of the best-known examples of a star surrounded by a dusty debris disc. Earlier observations showed a warp of the disc, a secondary inclined disc, and comets falling onto the star, all indirect, but tell-tale signs that strongly suggested the presence of a massive planet. Observations done with the NACO instrument on ESO’s Very Large Telescope in 2003, 2008, and 2009, have proven the presence of a planet around Beta Pictoris. It is located at a distance between 8 and 15 times the Earth-Sun separation — or Astronomical Units — which is about the distance Saturn is from the Sun. The planet has a mass of about nine Jupiter masses and the right mass and location to explain the observed warp in the inner parts of the disc. This image, based on data from the Digitized Sky Survey 2, shows a region of approximately 1.7 x 2.3 degrees around Beta Pictoris. Credit: ESO/Sky Survey II
Just like discovering planets through the transit method, astronomers believe exocomets can cause a disturbance in the amount of light we can see from a given star. When these icy travelers exhaust themselves, their gas and dust tails could absorb a portion of the star light passing through them. For nearly three decades scientists had been aware of minute changes in the light from Beta Pictoris, but attributing it to comets was next to impossible to prove. Their tiny light was simply overpowered by the light of the star and could not be imaged from Earth.
Enter HARPS…
Using more than a thousand observations taken by this sensitive equipment, astronomers chose a sample of 493 exocomets unrelated to each other, but sharing in the Beta Pictoris system. Of these, some were dutifully followed for hours at several different times. The size and speed of the gas clouds produced were carefully measured. Researchers were even able to document the orbital properties of some of these exocomets – the size and shape of their passage paths in relation to the parent star allowing scientists to infer their distances.
Knowing that comets exist around other stars is very exciting – and knowing that solar systems around other stars work much like our own is downright rewarding. Through this study, we’re able to take a unique look at what might be several hundreds of exocomets connected to a solitary exo-planet system. What the research has revealed is two distinct branches of the comet family tree. One of these is old comets – their orbit dictated by a single, massive planet. The other half of the family fork belongs to comets that might have arisen from the destruction of a larger object.
The older group behaves in a predictable manner. These exocomets have differing orbital patterns, and their gas and dust production is greatly reduced. If they follow the same rules as the ones in our solar system, it’s typical behavior for a comet which has exhausted its volatiles during multiple trips around the parent star and is also being controlled by the system’s massive planet. This is exciting because it confirms the planet’s presence and distance!
“Moreover, the orbits of these comets (eccentricity and orientation) are exactly as predicted for comets trapped in orbital resonance with a massive planet.” says the science team. “The properties of the comets of the first family show that this planet in resonance must be at about 700 million kilometres from the star – close to where the planet Beta Pictoris b was discovered.”
The second group also behaves in a predictable manner. These exocomets have nearly identical orbits and their emissions are active and radical. Observations of this cometary type tell us they more than likely originated from the destruction of a larger body and the rubble is caught in a orbit which allows the fragments to graze Beta Pictoris. According to the research team: “This makes them similar to the comets of the Kreutz family in the Solar System, or the fragments of Comet Shoemaker-Levy 9, which impacted Jupiter in July 1994.”
Flavien Kiefer concludes: “For the first time a statistical study has determined the physics and orbits for a large number of exocomets. This work provides a remarkable look at the mechanisms that were at work in the Solar System just after its formation 4.5 billion years ago.”
Artist’s impression of a fifth giant planet being ejected from the solar system. Image credit: Southwest Research Institute
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Earth’s place in the “Goldilocks” zone of our solar system may be the result of the expulsion of a fifth giant planet from our solar system during its first 600 million years, according to a recent journal publication.
“We have all sorts of clues about the early evolution of the solar system,” said author Dr. David Nesvorny of the Southwest Research Institute. “They come from the analysis of the trans-Neptunian population of small bodies known as the Kuiper Belt, and from the lunar cratering record.”
Nesvorny and his team used the clues they had to build computer simulations of the early solar system and test their theories. What resulted was an early solar system model that has quite a different configuration than today, and a jumbling of planets that may have given Earth the “preferred” spot for life to evolve.
Researchers interpret the clues as evidence that the orbits of Jupiter, Saturn, Uranus and Neptune were affected by a dynamical instability when our solar system was only about half a billion years old. This instability is believed to have helped increase the distance between the giant planets, along with scattering smaller bodies. The scattering of small bodies pushed objects both inward, and outward with some objects ending up in the Kuiper Belt and others impacting the terrestrial planets and the Moon. Jupiter is believed to have scattered objects outward as it moved in towards the sun.
One problem with this interpretation is that slow changes to Jupiter’s orbit would most likely add too much momentum to the orbits of the terrestrial planets. The additional momentum would have possibly caused a collision of Earth with Venus or Mars.
“Colleagues suggested a clever way around this problem,” said Nesvorny. “They proposed that Jupiter’s orbit quickly changed when Jupiter scattered off of Uranus or Neptune during the dynamical instability in the outer solar system.”
Basically if Jupiter’s early migration “jumps,” the orbital coupling between the terrestrial planets and Jupiter is weaker, and less harmful to the inner solar system.
Animation showing the evolution of the planetary system from 20 million years before the ejection to 30 million years after. Five initial planets are shown by red circles, small bodies are in green. After the fifth planet is ejected, the remaining four planets stabilize after a while, and looks like the outer solar system in the end, with giant planets at 5, 10, 20 and 30 astronomical units. Click image to view animation. Image Credit: Southwest Research Institute
Nesvorny and his team performed thousands of computer simulations that attempted to model the early solar system in an effort to test the “jumping-Jupiter” theory. Nesvorny found that Jupiter did in fact jump due to gravitational interactions from Uranus or Neptune, but when Jupiter jumped, either Uranus or Neptune were expelled from the solar system. “Something was clearly wrong,” he said.
Based on his early results, Nesvorny added a fifth giant planet, similar to Uranus or Neptune to his simulations. Once he ran the reconfigured simulations, everything fell into place. The simulation showed the fifth planet ejected from the solar system by Jupiter, with four giant planets remaining, and the inner, terrestrial planets untouched.
Nesvorny concluded with, “The possibility that the solar system had more than four giant planets initially, and ejected some, appears to be conceivable in view of the recent discovery of a large number of free-floating planets in interstellar space, indicating the planet ejection process could be a common occurrence.”
One of 42 new proplyds discovered in the Orion Nebula, 177-341E is one of the bright proplyds that lies relatively close to the nebula’s brightest star, Theta 1 Orionis C. The tadpole-shaped tail is actually a jet of matter flowing away from the excited cusp. Credit:NASA/ESA and L. Ricci (ESO)
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When it comes to solar systems, chances are good that we’re a lot more special than we thought. According to a German-British team led by Professor Pavel Kroupa of the University of Bonn, our orderly neighborhood of varied planet sizes quietly orbiting in a nearly circular path isn’t a standard affair. Their new models show that habitable planets might just get ejected in a violent scenario where forming solar systems mean highly inclined orbits where hot Jupiters rule.
Some 4600 million years ago, our local planetary system was surmised to have evolved from a blanket of dust surrounding a rather ordinary star. Its planets orbited the same direction as the solar spin and lined up neatly on a plane fairly close to the solar equator. We were good little children… But maybe other systems aren’t so hospitable. There could be systems where the planets cruise around in the opposite direction of their host star’s spin – and have highly inclined orbits. What could cause one protoplanetary disk to take on quiet properties while another is more radical? Try a cosmic crash.
This new study focuses on the theory of a protoplanetary disk colliding with another cloud of material… not unrealistic thinking since most stars form within a cluster. The results could mean the inclusion of up to thirty times the mass of Jupiter. This added “weight” of extra gas and dust could add a tilt to a forming system. Team member Dr Ingo Thies, also of the University of Bonn, has carried out computer simulations to test the new idea. What he has found is that adding extra material can not only incline a forming disk, but cause a reverse spin as well. It may even speed up the planetary formation, leaving the rogues in retrograde orbits. This inhospitable scenario means that smaller planets get ejected systematically, leaving only hot Jupiters to hug in close to the parent star. Thankfully our path was a bit less disturbing.
Says Dr Thies: “Like most stars, the Sun formed in a cluster, so probably did encounter another cloud of gas and dust soon after it formed. Fortunately for us, this was a gentle collision, so the effect on the disk that eventually became the planets was relatively benign. If things had been different, an unstable planetary system may have formed around the Sun, the Earth might have been ejected from the Solar System and none of us would be here to talk about it.”
Professor Kroupa sees the model as a big step forward. “We may be on the cusp of solving the mystery of why some planetary systems are tilted so much and lack places where life could thrive. The model helps to explain why our Solar System looks the way it does, with the Earth in a stable orbit and larger planets further out. Our work should help other scientists refine their search for life elsewhere in the Universe.”
