Chasing ‘Oumuamua

Chasing ‘Oumuamua

Written by Elizabeth Landau:

The interstellar object ‘Oumuamua perplexed scientists in October 2017 as it whipped past Earth at an unusually high speed. This mysterious visitor is the first object ever seen in our solar system that is known to have originated elsewhere.

What we know

-It came from outside the solar system — Because of its high speed (196,000 mph, or 87.3 kilometers per second) and the trajectory it followed as it whipped around the Sun, scientists are confident ‘Oumuamua originated beyond our solar system. The object flew by Earth so fast its speed couldn’t be due to the influence of the Sun’s gravity alone, so it must have approached the solar system at an already high speed and not interacted with any other planets. On its journey past our star, the object came within a quarter of the distance between the Sun and Earth.

-Its trajectory is hyperbolic — By tracking this object as it passed within view of telescopes, scientists can see that this high-speed object won’t be captured by our Sun’s gravity. It won’t circle back around again on an elliptical path. Instead, it will follow the shape of a hyperbola — that is, it will keep on going out of the solar system, and never come back.

-It doesn’t look like a comet, but it behaves like one — A comet is a small icy body that, when heated by the Sun, develops a coma — a fuzzy atmosphere and tail made of volatile material vaporizing off the comet body. At first, scientists assumed ‘Oumuamua was a comet. But because ‘Oumuamua appears in telescope images as a single point of light without a coma, scientists then concluded it was an asteroid. But when astronomers saw the object was accelerating ever so slightly, they realized that a coma and jets might not be visible to the telescopes used to observe it. The jetting of volatile materials or “outgassing” would explain why ‘Oumuamua was accelerating in a subtle, unexpected way when only gravity from our solar system is taken into account.

It must be elongated — While it is impossible to take a close-up photo of ‘Oumuamua, its dramatic variations in brightness over time suggest it is highly elongated. By calculating what kind of object could dim and brighten in this way, scientists realized the object must be up to 10 times as long as it is wide. Currently, ‘Oumuamua is estimated to be about half a mile (800 meters) long. Astronomers had never seen a natural object with such extreme proportions in the solar system before.

It tumbles through space — The unusual brightness variations also suggest the object does not rotate around just one axis. Instead, it is tumbling — not just end over end, but about a second axis at a different period, too. A small object’s rotation state can easily change, especially if it is outgassing, so this tumbling behavior could have started recently. The object appears to make a complete rotation every 7.3 hours.

What we don’t know

-What does it look like? All that astronomers have seen of ‘Oumuamua is a single point of light. But because of its trajectory and small-scale accelerations, it must be smaller than typical objects from the Oort Cloud, the giant group of icy bodies that orbit the solar system roughly 186 billion miles (300 billion kilometers) away from the Sun. Oort Cloud objects formed in our own solar system, but were kicked out far beyond the planets by the immense gravity of Jupiter. They travel slower than ‘Oumuamua and will forever be bound by the gravity of our Sun. But besides its elongated nature, scientists do not know what kinds of features ‘Oumuamua has on its surface, if any. An elongated shape would explain its rotation behavior, but its exact appearance is unknown.

-What is it made of? Comets from our solar system have a lot of dust, but because none is visible coming off ‘Oumuamua, scientists conclude it may not have very much at all. It is impossible to know what materials make up ‘Oumuamua, but it could have gases such as carbon monoxide or carbon dioxide coming off the surface that are less likely to produce a visible coma or tail.

Where did it come from? ‘Oumuamua came into our solar system from another star system in the galaxy, but which one? Scientists observe that its incoming speed was close to the average motion of stars near our own, and since the speed of younger stars is more stable than older stars, ‘Oumuamua may have come from a relatively young system. But this is still a guess — it is possible the object has been wandering around the galaxy for billions of years.

What is it doing now? After January 2018, ‘Oumuamua was no longer visible to telescopes, even in space. But scientists continue to analyze it and crack open more mysteries about this unique interstellar visitor.

 

SETI Observations of ‘Oumuamua

Radio SETI Observations of the Interstellar Object ‘Oumuamua

Motivated by the hypothesis that ‘Oumuamua could conceivably be an interstellar probe, we used the Allen Telescope Array to search for radio transmissions that would indicate a non-natural origin for this object. Observations were made at radio frequencies between 1-10 GHz using the Array’s correlator receiver with a channel bandwidth of 100 kHz. In frequency regions not corrupted by man-made interference, we find no signal flux with frequency-dependent lower limits of 0.01 Jy at 1 GHz and 0.1 Jy at 7 GHz. For a putative isotropic transmitter on the object, these limits correspond to transmitter powers of 30 mW and 300 mW, respectively. In frequency ranges that are heavily utilized for satellite communications, our sensitivity to weak signals is badly impinged, but we can still place an upper limit of 10 W for a transmitter on the asteroid. For comparison and validation should a transmitter be discovered, contemporaneous measurements were made on the solar system asteroids uz2017 and wc2017 with comparable sensitivities. Because they are closer to Earth, we place upper limits on transmitter power to be 0.1 and 0.001 times the limits for ‘Oumuamua. A concurrent set of observations over the same frequency range were made with a narrow-band (1 Hz) beamformer/spectrometer. Setting a 6.5 sigma threshold, the (frequency dependent) sensitivity limits on ‘Oumuamua were in the range 175 +/- 25 Jy into a 1 Hz bin. This rules out 1 Hz transmitters on ‘Oumuamua, 2017uz, and 2017wc to less than 500 mW, 50 mW, and 0.5 mW respectively over the frequency range from 1-10 GHz.

 

Another interstellar object

This asteroid came from another solar system—and it’s here to stay

By Daniel Clery |

While astronomers around the world had their eyes fixed last year on ‘Oumuamua, a lump of rock from another planetary system that whizzed through ours, little did they know that another interstellar interloper was quietly living among us. And this one appears to have been here for billions of years.

Astronomers first spotted the object, an asteroid called 2015 BZ509 that is orbiting close to Jupiter, in 2014. They knew it was unusual because it was traveling around the solar system in the opposite direction as almost everything else. (Its motion is shown in the animations above, with 2015 BZ509 circled.) Astronomers have found other objects in “retrograde” orbits, perhaps knocked off course by passing too close to a giant planet, but 2015 BZ509’s orbit was the weirdest of all because it is also elongated and out of alignment with the planets and other bodies.

To find out why, a pair of astronomers ran a series of 1 million simulations of the asteroid’s orbit, each with slightly different parameters. Jupiter’s orbit is a busy part of the solar system where the risk of being knocked off course is high, so eccentric orbits that are stable long term are unlikely. But the researchers found a number of possible orbits that were stable and concluded it is much more likely that 2015 BZ509 is in one of them, rather than that it happened to arrive for a short-term visit. Some of those stable orbits, if wound back in time, would mean that 2015 BZ509 has been with us since the beginning of our solar system, about 4.5 billion years ago.

There is no known mechanism that could have produced 2015 BZ509 in such an orbit when the planets were forming, the researchers report today in the Monthly Notices of the Royal Astronomical Society: Letters. Instead, the asteroid must have been drifting through space and was captured by the sun’s gravity.

That’s not as far-fetched as it seems. The sun and its planets formed inside a closely packed cluster of stars—which have since moved on—and any object ejected from one planetary system by gravitational interaction could conceivably end up in another. The discovery of 2015 BZ509 quietly living in the solar system suggests we should look again at some other oddball asteroids; the team’s simulations suggest that some of them may be interstellar interlopers as well. And if space agencies deemed it worth visiting one of them, we could find out whether other planetary systems are made of the same stuff as ours.

 

The Excited Spin State of `Oumuamua

The Excited Spin State of 1I/2017 U1 `Oumuamua

We show that `Oumuamua’s excited spin could be in a high energy Long Axis Mode (LAM) state, which implies that its shape could be far from the highly elongated shape found in previous studies. CLEAN and ANOVA algorithms are used to analyze `Oumuamua’s lightcurve using 818 observations over 29.3 days. Two fundamental periodicities are found at frequencies (2.77±0.11) and (6.42±0.18) cycles/day, corresponding to (8.67±0.34) h and (3.74±0.11) h, respectively. The phased data show that the lightcurve does not repeat in a simple manner, but approximately shows a double minimum at 2.77 cycles/day and a single minimum at 6.42 cycles/day. This is characteristic of an excited spin state. `Oumuamua could be spinning in either the long (LAM) or short (SAM) axis mode. For both, the long axis precesses around the total angular momentum vector (TAMV) with an average period of (8.67±0.34) h. For the three LAMs we have found, the possible rotation periods around the long axis are 6.58, 13.15, or 54.48 h, with 54.48 h being the most likely. `Oumuamua may also be nutating with respective periods of half of these values. We have also found two possible SAM states where `Oumuamua oscillates around the long axis with possible periods at 13.15 and 54.48 h, the latter as the most likely. In this case any nutation will occur with the same periods. Determination of the spin state, the amplitude of the nutation, the direction of the TAMV, and the average total spin period may be possible with a direct model fit to the lightcurve. We find that ‘Oumuamua is “cigar-shaped”, if close to its lowest rotational energy, and an extremely “oblate spheroid” if close to its highest energy state for its total angular momentum.

 

‘Oumuamua came from two stars

Mysterious asteroid from beyond our solar system probably came from a place with two stars

Late last year, astronomers spotted the first object to enter our solar system from interstellar space—a somewhat reddish, cigar-shaped body named ‘Oumuamua. Now, a new study hints that this exotic interloper most likely began its voyage after being cast out of a double-star system.

Astronomers first classified ‘Oumuamua (Hawaiian for “scout”) as a comet, but later observations didn’t reveal the telltale signs, including clouds of dust or water vapor. That, plus the 400-meter-long object’s high speed and odd trajectory, strongly suggested that ‘Oumuamua was an asteroid, not a comet, from beyond our solar system.

But very few single-star solar systems would be able to cast out a waterless object like an asteroid, a new study suggests. That’s because such a feat would require gravitational interactions with a planet the size of Saturn or larger, something present in only about 10% of single-star solar systems near us in the Milky Way.

But solar systems that have two suns, especially those in which the stars orbit each other tightly, are much more likely to cast out asteroids, the researchers report today in the Monthly Notices of the Royal Astronomical Society. The team’s computer simulations suggest that up to 36% of binary stars can eject asteroids. When the researchers take into account the numbers of single-star versus binary systems and the numbers and sizes of planets they’re likely to have, they estimate that more than three-fourths of the asteroids cast into interstellar space come from solar systems that have two suns.

 

Exploration of `Oumuamua-like objects

The Feasibility and Benefits of In Situ Exploration of `Oumuamua-like objects

A rapid accumulation of observations and interpretation have followed in the wake of 1I `Oumuamua’s passage through the inner Solar System. We briefly outline the consequences that this first detection of an interstellar asteroid implies for the planet-forming process, and we assess the near-term prospects for detecting and observing (both remotely and in situ) future Solar System visitors of this type. Drawing on detailed heat-transfer calculations that take both `Oumuamua’s unusual shape and its chaotic tumbling into account, we affirm that the lack of a detectable coma in deep images of the object very likely arises from the presence of a radiation-modified coating of high molecular weight material (rather than a refractory bulk composition). Assuming that `Oumuamua is a typical representative of a larger population with a kinematic distribution similar to Population I stars in the local galactic neighborhood, we calculate expected arrival rates, impact parameters and velocities of similar objects and assess their prospects for detection using operational and forthcoming facilities. Using `Oumuamua as a proof-of-concept, we assess the prospects for missions that intercept interstellar objects (ISOs) using conventional chemical propulsion. Using a “launch on detection” paradigm, we estimate wait times of order 10 years between favorable mission opportunities with the detection capabilities of the Large-Scale Synoptic Survey Telescope (LSST), a figure that will be refined as the population of interstellar asteroids becomes observationally better constrained.

 

Radiants of hyperbolic minor bodies

Where the Solar system meets the solar neighbourhood: patterns in the distribution of radiants of observed hyperbolic minor bodies

Observed hyperbolic minor bodies might have an interstellar origin, but they can be natives of the Solar system as well. Fly-bys with the known planets or the Sun may result in the hyperbolic ejection of an originally bound minor body; in addition, members of the Oort cloud could be forced to follow inbound hyperbolic paths as a result of secular perturbations induced by the Galactic disc or, less frequently, due to impulsive interactions with passing stars. These four processes must leave distinctive signatures in the distribution of radiants of observed hyperbolic objects, both in terms of coordinates and velocity. Here, we perform a systematic numerical exploration of the past orbital evolution of known hyperbolic minor bodies using a full N-body approach and statistical analyses to study their radiants. Our results confirm the theoretical expectations that strong anisotropies are present in the data. We also identify a statistically significant overdensity of high-speed radiants towards the constellation of Gemini that could be due to the closest and most recent known fly-by of a star to the Solar system, that of the so-called Scholz’s star. In addition to and besides 1I/2017 U1 (`Oumuamua), we single out eight candidate interstellar comets based on their radiants’ velocities.

 

The Three Surprises of ‘Oumuamua

The Three Surprises of ‘Oumuamua

January 30, 2018, by Matija Cuk

One of the defining moments in planetary astronomy in 2017 is that this is the year we discovered the first astronomical object to enter the Solar System from interstellar space. Now known as ‘Oumuamua (Hawaiian for “scout”), the object was discovered by the Pan-STARRS survey team in Hawaii on October 19th. Over the next three weeks it was in turn classified as a comet, a long-period asteroid and finally, the first of the new class of interstellar objects.

As soon as `Oumuamua’s true trajectory was confirmed, all available telescopes were used to study it as quickly as possible because it was moving away from Earth at a very high rate of speed. `Oumuamua was actually discovered already on its way out of our solar system, after it passed Earth and could finally be seen in the nighttime sky (when it was on the same side as the sun, it wasn’t visible). Now (late January 2018), `Oumuamua is too faint to see even through the largest telescopes, but its brief passage has given us some rare firsthand information on a distant solar system, and also left us with three surprises.

Before discussing the surprising aspects of `Oumuamua, here are some of the less unexpected facts of `Oumuamua:

It wasn’t moving very fast relative to nearby stars – in fact, it was the Solar System that ran into `Oumuamua, rather than the other way around. This means that the star `Oumuamua originated from orbits the galaxy on an orderly orbit in the galactic disk, like most other local stars.

`Oumuamua is faint and small. We are not sure how small exactly as we don’t know how reflective its surface is, but it’s definitely less than a kilometer long.

Another unremarkable quality of `Oumuamua is its color, which is somewhat red and therefore very similar to that of some of our own comets and distant asteroids.

The first surprise of `Oumuamua is that it is not a comet. `Oumuamua was initially classified as a comet not because of having coma, or a tail (it has neither), but because we expected interstellar objects to be comets. Our giant planets have ejected many, many comets (and many fewer asteroids) into interstellar space during Solar System’s formation. We know this because some of them were not quite lost, but were “stuck” in the Oort cloud, a giant swarm of comets orbiting the Sun at very large distances. Combined with the fact that comets are easier to see than asteroids for the same size of the nucleus (comets were known in antiquity and asteroids were discovered only in the 19th century), we expected the first interstellar visitor to be a comet, but we were wrong.

The second surprise of `Oumuamua is how elongated it is. `Oumuamua’s changes in brightness over time imply that it is roughly cigar-shaped, with an axis ratio of 5:1 to 10:1. This is very extreme among asteroids in the Solar System, and would certainly not be expected if we randomly select one body from over hundred thousand known asteroids. If `Oumaumua’s shape is typical of the population it comes from, things must be very different in its parent system from how they are here.

The third surprise was the fact that `Oumuamua is tumbling. At first it was noted that ‘Oumuamua had a 7 or 8-hour spin period, but different measurements did not quite agree. It turned out that `Oumuamua’s spin is not regular, but it executes a complex tumbling motion that shows different views of the body at different times. Some asteroids in our Solar System do tumble, but vast majority do not. We think that this is because internal motions of material inside asteroids (which are often just piles of rocks and sand loosely held together by gravity) damp this tumbling relatively quickly (astronomically speaking), leaving only asteroids that suffered recent collisions as tumblers. `Oumuamua spent many millions of years in the interstellar void, so it should have damped its tumbling, but it apparently did not. This made planetary scientists conclude that `Oumuamua is likely a solid chunk of rock or metal, without any internal structure or lose material.

So why is `Oumuamua the way it is? We do not know, but we do have some ideas. My own favorite hypothesis is that `Oumuamua is a piece of a planet destroyed by tides as it was passing close to a red dwarf star in a binary system. The idea is that the planet formed around the red dwarf’s companion, but its orbit was destabilized and the planet swung past the red dwarf, about to be hurled into interstellar space. Red dwarf stars can be surprisingly dense, some of them are the size of Jupiter, but with a hundred times larger mass. This makes their tides very strong, and tides can disrupt bodies that come too close (like Jupiter disrupted comet Shoemaker-Levy 9 in 1994). If a planet can be shredded into trillions of fragments which are then ejected into interstellar space, such catastrophic events could produce more interstellar objects than regular ejections of comets and asteroids by planets.

So, what do we do now? Well, we wait for more interstellar objects to see what they are like, and we probably won’t have to wait too long. A new telescope, the Large Synoptic Survey Telescope (LSST) is under construction in Chile, and it should become operational in 2022. LSST will be a robotic telescope that will take a complete scan of the whole sky down to very faint objects every three days, so it will very literally catch anything that moves. If `Oumuamua is not a complete fluke, LSST should detect about one such object every year.

`Oumuamua is the first and almost certainly won’t be the last interstellar visitor we have discovered. And we are anxiously awaiting the next visitor.

 

Captured Interstellar Objects

Implications of Captured Interstellar Objects for Panspermia and Extraterrestrial Life

ABSTRACT: We estimate the capture rate of interstellar objects by means of three-body gravitational interactions. We apply this model to the Sun-Jupiter system and the Alpha Centauri A&B binary system, and find that the radius of the largest captured object is a few tens of km and Earth-sized respectively. We explore the implications of our model for the transfer of life by means of rocky material. The interstellar comets captured by the “fishing net” of the Solar system can be potentially distinguished by their differing ratios of oxygen isotopes through high-resolution spectroscopy of water vapor in their tails.

 

Origins of ‘Oumuamua-like Objects

Interstellar Interlopers: Number Density and Origins of ‘Oumuamua-like Objects

ABSTRACT: We provide a calculation of Pan-STARRS’ ability to detect objects similar to the interstellar object 1I/2017 U1 (hereafter ‘Oumuamua), including the most detectable approach vectors and the effect of object size on detection efficiency. Using our updated detection cross-section, we infer an interstellar number density of such objects (nIS ≈ 0.2 au-3). This translates to a mass density of ρIS ≈ 4M pc-3 which cannot be populated unless every star is contributing. We find that given current models, such a number density cannot arise from the ejection of inner solar system material during planet formation. We note that a stellar system’s Oort cloud will be released after a star’s main sequence life time and may provide enough material to obtain the observed density. The challenge is that Oort cloud bodies are icy and ‘Oumuamua was observed to be dry which necessitates a crust generation mechanism.