Astronomy

Could an ejected “extra ice-giant” still be lurking in distant solar orbit?

Could an ejected “extra ice-giant” still be lurking in distant solar orbit?

BACKGROUND

Hot Jupiters are thought to have migrated inwards, implying that another giant planet has been ejected in order to conserve the orbital momentum of those planetary systems. The number of interstellar "vagabond planets" has been estimated to be as many as the number of stars. And simulations show that if the Solar System started out with an extra gas giant next to Jupiter which was ejected, then one can end up with a very good fit with today's Solar System.

QUESTIONS

Where would such a hypothetical ejected ice giant be today? Should it be closer or further away than hot Jupiter-ejected planets because it is much smaller? Could it stay in an Oort cloud type orbit during several billion years (maybe ~15 orbits around the Milky Way) without being pulled off by passing stars? At what distance could we today observe a Uranus sized planet? Should it stay in the ecliptical plane or could its orbit have become greatly inclined when it was ejected?


Since I'm a hobbyist, I usually wait to see if someone a bit smarter wants to answer first, but I can give a couple thoughts on this.

Hot Jupiters are thought to have migrated inwards, implying that another giant planet has been ejected in order to conserve the orbital momentum of those planetary systems.

In the article you posted (I'll pull the quote)

The conservation of orbital momentum dictates, as planetesimals move from the outer disk inward, that Saturn, Uranus and Neptune must move outward. This process is known as the planetesimal driven migration

The conservation of orbital momentum he's talking about happens when objects cross each others orbits and interact, one gains orbital energy, the other loses energy. That's not the only way planetary orbits can change. Over time, there's tidal effects, orbital resonance and a sun can lose mass leading to gradual drifting or perhaps a sun can gain mass if the solar system passes through a dense dust/gas cloud.

But mostly what we're talking about here is a form of gravity assist as 2 planets get close enough that they kind of swing past each other, one moves in the other out.

More on that here: http://science.howstuffworks.com/nomad-planet2.htm

Now, were gas giants ejected from our solar system, according to this question, somebody said no:

Is there any evidence that the Gas Giant planets in our solar system are experiencing orbital migration?

But, this article, and the article you posted that suggests otherwise, that it probobly did happen based on models, but when the solar system was young, like about 4 billion years ago, this model suggests the Solar system could have lost 1 or 2 ice giants.

http://www.space.com/13584-extra-giant-planet-solar-system.html

and we could still lose Mercury (though we probobly won't). http://www.universetoday.com/14032/could-jupiter-wreck-the-solar-system/

Where would such a hypothetical ejected ice giant be today? Should it be closer or further away than hot Jupiter-ejected planets because it is much smaller?

Tough call. Using basic gravity assist mathematics, the closer to the sun the faster an object could be ejected from the solar system so a hot Jupiter probobly eject inner planets at a faster velocity on average than more distant Jupiter, but the bigger factor is the size of the star system, not the size of the planet. The more massive the star, the faster the orbital velocities, the faster the Ejection is likely to be, even after accounting for escape velocity.

it's also important that the object being ejected is smaller than the object doing the ejecting, for example, Jupiter could pull Mercury away from the sun but Mercury couldn't pull Jupiter into the sun. More massive wins the tug of war, but how much smaller the ejected object needs to be isn't that relevant. Jupiter could throw a Neptune sized planet out of a solar system at almost the same speed as it could throw a Mercury sized planet out of a solar system.

Now, as to where it would be, planets aren't likely to escape a solar system at too fast a speed. Jupiter orbits the sun at 47,000 KM/Hour and an optimal gravity assist could eject something way from Jupiter at 94,000 KM/Hour, (plus any velocity of the object's own, which would depend largely on eccentricity, but since it would cross Jupiter's path, probably wouldn't be hugely different than Jupiter's), and if you subtract escape velocity from the sun at that distance (47,000 * square root of 2), or 66.4 KM/Hour, your left with a maximum exit the solar system velocity of, roughly 28,000 KM/Hour, plus a bit - and that's maximum. Average escape velocity would likely be less, but lets say 20,000 KM/Hour, which works out to be 1 light year every 50,000 years. considering it left the solar system an estimated 4 billion years ago, that's 80,000 light years, which from where we are, is pretty much the far end of the Milky way and who can say how it's orbital velocity might have changed over that time as it passed by other stars, but basically, it could be anywhere in the Milky way, and, if you consider, our sun might have come from the Sagittarius dwarf galaxy, not the Milky way, it could be practically anywhere, even ejected from the Milky way. 4 billion years is a long time, even for a slow ejection.

or, it could be orbiting another sun - article on that here: http://www.universetoday.com/94656/rogue-planets-can-find-homes-around-other-stars/

Could it stay in an Oort cloud type orbit during several billion years (maybe ~15 orbits around the Milky Way) without being pulled off by passing stars?

Entirely possible though billions of years is a long time. Passing stars usually don't pass very close, but I'm sure it's happened at least a few times in the past 4 billion years.

close enough to disrupt the inner planets - that's different and more rare. Close enough to disrupt the Oort cloud, probobly happens every so often - but I wouldn't want to guess how often.

At what distance could we today observe a Uranus sized planet? Should it stay in the ecliptical plane or could its orbit have become greatly inclined when it was ejected?

it depends if we know where to look. If we saw it pass in-front of a star and we could plot it's path, then a number of light-years. Looking into the blackness of space, much less distance, and it would depend on the planet's reflectivity too. I wouldn't even know what to guess, but Eris, for example is 1/20th the diameter of Uranus and it's one of the larger Kuiper belt objects. With relative ease, they could likely spot a Uranus sized object at well over 20 times the distance of Eris. Now, if the question is, what's the furthest a Uranus sized object could be and not have been seen yet - that's hard for me to say but an object that size and at that distance would likely have it's own Trojans and I find it hard to believe it would have gone unnoticed.

as to "would it stay in an elliptical plane" - not necessarily. A gravitational assist around a planet can alter the elliptical plane depending on the angle of approach.

hope that's not too wordy. I can try to tidy up later.


Distant Dwarf Planet Discovered Beyond the Known Edge of Our Solar System

This is an orbit diagram for the outer solar system. The Sun and Terrestrial planets are at the center. The orbits of the four giant planets, Jupiter, Saturn, Uranus and Neptune, are shown by blue solid circles. The Kuiper Belt, including Pluto, is shown by the gray region just beyond the giant planets. Sedna’s orbit is shown in orange while 2012 VP113’s orbit is shown in green.

Using ground based observatories, scientists discovered a distant dwarf planet, dubbed 2012 VP113, located beyond the known edge of our solar system.

Washington, D.C.—The Solar System has a new most-distant member, bringing its outer frontier into focus.

New work from Carnegie’s Scott Sheppard and Chadwick Trujillo of the Gemini Observatory reports the discovery of a distant dwarf planet, called 2012 VP113, which was found beyond the known edge of the Solar System. This is likely one of thousands of distant objects that are thought to form the so-called inner Oort cloud. What’s more, their work indicates the potential presence of an enormous planet, perhaps up to 10 times the size of Earth, not yet seen, but possibly influencing the orbit of 2012 VP113, as well as other inner Oort cloud objects.

The known Solar System can be divided into three parts: the rocky planets like Earth, which are close to the Sun the gas giant planets, which are further out and the frozen objects of the Kuiper belt, which lie just beyond Neptune’s orbit. Beyond this, there appears to be an edge to the Solar System where only one object, Sedna, was previously known to exist for its entire orbit. But the newly found 2012 VP113 has an orbit that stays even beyond Sedna, making it the furthest known in the Solar System.

“This is an extraordinary result that redefines our understanding of our Solar System,” says Linda Elkins-Tanton, director of Carnegie’s Department of Terrestrial Magnetism.

Three images of the night sky, each taken about two hours apart, were combined into one. The first image was artificially colored red, second green and third blue. 2012 VP113 moved between each image as seen by the red, green and blue dots. The background stars and galaxies did not move and thus their red, green and blue images combine to showup as white sources. Courtesy Scott Sheppard and Chad Trujillo.

Sedna was discovered beyond the Kuiper Belt edge in 2003, and it was not known if Sedna was unique, as Pluto once was thought to be before the Kuiper Belt was discovered. With the discovery of 2012 VP113 it is now clear Sedna is not unique and is likely the second known member of the hypothesized inner Oort cloud, the likely origin of some comets.

2012 VP113’s closest orbit point to the Sun brings it to about 80 times the distance of the Earth from the Sun, a measurement referred to as an astronomical unit or AU. For context, the rocky planets and asteroids exist at distances ranging between .39 and 4.2 AU. Gas giants are found between 5 and 30 AU, and the Kuiper belt (composed of thousands of icy objects, including Pluto) ranges from 30 to 50 AU. In our solar system there is a distinct edge at 50 AU. Only Sedna was known to stay significantly beyond this outer boundary at 76 AU for its entire orbit.

“The search for these distant inner Oort cloud objects beyond Sedna and 2012 VP113 should continue, as they could tell us a lot about how our Solar System formed and evolved,” says Sheppard.

Sheppard and Trujillo used the new Dark Energy Camera (DECam) on the NOAO 4 meter telescope in Chile for discovery. DECam has the largest field-of-view of any 4-meter or larger telescope, giving it unprecedented ability to search large areas of sky for faint objects. The Magellan 6.5-meter telescope at Carnegie’s Las Campanas Observatory was used to determine the orbit of 2012 VP113 and obtain detailed information about its surface properties.

From the amount of sky searched, Sheppard and Trujillo determine that about 900 objects with orbits like Sedna and 2012 VP113 and sizes larger than 1000 km may exist and that the total population of the inner Oort cloud is likely bigger than that of the Kuiper Belt and main asteroid belt.

“Some of these inner Oort cloud objects could rival the size of Mars or even Earth. This is because many of the inner Oort cloud objects are so distant that even very large ones would be too faint to detect with current technology”, says Sheppard.

Both Sedna and 2012 VP113 were found near their closest approach to the Sun, but they both have orbits that go out to hundreds of AU, at which point they would be too faint to discover. In fact, the similarity in the orbits found for Sedna, 2012 VP113 and a few other objects near the edge of the Kuiper Belt suggests that an unknown massive perturbing body may be shepherding these objects into these similar orbital configurations. Sheppard and Trujillo suggest a Super Earth or an even larger object at hundreds of AU could create the shepherding effect seen in the orbits of these objects, which are too distant to be perturbed significantly by any of the known planets.

There are three competing theories for how the inner Oort cloud might have formed. As more objects are found, it will be easier to narrow down which of these theories is most likely accurate. One theory is that a rogue planet could have been tossed out of the giant planet region and could have perturbed objects out of the Kuiper Belt to the inner Oort cloud on its way out. This planet could have been ejected or still be in the distant solar system today. The second theory is that a close stellar encounter could have put objects into the inner Oort cloud region. A third theory suggests inner Oort cloud objects are captured extra-solar planets from other stars that were near our Sun in its birth cluster.

The outer Oort cloud is distinguished from the inner Oort cloud because in the outer Oort cloud, starting around 1500 AU, the gravity from other nearby stars perturbs the orbits of the objects, causing objects in the outer Oort cloud to have orbits that change drastically over time. Many of the comets we see were objects that were perturbed out of the outer Oort cloud. Inner Oort cloud objects are not highly affected by the gravity of other stars and thus have more stable and more primordial orbits.

Publication: Chadwick A. Trujillo & Scott S. Sheppard, “A Sedna-like body with a perihelion of 80 astronomical units,” Nature 507, 471–474 (27 March 2014) doi:10.1038/nature13156

Image: Carnegie Institution for Science Scott Sheppard and Chad Trujillo


Astrophysicists find Jupiter likely bumped giant planet from solar system

This is Jupiter's Great Red Spot in 2000 as seen by NASA's Cassini orbiter. Credit: NASA/JPL/Space Science Institute

It's like something out of an interplanetary chess game. Astrophysicists at the University of Toronto have found that a close encounter with Jupiter about four billion years ago may have resulted in another planet's ejection from the Solar System altogether.

The existence of a fifth giant gas planet at the time of the Solar System's formation - in addition to Jupiter, Saturn, Uranus and Neptune that we know of today - was first proposed in 2011. But if it did exist, how did it get pushed out?

For years, scientists have suspected the ouster was either Saturn or Jupiter.

"Our evidence points to Jupiter," said Ryan Cloutier, a PhD candidate in U of T's Department of Astronomy & Astrophysics and lead author of a new study published in The Astrophysical Journal.

Planet ejections occur as a result of a close planetary encounter in which one of the objects accelerates so much that it breaks free from the massive gravitational pull of the Sun. However, earlier studies which proposed that giant planets could possibly eject one another did not consider the effect such violent encounters would have on minor bodies, such as the known moons of the giant planets, and their orbits.

So Cloutier and his colleagues turned their attention to moons and orbits, developing computer simulations based on the modern-day trajectories of Callisto and lapetus, the regular moons orbiting around Jupiter and Saturn respectively. They then measured the likelihood of each one producing its current orbit in the event that its host planet was responsible for ejecting the hypothetical planet, an incident which would have caused significant disturbance to each moon's original orbit.

"Ultimately, we found that Jupiter is capable of ejecting the fifth giant planet while retaining a moon with the orbit of Callisto," said Cloutier, who is also a graduate fellow at the Centre for Planetary Sciences at the University of Toronto at Scarborough. "On the other hand, it would have been very difficult for Saturn to do so because Iapetus would have been excessively unsettled, resulting in an orbit that is difficult to reconcile with its current trajectory."

The findings are reported in a paper titled "Could Jupiter or Saturn have ejected a fifth giant planet?" published in the November 1 issue of The Astrophysical Journal.


Solar system has a new most-distant member

The Solar System has a new most-distant member, bringing its outer frontier into focus.

New work from Carnegie's Scott Sheppard and Chadwick Trujillo of the Gemini Observatory reports the discovery of a distant dwarf planet, called 2012 VP113, which was found beyond the known edge of the Solar System. This is likely one of thousands of distant objects that are thought to form the so-called inner Oort cloud. What's more, their work indicates the potential presence of an enormous planet, perhaps up to 10 times the size of Earth, not yet seen, but possibly influencing the orbit of 2012 VP113, as well as other inner Oort cloud objects.

Their findings are published March 27 in Nature.

The known Solar System can be divided into three parts: the rocky planets like Earth, which are close to the Sun the gas giant planets, which are further out and the frozen objects of the Kuiper belt, which lie just beyond Neptune's orbit. Beyond this, there appears to be an edge to the Solar System where only one object, Sedna, was previously known to exist for its entire orbit. But the newly found 2012 VP113 has an orbit that stays even beyond Sedna, making it the furthest known in the Solar System.

"This is an extraordinary result that redefines our understanding of our Solar System," says Linda Elkins-Tanton, director of Carnegie's Department of Terrestrial Magnetism.

Sedna was discovered beyond the Kuiper Belt edge in 2003, and it was not known if Sedna was unique, as Pluto once was thought to be before the Kuiper Belt was discovered. With the discovery of 2012 VP113 it is now clear Sedna is not unique and is likely the second known member of the hypothesized inner Oort cloud, the likely origin of some comets.

2012 VP113's closest orbit point to the Sun brings it to about 80 times the distance of Earth from the Sun, a measurement referred to as an astronomical unit or AU. For context, the rocky planets and asteroids exist at distances ranging between .39 and 4.2 AU. Gas giants are found between 5 and 30 AU, and the Kuiper belt (composed of thousands of icy objects, including Pluto) ranges from 30 to 50 AU. In our solar system there is a distinct edge at 50 AU. Only Sedna was known to stay significantly beyond this outer boundary at 76 AU for its entire orbit.

"The search for these distant inner Oort cloud objects beyond Sedna and 2012 VP113 should continue, as they could tell us a lot about how our Solar System formed and evolved," says Sheppard.

Sheppard and Trujillo used the new Dark Energy Camera (DECam) on the NOAO 4 meter telescope in Chile for discovery. DECam has the largest field-of-view of any 4-meter or larger telescope, giving it unprecedented ability to search large areas of sky for faint objects. The Magellan 6.5-meter telescope at Carnegie's Las Campanas Observatory was used to determine the orbit of 2012 VP113 and obtain detailed information about its surface properties.

From the amount of sky searched, Sheppard and Trujillo determine that about 900 objects with orbits like Sedna and 2012 VP113 and sizes larger than 1000 km may exist and that the total population of the inner Oort cloud is likely bigger than that of the Kuiper Belt and main asteroid belt.

"Some of these inner Oort cloud objects could rival the size of Mars or even Earth. This is because many of the inner Oort cloud objects are so distant that even very large ones would be too faint to detect with current technology," says Sheppard.

Both Sedna and 2012 VP113 were found near their closest approach to the Sun, but they both have orbits that go out to hundreds of AU, at which point they would be too faint to discover. In fact, the similarity in the orbits found for Sedna, 2012 VP113 and a few other objects near the edge of the Kuiper Belt suggests that an unknown massive perturbing body may be shepherding these objects into these similar orbital configurations. Sheppard and Trujillo suggest a Super Earth or an even larger object at hundreds of AU could create the shepherding effect seen in the orbits of these objects, which are too distant to be perturbed significantly by any of the known planets.

There are three competing theories for how the inner Oort cloud might have formed. As more objects are found, it will be easier to narrow down which of these theories is most likely accurate. One theory is that a rogue planet could have been tossed out of the giant planet region and could have perturbed objects out of the Kuiper Belt to the inner Oort cloud on its way out. This planet could have been ejected or still be in the distant solar system today. The second theory is that a close stellar encounter could have put objects into the inner Oort cloud region. A third theory suggests inner Oort cloud objects are captured extra-solar planets from other stars that were near our Sun in its birth cluster.

The outer Oort cloud is distinguished from the inner Oort cloud because in the outer Oort cloud, starting around 1500 AU, the gravity from other nearby stars perturbs the orbits of the objects, causing objects in the outer Oort cloud to have orbits that change drastically over time. Many of the comets we see were objects that were perturbed out of the outer Oort cloud. Inner Oort cloud objects are not highly affected by the gravity of other stars and thus have more stable and more primordial orbits.


Solar System's Edge Redefined

Washington, D.C.—The Solar System has a new most-distant member, bringing its outer frontier into focus.

New work from Carnegie's Scott Sheppard and Chadwick Trujillo of the Gemini Observatory reports the discovery of a distant dwarf planet, called 2012 VP113, which was found beyond the known edge of the Solar System. This is likely one of thousands of distant objects that are thought to form the so-called inner Oort cloud. What's more, their work indicates the potential presence of an enormous planet, perhaps up to 10 times the size of Earth, not yet seen, but possibly influencing the orbit of 2012 VP113, as well as other inner Oort cloud objects.

Their findings are published March 27 in Nature.

The known Solar System can be divided into three parts: the rocky planets like Earth, which are close to the Sun the gas giant planets, which are further out and the frozen objects of the Kuiper belt, which lie just beyond Neptune's orbit. Beyond this, there appears to be an edge to the Solar System where only one object, Sedna, was previously known to exist for its entire orbit. But the newly found 2012 VP113 has an orbit that stays even beyond Sedna, making it the furthest known in the Solar System.

"This is an extraordinary result that redefines our understanding of our Solar System," says Linda Elkins-Tanton, director of Carnegie's Department of Terrestrial Magnetism.

Sedna was discovered beyond the Kuiper Belt edge in 2003, and it was not known if Sedna was unique, as Pluto once was thought to be before the Kuiper Belt was discovered. With the discovery of 2012 VP113 it is now clear Sedna is not unique and is likely the second known member of the hypothesized inner Oort cloud, the likely origin of some comets.

2012 VP113’s closest orbit point to the Sun brings it to about 80 times the distance of the Earth from the Sun, a measurement referred to as an astronomical unit or AU. For context, the rocky planets and asteroids exist at distances ranging between .39 and 4.2 AU. Gas giants are found between 5 and 30 AU, and the Kuiper belt (composed of thousands of icy objects, including Pluto) ranges from 30 to 50 AU. In our solar system there is a distinct edge at 50 AU. Only Sedna was known to stay significantly beyond this outer boundary at 76 AU for its entire orbit.

“The search for these distant inner Oort cloud objects beyond Sedna and 2012 VP113 should continue, as they could tell us a lot about how our Solar System formed and evolved," says Sheppard.

Sheppard and Trujillo used the new Dark Energy Camera (DECam) on the NOAO 4 meter telescope in Chile for discovery. DECam has the largest field-of-view of any 4-meter or larger telescope, giving it unprecedented ability to search large areas of sky for faint objects. The Magellan 6.5-meter telescope at Carnegie’s Las Campanas Observatory was used to determine the orbit of 2012 VP113 and obtain detailed information about its surface properties.

From the amount of sky searched, Sheppard and Trujillo determine that about 900 objects with orbits like Sedna and 2012 VP113 and sizes larger than 1000 km may exist and that the total population of the inner Oort cloud is likely bigger than that of the Kuiper Belt and main asteroid belt.

“Some of these inner Oort cloud objects could rival the size of Mars or even Earth. This is because many of the inner Oort cloud objects are so distant that even very large ones would be too faint to detect with current technology”, says Sheppard.

Both Sedna and 2012 VP113 were found near their closest approach to the Sun, but they both have orbits that go out to hundreds of AU, at which point they would be too faint to discover. In fact, the similarity in the orbits found for Sedna, 2012 VP113 and a few other objects near the edge of the Kuiper Belt suggests that an unknown massive perturbing body may be shepherding these objects into these similar orbital configurations. Sheppard and Trujillo suggest a Super Earth or an even larger object at hundreds of AU could create the shepherding effect seen in the orbits of these objects, which are too distant to be perturbed significantly by any of the known planets.

There are three competing theories for how the inner Oort cloud might have formed. As more objects are found, it will be easier to narrow down which of these theories is most likely accurate. One theory is that a rogue planet could have been tossed out of the giant planet region and could have perturbed objects out of the Kuiper Belt to the inner Oort cloud on its way out. This planet could have been ejected or still be in the distant solar system today. The second theory is that a close stellar encounter could have put objects into the inner Oort cloud region. A third theory suggests inner Oort cloud objects are captured extra-solar planets from other stars that were near our Sun in its birth cluster.

The outer Oort cloud is distinguished from the inner Oort cloud because in the outer Oort cloud, starting around 1500 AU, the gravity from other nearby stars perturbs the orbits of the objects, causing objects in the outer Oort cloud to have orbits that change drastically over time. Many of the comets we see were objects that were perturbed out of the outer Oort cloud. Inner Oort cloud objects are not highly affected by the gravity of other stars and thus have more stable and more primordial orbits.

Caption: This is an orbit diagram for the outer solar system. The Sun and Terrestrial planets are at the center. The orbits of the four giant planets, Jupiter, Saturn, Uranus and Neptune, are shown by purple solid circles. The Kuiper Belt, including Pluto, is shown by the dotted light blue region just beyond the giant planets. Sedna's orbit is shown in orange while 2012 VP113's orbit is shown in red. A larger version is available here. Another image is available here.
__________________

Acquisition of data used in this study was supported by NASA. Observations were partly obtained at Cerro Tololo Inter-American Observatory, National Optical Astronomy Observatory, operated by the Association of Universities for Research in Astronomy, under contract with the National Science Foundation. This paper also includes data gathered with the 6.5-meter Magellan Telescopes located at Las Campanas Observatory, Chile.


A new object at the edge of our Solar System discovered

These are the discovery images of 2012 VP113, affectionately called 'Biden' because of the VP in the provisional name. It has the most distant orbit known in our Solar System. Three images of the night sky, each taken about two hours apart, were combined into one. The first image was artificially colored red, second green and third blue. 2012 VP113 moved between each image as seen by the red, green and blue dots. The background stars and galaxies did not move and thus their red, green and blue images combine to showup as white sources. Credit: Scott Sheppard and Chad Trujillo

The Solar System has a new most-distant member, bringing its outer frontier into focus.

New work from Carnegie's Scott Sheppard and Chadwick Trujillo of the Gemini Observatory reports the discovery of a distant dwarf planet, called 2012 VP113, which was found beyond the known edge of the Solar System. This is likely one of thousands of distant objects that are thought to form the so-called inner Oort cloud. What's more, their work indicates the potential presence of an enormous planet, perhaps up to 10 times the size of Earth, not yet seen, but possibly influencing the orbit of 2012 VP113, as well as other inner Oort cloud objects.

Their findings are published March 27 in Nature.

The known Solar System can be divided into three parts: the rocky planets like Earth, which are close to the Sun the gas giant planets, which are further out and the frozen objects of the Kuiper belt, which lie just beyond Neptune's orbit. Beyond this, there appears to be an edge to the Solar System where only one object, Sedna, was previously known to exist for its entire orbit. But the newly found 2012 VP113 has an orbit that stays even beyond Sedna, making it the furthest known in the Solar System.

"This is an extraordinary result that redefines our understanding of our Solar System," says Linda Elkins-Tanton, director of Carnegie's Department of Terrestrial Magnetism.

These images show the discovery of the new inner Oort cloud object 2012 VP113 taken about 2 hours apart on UT November 5, 2012. The motion of 2012 VP113 clearly stands out compared to the steady state background stars and galaxies. Credit: Scott S. Sheppard: Carnegie Institution for Science

Sedna was discovered beyond the Kuiper Belt edge in 2003, and it was not known if Sedna was unique, as Pluto once was thought to be before the Kuiper Belt was discovered. With the discovery of 2012 VP113 it is now clear Sedna is not unique and is likely the second known member of the hypothesized inner Oort cloud, the likely origin of some comets.

2012 VP113's closest orbit point to the Sun brings it to about 80 times the distance of the Earth from the Sun, a measurement referred to as an astronomical unit or AU. For context, the rocky planets and asteroids exist at distances ranging between .39 and 4.2 AU. Gas giants are found between 5 and 30 AU, and the Kuiper belt (composed of thousands of icy objects, including Pluto) ranges from 30 to 50 AU. In our solar system there is a distinct edge at 50 AU. Only Sedna was known to stay significantly beyond this outer boundary at 76 AU for its entire orbit.

This is an orbit diagram for the outer solar system. The Sun and Terrestrial planets are at the center. The orbits of the four giant planets, Jupiter, Saturn, Uranus and Neptune, are shown by purple solid circles. The Kuiper Belt, including Pluto, is shown by the dotted light blue region just beyond the giant planets. Sedna's orbit is shown in orange while 2012 VP113's orbit is shown in red. Both objects are currently near their closest approach to the Sun (perihelion). They would be too faint to detect when in the outer parts of their orbits. Notice that both orbits have similar perihelion locations on the sky and both are far away from the giant planet and Kuiper Belt regions. Credit: Scott Sheppard and Chad Trujillo

"The search for these distant inner Oort cloud objects beyond Sedna and 2012 VP113 should continue, as they could tell us a lot about how our Solar System formed and evolved," says Sheppard.

Sheppard and Trujillo used the new Dark Energy Camera (DECam) on the NOAO 4 meter telescope in Chile for discovery. DECam has the largest field-of-view of any 4-meter or larger telescope, giving it unprecedented ability to search large areas of sky for faint objects. The Magellan 6.5-meter telescope at Carnegie's Las Campanas Observatory was used to determine the orbit of 2012 VP113 and obtain detailed information about its surface properties.

From the amount of sky searched, Sheppard and Trujillo determine that about 900 objects with orbits like Sedna and 2012 VP113 with sizes larger than 1000 km may exist and that the total population of the inner Oort cloud is likely bigger than that of the Kuiper Belt and main asteroid belt.

"Some of these inner Oort cloud objects could rival the size of Mars or even Earth. This is because many of the inner Oort cloud objects are so distant that even very large ones would be too faint to detect with current technology", says Sheppard.

Both Sedna and 2012 VP113 were found near their closest approach to the Sun, but they both have orbits that go out to hundreds of AU, at which point they would be too faint to discover. In fact, the similarity in the orbits found for Sedna, 2012 VP113 and a few other objects near the edge of the Kuiper Belt suggests that an unknown massive perturbing body may be shepherding these objects into these similar orbital configurations. Sheppard and Trujillo suggest a Super Earth or an even larger object at hundreds of AU could create the shepherding effect seen in the orbits of these objects, which are too distant to be perturbed significantly by any of the known planets.

There are three competing theories for how the inner Oort cloud might have formed. As more objects are found, it will be easier to narrow down which of these theories is most likely accurate. One theory is that a rogue planet could have been tossed out of the giant planet region and could have perturbed objects out of the Kuiper Belt to the inner Oort cloud on its way out. This planet could have been ejected or still be in the distant solar system today. The second theory is that a close stellar encounter could put objects into the inner Oort cloud region. A third theory suggests inner Oort cloud objects are captured extra-solar planets from other stars that were near our Sun in its birth cluster.

The outer Oort cloud is distinguished from the inner Oort cloud because in the outer Oort cloud, starting around 1500 AU, the gravity from other nearby stars perturbs the orbits of the objects, causing objects in the outer Oort cloud to have orbits that change drastically over time. Many of the comets we see were objects that were perturbed out of the outer Oort cloud. Inner Oort cloud objects are not highly affected by the gravity of other stars and thus have more stable and more primordial orbits.


Formation

The leading idea for the formation of the Oort Cloud says that these icy objects were not always so far from the Sun. After the planets formed 4.6 billion years ago, the region in which they formed still contained lots of leftover chunks called planetesimals. Planetesimals formed from the same material as the planets did. The gravity of the planets (primarily Jupiter) then scattered the planetesimals every which way.

Some planetesimals were ejected from the solar system entirely, while others were flung into eccentric orbits where they were still held by the Sun&rsquos gravity, but were far enough out that galactic influences also tugged on them. Likely the strongest influence was the tidal force from our galaxy itself.

In short, gravity from the planets shoved many icy planetesimals away from the Sun, and gravity from the galaxy likely caused them to settle in the borderlands of the solar system, where the planets couldn&rsquot perturb them anymore. And they became what we now call the Oort Cloud. Again, that&rsquos the leading idea, but the Oort Cloud could also capture objects that didn&rsquot form in the solar system.


A new ’Oumuamua theory could mean many more interstellar visitors are headed our way

‘Oumuamua sure looked like an oddball, but similar planetary tatters could be floating everywhere.

A close approach to its host star could have torn a comet into smaller pieces. Perhaps one of those slivers found its way into our galaxy. ESO/M. Kornmesser

‘Oumuamua shattered astronomers’ expectations when it streaked past the sun in 2017. It was skinny, not round. It looked rather dry with ruddy hue—nothing like the ice ball it should have resembled. To make matters worse, it jetted away as if it were moving under its own power. The first interstellar object to be spotted in our solar system exploded researchers’ assumptions about what sorts of bodies are most likely to escape their host star, and how. Three years later, they’re still trying to figure out where they went wrong.

New results based on computer simulations, which appeared Monday in Nature Astronomy, aim to tie up all ‘Oumuamua’s mysteries into one explanatory package: once upon a time, a distant star shredded a comet or planetary fragment, spraying thin comet-asteroid hybrids out into the void. If the theory proves accurate, then ‘Oumuamua represents just one of a countless number of such objects, ejected by similar stars across the galaxy.

“We are confident that this scenario we proposed is common,” says Yun Zhang, a researcher at the Observatoire de la Côte d’Azur in France and co-author of the work. “We expect we will see more things like ‘Oumuamua in the future.”

Astronomers have been anxiously awaiting the first signs of an interstellar interloper for the better part of a decade, but it wasn’t supposed to look anything like ‘Oumuamua. It was supposed to be a comet—a dusty snowball that would cast off a dramatic tail as it melted in the sun’s glow. Comets in our solar system follow looping orbits that push them right to the edge of the sun’s domain, where the slightest gravitational nudge could knock them free. (Nature obliged this intuition last year when Borisov—a classic comet—became the second interstellar object to be discovered in our solar system. It has since sizzled into pieces after swinging around the sun.)

‘Oumuamua looked more like an asteroid, the other type of small body common in the solar system. These pebbly sand piles are dark and dry, with little water or ice for the sun to mess with. Numerous attempts have failed to find signs of a tail behind ‘Oumuamua, although it did inexplicably speed off as if being pushed by a jet of gas. What was driving its hasty retreat? And if it was an asteroid, which tend to huddle close to their host star, what had kicked it out into space?

Neither the “comet” nor the “asteroid” label fit well, suggesting that something new had formed around ‘Oumuamua’s host star. Guesses ranged from alien spaceship to fluffy ice cloud. “It’s really a mystery,” says Matija Ćuk, an astronomer at the SETI Institute. “We really don’t understand it.”

In late 2017, Ćuk suggested that a phenomenon called “tidal disruption” could have birthed the cigar-shaped object. Tidal forces lie behind gravity’s ability to distort a body, such as when the moon stretches the Earth’s girth, resulting in the daily rise and fall of oceans against the shore. They’re also one of the many ways a black hole could kill an astronaut, stretching her out like a piece of spaghetti. Ćuk proposed that a similar event could have shredded an entire planet into ribbon-like strands, and that ‘Oumuamua was a piece of flotsam from one such Armageddon.

Zhang’s work took Ćuk’s idea further, fleshing it out with physical details to see if it was possible. She and a colleague modified a popular simulation that considers how the particles in an asteroid-like object—which she likens to “a sandcastle floating in space”—rearrange themselves when poked and prodded by gravity. They took a variety of objects and repeatedly hurled them at a digital star until they got a sense of what might have happened to ‘Oumuamua.

A sample product of the tidal disruption simulation does its best Oumuamua impression. NAOC/Y. Zhang (background: ESO/M. Kornmesser)

It probably started out life as a comet, Zhang says. A planet or baby planet is also possible, but those objects would be less likely to match ‘Oumuamua’s inferred composition. As the comet swung by its host star, which was perhaps half as massive as the sun, it received a tidal death squeeze and got shredded. Zhang also analyzed the fragments with a heat model, and found that the host star’s warmth would have baked the surface of each fragment into a dry and crispy crust. The same squeeze would have sent some splinters, including ‘Oumuamua, flying out into interstellar space.

While she set out only to explain the ex-comet’s shape, Zhang was surprised to find that the theory could also handle ‘Oumuamua’s bizarre acceleration. Upon arrival in our solar system, additional heat from our heavier, brighter sun reached deep through the crust and liberated residual ice, the thinking goes. Those evaporating materials then gave the object a push as they escaped unseen into space. “Our analysis showed that the simulation can explain all of ‘Oumuamua’s features,” she said, its dry asteroid-like appearance as well as its comet-like activity.

The researchers can’t be sure that this precise sequence of events definitely produced the collection of rubble that passed through three years ago. But if the process plays out as easily as the simulation suggests, interstellar space could be littered with a truly incomprehensible number of such shards. Each solar system could spew out one hundred trillion ‘Oumuamua-like objects, Zhang estimates.

For other researchers, however, ‘Oumuamua remains enigmatic. Ćuk says that it’s nice to see a simulation confirming that tidal disruption can roll cigar-shaped bodies, but wonders how often stars can really tear up their comets. “If you shred one planet you get a lot of mass,” he says. “If you’re playing with comets, you need to shred something like 10 earth masses per star, which is basically all of [the comets].”

Zhang also points out that the whole theory rests upon the assumption that ‘Oumuamua is long and skinny. If the object is more pancake than sausage, as one analysis suggested last summer, the whole tidal origin story falls to pieces.

The scout is currently zooming past Uranus, beyond the reach of any telescope, so researchers will never know for sure what ‘Oumuamua looks like or how it formed. But the Vera C. Rubin Observatory, which is slated to begin sweeping and regular surveys in 2022, should spot one interstellar object each year, according to Ćuk. Only then will astronomers be able to get a sense of what sorts of objects are whizzing around out there. “Before you do physics, you do stamp collecting,” he says. “Let’s see what else falls out of the sky.”

Charlie Woodis a journalist covering developments in the physical sciences both on and off the planet. In addition to Popular Science, his work has appeared in Quanta Magazine, Scientific American, The Christian Science Monitor, and other publications. Previously, he taught physics and English in Mozambique and Japan, and studied physics at Brown University. You can view his website here.


Contents

When a galaxy or a planetary system forms, its material takes the shape of a disk. Most of the material orbits and rotates in one direction. This uniformity of motion is due to the collapse of a gas cloud. [1] The nature of the collapse is explained by conservation of angular momentum. In 2010 the discovery of several hot Jupiters with backward orbits called into question the theories about the formation of planetary systems. [2] This can be explained by noting that stars and their planets do not form in isolation but in star clusters that contain molecular clouds. When a protoplanetary disk collides with or steals material from a cloud this can result in retrograde motion of a disk and the resulting planets. [3] [4]

Orbital inclination Edit

A celestial object's inclination indicates whether the object's orbit is prograde or retrograde. The inclination of a celestial object is the angle between its orbital plane and another reference frame such as the equatorial plane of the object's primary. In the Solar System, inclination of the planets is measured from the ecliptic plane, which is the plane of Earth's orbit around the Sun. [5] The inclination of moons is measured from the equator of the planet they orbit. An object with an inclination between 0 and 90 degrees is orbiting or revolving in the same direction as the primary is rotating. An object with an inclination of exactly 90 degrees has a perpendicular orbit that is neither prograde nor retrograde. An object with an inclination between 90 degrees and 180 degrees is in a retrograde orbit.

Axial tilt Edit

A celestial object's axial tilt indicates whether the object's rotation is prograde or retrograde. Axial tilt is the angle between an object's rotation axis and a line perpendicular to its orbital plane passing through the object's centre. An object with an axial tilt up to 90 degrees is rotating in the same direction as its primary. An object with an axial tilt of exactly 90 degrees has a perpendicular rotation that is neither prograde nor retrograde. An object with an axial tilt between 90 degrees and 180 degrees is rotating in the opposite direction to its orbital direction. Regardless of inclination or axial tilt, the north pole of any planet or moon in the Solar System is defined as the pole that is in the same celestial hemisphere as Earth's north pole.

Planets Edit

All eight planets in the Solar System orbit the Sun in the direction of the Sun's rotation, which is counterclockwise when viewed from above the Sun's north pole. Six of the planets also rotate about their axis in this same direction. The exceptions – the planets with retrograde rotation – are Venus and Uranus. Venus's axial tilt is 177°, which means it is rotating almost exactly in the opposite direction to its orbit. Uranus has an axial tilt of 97.77°, so its axis of rotation is approximately parallel with the plane of the Solar System. The reason for Uranus's unusual axial tilt is not known with certainty, but the usual speculation is that during the formation of the Solar System, an Earth-sized protoplanet collided with Uranus, causing the skewed orientation. [6]

It is unlikely that Venus was formed with its present slow retrograde rotation, which takes 243 days. Venus probably began with a fast prograde rotation with a period of several hours much like most of the planets in the Solar System. Venus is close enough to the Sun to experience significant gravitational tidal dissipation, and also has a thick enough atmosphere to create thermally driven atmospheric tides that create a retrograde torque. Venus's present slow retrograde rotation is in equilibrium balance between gravitational tides trying to tidally lock Venus to the Sun and atmospheric tides trying to spin Venus in a retrograde direction. In addition to maintaining this present day equilibrium, tides are also sufficient to account for evolution of Venus's rotation from a primordial fast prograde direction to its present-day slow retrograde rotation. [7] In the past, various alternative hypotheses have been proposed to explain Venus's retrograde rotation, such as collisions or it having originally formed that way. [a]

Despite being closer to the Sun than Venus, Mercury is not tidally locked because it has entered a 3:2 spin–orbit resonance due to the eccentricity of its orbit. Mercury's prograde rotation is slow enough that due to its eccentricity, its angular orbital velocity exceeds its angular rotational velocity near perihelion, causing the motion of the sun in Mercury's sky to temporarily reverse. [8] The rotations of Earth and Mars are also affected by tidal forces with the Sun, but they have not reached an equilibrium state like Mercury and Venus because they are further out from the Sun where tidal forces are weaker. The gas giants of the Solar System are too massive and too far from the Sun for tidal forces to slow down their rotations. [7]

Dwarf planets Edit

All known dwarf planets and dwarf planet candidates have prograde orbits around the Sun, but some have retrograde rotation. Pluto has retrograde rotation its axial tilt is approximately 120 degrees. [9] Pluto and its moon Charon are tidally locked to each other. It is suspected that the Plutonian satellite system was created by a massive collision. [10] [11]

Natural satellites and rings Edit

If formed in the gravity field of a planet as the planet is forming, a moon will orbit the planet in the same direction as the planet is rotating and is a regular moon. If an object is formed elsewhere and later captured into orbit by a planet's gravity, it can be captured into either a retrograde or prograde orbit depending on whether it first approaches the side of the planet that is rotating towards or away from it. This is an irregular moon. [12]

In the Solar System, many of the asteroid-sized moons have retrograde orbits, whereas all the large moons except Triton (the largest of Neptune's moons) have prograde orbits. [13] The particles in Saturn's Phoebe ring are thought to have a retrograde orbit because they originate from the irregular moon Phoebe.

All retrograde satellites experience tidal deceleration to some degree. The only satellite in the Solar System for which this effect is non-negligible is Neptune's moon Triton. All the other retrograde satellites are on distant orbits and tidal forces between them and the planet are negligible.

Within the Hill sphere, the region of stability for retrograde orbits at a large distance from the primary is larger than that for prograde orbits. This has been suggested as an explanation for the preponderance of retrograde moons around Jupiter. Because Saturn has a more even mix of retrograde/prograde moons, however, the underlying causes appear to be more complex. [14]

With the exception of Hyperion, all the known regular planetary natural satellites in the Solar System are tidally locked to their host planet, so they have zero rotation relative to their host planet, but have the same type of rotation as their host planet relative to the Sun because they have prograde orbits around their host planet. That is, they all have prograde rotation relative to the Sun except those of Uranus.

If there is a collision, material could be ejected in any direction and coalesce into either prograde or retrograde moons, which may be the case for the moons of dwarf planet Haumea, although Haumea's rotation direction is not known. [15]

Asteroids Edit

Asteroids usually have a prograde orbit around the Sun. Only a few dozen asteroids in retrograde orbits are known.

Some asteroids with retrograde orbits may be burnt-out comets, [16] but some may acquire their retrograde orbit due to gravitational interactions with Jupiter. [17]

Due to their small size and their large distance from Earth it is difficult to telescopically analyse the rotation of most asteroids. As of 2012, data is available for less than 200 asteroids and the different methods of determining the orientation of poles often result in large discrepancies. [18] The asteroid spin vector catalog at Poznan Observatory [19] avoids use of the phrases "retrograde rotation" or "prograde rotation" as it depends which reference plane is meant and asteroid coordinates are usually given with respect to the ecliptic plane rather than the asteroid's orbital plane. [20]

Asteroids with satellites, also known as binary asteroids, make up about 15% of all asteroids less than 10 km in diameter in the main belt and near-Earth population and most are thought to be formed by the YORP effect causing an asteroid to spin so fast that it breaks up. [21] As of 2012, and where the rotation is known, all satellites of asteroids orbit the asteroid in the same direction as the asteroid is rotating. [22]

Most known objects that are in orbital resonance are orbiting in the same direction as the objects they are in resonance with, however a few retrograde asteroids have been found in resonance with Jupiter and Saturn. [23]

Comets Edit

Comets from the Oort cloud are much more likely than asteroids to be retrograde. [16] Halley's Comet has a retrograde orbit around the Sun. [24]

Kuiper belt objects Edit

Most Kuiper belt objects have prograde orbits around the Sun. The first Kuiper belt object discovered to have a retrograde orbit was 2008 KV 42 . [25] Other Kuiper belt objects with retrograde orbits are (471325) 2011 KT19, [26] (342842) 2008 YB 3 , (468861) 2013 LU 28 and 2011 MM4. [27] All of these orbits are highly tilted, with inclinations in the 100°–125° range.

Meteoroids Edit

Meteoroids in a retrograde orbit around the Sun hit the Earth with a faster relative speed than prograde meteoroids and tend to burn up in the atmosphere and are more likely to hit the side of the Earth facing away from the Sun (i.e. at night) whereas the prograde meteoroids have slower closing speeds and more often land as meteorites and tend to hit the Sun-facing side of the Earth. Most meteoroids are prograde. [28]

Orbital motion of the Sun Edit

The Sun's motion about the centre of mass of the Solar System is complicated by perturbations from the planets. Every few hundred years this motion switches between prograde and retrograde. [29]

Retrograde motion, or retrogression, within the Earth's atmosphere is seen in weather systems whose motion is opposite the general regional direction of airflow, i.e. from east to west against the westerlies or from west to east through the trade wind easterlies. Prograde motion with respect to planetary rotation is seen in the atmospheric super-rotation of the thermosphere of Earth and in the upper troposphere of Venus. Simulations indicate that the atmosphere of Pluto should be dominated by winds retrograde to its rotation. [30]

Artificial satellites destined for low inclination orbits are usually launched in the prograde direction, since this minimizes the amount of propellant required to reach orbit by taking advantage of the Earth's rotation (an equatorial launch site is optimal for this effect). However, Israeli Ofeq satellites are launched in a westward, retrograde direction over the Mediterranean to ensure that launch debris does not fall onto populated land areas.

Stars and planetary systems tend to be born in star clusters rather than forming in isolation. Protoplanetary disks can collide with or steal material from molecular clouds within the cluster and this can lead to disks and their resulting planets having inclined or retrograde orbits around their stars. [3] [4] Retrograde motion may also result from gravitational interactions with other celestial bodies in the same system (See Kozai mechanism) or a near-collision with another planet, [1] or it may be that the star itself flipped over early in their system's formation due to interactions between the star's magnetic field and the planet-forming disk. [31] [32]

The accretion disk of the protostar IRAS 16293-2422 has parts rotating in opposite directions. This is the first known example of a counterrotating accretion disk. If this system forms planets, the inner planets will likely orbit in the opposite direction to the outer planets. [33]

WASP-17b was the first exoplanet that was discovered to be orbiting its star opposite to the direction the star is rotating. [34] A second such planet was announced just a day later: HAT-P-7b. [35]

In one study more than half of all the known hot Jupiters had orbits that were misaligned with the rotation axis of their parent stars, with six having backwards orbits. [2]

The last few giant impacts during planetary formation tend to be the main determiner of a terrestrial planet's rotation rate. During the giant impact stage, the thickness of a protoplanetary disk is far larger than the size of planetary embryos so collisions are equally likely to come from any direction in three dimensions. This results in the axial tilt of accreted planets ranging from 0 to 180 degrees with any direction as likely as any other with both prograde and retrograde spins equally probable. Therefore, prograde spin with small axial tilt, common for the solar system's terrestrial planets except for Venus, is not common for terrestrial planets in general. [36]

The pattern of stars appears fixed in the sky, insofar as human vision is concerned this is because their massive distances relative to the Earth result in motion imperceptible to the naked eye. In reality, stars orbit the center of their galaxy.

Stars with an orbit retrograde relative to a disk galaxy's general rotation are more likely to be found in the galactic halo than in the galactic disk. The Milky Way's outer halo has many globular clusters with a retrograde orbit [37] and with a retrograde or zero rotation. [38] The structure of the halo is the topic of an ongoing debate. Several studies have claimed to find a halo consisting of two distinct components. [39] [40] [41] These studies find a "dual" halo, with an inner, more metal-rich, prograde component (i.e. stars orbit the galaxy on average with the disk rotation), and a metal-poor, outer, retrograde (rotating against the disc) component. However, these findings have been challenged by other studies, [42] [43] arguing against such a duality. These studies demonstrate that the observational data can be explained without a duality, when employing an improved statistical analysis and accounting for measurement uncertainties.

The nearby Kapteyn's Star is thought to have ended up with its high-velocity retrograde orbit around the galaxy as a result of being ripped from a dwarf galaxy that merged with the Milky Way. [44]

Satellite galaxies Edit

Close-flybys and mergers of galaxies within galaxy clusters can pull material out of galaxies and create small satellite galaxies in either prograde or retrograde orbits around larger galaxies. [45]

A galaxy called Complex H, which was orbiting the Milky Way in a retrograde direction relative to the Milky Way's rotation, is colliding with the Milky Way. [46] [47]

Counter-rotating bulges Edit

NGC 7331 is an example of a galaxy that has a bulge that is rotating in the opposite direction to the rest of the disk, probably as a result of infalling material. [48]

Central black holes Edit

The center of a spiral galaxy contains at least one supermassive black hole. [49] A retrograde black hole – one whose spin is opposite to that of its disk – spews jets much more powerful than those of a prograde black hole, which may have no jet at all. Scientists have produced a theoretical framework for the formation and evolution of retrograde black holes based on the gap between the inner edge of an accretion disk and the black hole. [50] [51] [52]


'Oumuamua is not an alien spacecraft: study

In this artist's concept, the interstellar object 'Oumuamua is depicted as a cigar-shaped body. A new analysis strongly suggests that 'Oumuamua has a natural origin and is not an alien spacecraft. Credit: ESO/M. Kornmesser

On October 19, 2017, astronomers discovered the first known interstellar object to visit our solar system. First spotted by the Panoramic Survey Telescope and Rapid Response System 1 (PanSTARRS1) telescope located at the University of Hawaii's Haleakala Observatory, the object defied easy description, simultaneously displaying characteristics of both a comet and an asteroid.

Astronomers formally named the object 1I/2017 U1 and appended the common name 'Oumuamua, which roughly translates to "scout" in Hawaiian. Researchers from around the world raced to collect as much data as possible before 'Oumuamua traveled beyond the reach of Earth's telescopes. In all, they had only a few weeks to observe the strange visitor.

Early reports of 'Oumuamua's odd characteristics led some to speculate that the object could be an alien spacecraft, sent from a distant civilization to examine our star system. But a new analysis co-led by Matthew Knight, an associate research scientist in the University of Maryland Department of Astronomy, strongly suggests that 'Oumuamua has a purely natural origin. The research team reported their findings in the July 1, 2019, issue of the journal Nature Astronomy.

"We have never seen anything like 'Oumuamua in our solar system. It's really a mystery still," Knight said. "But our preference is to stick with analogs we know, unless or until we find something unique. The alien spacecraft hypothesis is a fun idea, but our analysis suggests there is a whole host of natural phenomena that could explain it."

As Knight and his colleagues summarized in their study, 'Oumuamua is red in color, similar to many small objects observed in our solar system. But that's where the familiarity ends.

'Oumuamua likely has an elongated, cigarlike shape and an odd spin pattern—much like a soda bottle laying on the ground, spinning on its side. According to Knight, its motion through our solar system is particularly puzzling. While it appeared to accelerate along its trajectory—a typical feature of comets—astronomers could find no evidence of the gaseous emissions that typically create this acceleration.

"The motion of 'Oumuamua didn't simply follow gravity along a parabolic orbit as we would expect from an asteroid," Knight said. "But visually, it hasn't ever displayed any of the cometlike characteristics we'd expect. There is no discernable coma—the cloud of ice, dust and gas that surrounds active comets—nor a dust tail or gas jets."

This artist's impression shows the first interstellar object discovered in the Solar System, Oumuamua. Observations made with the NASA/ESA Hubble Space Telescope, CFHT, and others, show that the object is moving faster than predicted while leaving the Solar System.The inset shows a color composite produced by combining 192 images obtained through three visible and two near-infrared filters totaling 1.6 hours of integration on October 27, 2017, at the Gemini South telescope. Credit: ESA/Hubble, NASA, ESO/M. Kornmesser, Gemini Observatory/AURA/NSF

Knight worked with Alan Fitzsimmons, an astronomer at Queen's University Belfast in Northern Ireland, to assemble a team of 14 astronomers hailing from the U.S. and Europe. The International Space Science Institute in Bern, Switzerland, served as a virtual home base for the collaboration.

"We put together a strong team of experts in various different areas of work on 'Oumuamua. This cross-pollination led to the first comprehensive analysis and the best big-picture summary to date of what we know about the object," Knight explained. "We tend to assume that the physical processes we observe here, close to home, are universal. And we haven't yet seen anything like 'Oumuamua in our solar system. This thing is weird and admittedly hard to explain, but that doesn't exclude other natural phenomena that could explain it."

The new research paper is primarily an analysis of existing data, including a December 2017 study of 'Oumuamua's shape and spin pattern co-authored by Knight and a team of UMD astronomers. This paper, published in The Astrophysical Journal Letters, relied on data from the Discovery Channel Telescope (DCT) at the Lowell Observatory in Arizona. UMD is a scientific partner of the DCT, along with Boston University, the University of Toledo and Northern Arizona University.

Knight, Fitzsimmons and their colleagues considered a number of mechanisms by which 'Oumuamua could have escaped from its home system. For example, the object could have been ejected by a gas giant planet orbiting another star. According to theory, Jupiter may have created the Oort cloud—a massive shell of small objects at the outer edge of our solar system—in this way. Some of those objects may have slipped past the influence of the sun's gravity to become interstellar travelers themselves.

The research team suspects that 'Oumuamua could be the first of many interstellar visitors. Knight is looking forward to data from the Large Synoptic Survey Telescope (LSST), which is scheduled to be operational in 2022.

"In the next 10 years, we expect to begin seeing more objects like 'Oumuamua. The LSST will be leaps and bounds beyond any other survey we have in terms of capability to find small interstellar visitors," Knight said. "We may start seeing a new object every year. That's when we'll start to know whether 'Oumuamua is weird, or common. If we find 10-20 of these things and 'Oumuamua still looks unusual, we'll have to reexamine our explanations."