# How is it possible that we haven't discovered anything in the Oort cloud yet?

The Oort cloud is a hypothetical cloud of small icy bodies surrounding the Sun at more than 1000 AU. It is thought to be a vast reservoir of comets that occasionally get disrupted, sending comets towards the inner solar system.

I would guess that there are stellar occultations where a body from the Oort cloud passes in front of a background star, obscuring its light. This makes it surprising to me that we haven't detected any objects in the Oort cloud. Perhaps the sensitivity of our instruments is not high enough? Perhaps that in spite of the large number of objects that should exist in the Oort cloud, these occultations are actually infrequent, and you would have to wait tens of years before seeing one? Perhaps nobody has combed the data to find such signals?

Would any of our existing telescopes have been able to pick up the occultation if it was pointed in the right direction at the right time?

It's not very surprising that no Oort Cloud objects have been detected via occultations. They're extremely rare, even for our most advanced space-based observatories.

According to Ofek & Nakar 2010, published about one year after the launch of Kepler, the telescope would be able to detect between $$0$$ and $$sim100$$ occultation events of Oort Cloud objects, assuming that it monitored $$sim10^5$$ stars for a time $$au=3$$ years - both of which ended up being underestimates.$$^{dagger}$$ Part of the reason for the uncertainty is that the parameters of the Oort Cloud are not tightly constrained. Varying the inner radius $$r_{ ext{min}}$$ from 1000 to 5000 AU, or varying the index $$alpha$$ of the number density distribution $$npropto r^{alpha}$$ from $$-4$$ to $$-3$$, can produce changes on the expected occultations by a couple orders of magnitude.

The problem becomes apparent when you consider that 1) occultations are intrinsically short and infrequent and 2) they're one-off events. It's not the same as detecting a transiting exoplanet, or observing an expected occultation from a minor planet whose orbit is already known. Therefore, it's hard to tell whether an event is a true occultation or simply a glitch of some sort. The second point of the paper was to present methods for validating possible events.

Ground-based surveys of Kuiper Belt Objects (KBOs) have been performed, but they suffer from atmospheric effects, as you might expect. Given that occultations would last on the order of 1 second, atmospheric scintillation becomes a problem for the high-cadence searches required (Alcock et al.). Given that some ground-based surveys just for KBOs a few kilometers across, like the Taiwanese-American Occultation Survey, have returned no detections (Zhang et al. 2008), it might not be surprising that ground-based Oort Cloud searches are also extremely difficult.

$${dagger}$$ By comparison, the CoRoT mission, assuming it monitored $$sim10^4$$ stars over the same period, was expected to detect essentially no occultations, according to the same analysis.

## ʻOumuamua is Not an Alien Spacecraft

This artist's impression shows the first interstellar object discovered in the Solar System, ʻOumuamua. 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.

An international team of asteroid and comet experts, including two from the University of Hawaiʻi, agrees on a natural origin for our first interstellar visitor.

On October 19, 2017, the Panoramic Survey Telescope and Rapid Response System 1 (Pan-STARRS1) telescope, located at the University of Hawaiʻi's Haleakala Observatory, discovered the first known interstellar object to pass through our solar system. Researchers from around the world raced to collect as much data as possible before ʻOumuamua traveled beyond the reach of Earth's telescopes. In all, they had only a few weeks to observe the strange visitor.

The object is now usually known as ʻOumuamua, a name chosen in consultation with Hawaiian language experts Kaʻiu Kimura and Larry Kimura, which reflects the way this object can be considered a scout or messenger sent from the distant past to reach out to us (`ou means "reach out for", and mua, with the second mua placing emphasis, means "first, in advance of").

Early reports of ʻOumuamua's 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 review of all the available evidence by an international team of 14 experts strongly suggests that ʻOumuamua 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 ʻOumuamua in our solar system," said Dr. Matthew Knight, the team leader from the University of Maryland "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."

The team of 14 astronomers hailing from the U.S. and Europe met late last year at the International Space Science Institute (ISSI) in Bern, Switzerland, to critically assess all the available research and observations on ʻOumuamua and will meet again late this year. Their first priority was to determine whether there is any evidence to support the hypothesis that ʻOumuamua is a spacecraft built by an alien civilization.

"We put together a strong team of experts in various different areas of work on ʻOumuamua. 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 ʻOumuamua 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."

"While ʻOumuamua's interstellar origin makes it unique, many of its other properties are perfectly consistent with objects in our own solar system" said Dr. Robert Jedicke of the University of Hawai'i's Institute for Astronomy (IfA). In fact, ʻOumuamua's orbit, its path through our solar system, matches a prediction published in a scientific journal by Jedicke and his colleagues half a year before ʻOumuamua's discovery.

The ISSI team considered all the available information in peer-reviewed scientific journals and paid special attention to the research published by IfA researchers. In particular, Dr. Karen Meech's research paper in the journal Nature that first reported on ʻOumuamua's discovery and characteristics in December 2017, just two months after the unusual object was identified by Pan-STARRS1. "It was exciting and exhausting to coordinate all the ʻOumuamua observations with my co-authors from all around the world. It really was a 24 hour a day effort for the better part of two months. In that paper we established that ʻOumuamua rotates once in about seven hours and that is had a red color similar to many objects locked within our own solar system." said Meech. That work also showed that ʻOumuamua must have an extremely elongated shape, like a cigar or maybe a frisbee, unlike any known object in our solar system based on changes in its apparent brightness while it rotated.

Meech and other UH researchers were critical to another paper published in Nature a year ago that indicated ʻOumuamua is accelerating along its trajectory as it leaves our solar system. This behavior is typical of comets but astronomers have found no other visual evidence of the gas or dust emissions that create this acceleration. Meech explained that "while it is disappointing that we could not confirm the cometary activity with telescopic observations it is consistent with the fact that ʻOumuamua's acceleration is very small and must therefore be due to the ejection of just a small amount of gas and dust."

The ISSI team considered a number of mechanisms by which ʻOumuamua 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 this theory, Jupiter created our own solar system's Oort cloud, a population of small objects only loosely gravitationally bound to our Sun in a gigantic shell extending to about a third of the distance to the nearest star. Some of the objects in our Oort cloud eventually make it back into our solar system as long period comets while others may have slipped past the influence of the Sun's gravity to become interstellar travelers themselves.

The research team expects that ʻOumuamua will be the first of many interstellar visitors discovered passing through our solar system and they are collectively looking forward to data from the Large Synoptic Survey Telescope (LSST) which is scheduled to be operational in 2022. The LSST, located in Chile, may detect one interstellar object every year and allow astronomers to study the properties of objects from many other solar systems.

While the ISSI team hopes that LSST will detect more interstellar objects they think it is unlikely that astronomers will ever detect an alien spacecraft passing through our solar system and they are convinced that ʻOumuamua was a unique and extremely interesting but completely natural object.

The research paper, "The Natural History of ʻOumuamua," the ʻOumuamua ISSI Team (Michele Bannister, Asmita Bhandare, Piotr Dybczyński, Alan Fitzsimmons, Aurélie Guilbert-Lepoutre, Robert Jedicke, Matthew Knight, Karen Meech, Andrew McNeill, Susanne Pfalzner, Sean Raymond, Colin Snodgrass, David Trilling and Quanzhi Ye), was published in the journal Nature Astronomy on July 1, 2019.

This work was supported by the UK Science and Technology Facilities Council (Award Nos. ST/P0003094/1 and ST/L004569/1), the National Science Foundation (Award Nos. AST1617015 and 1545949), NASA (Award Nos. GO/DD-15405, GO/DD-15447, NAS 5-26555, NNX17AK15G and 80NSSC18K0829), the National Science Centre in Poland (Award No. 2015/17/B/ST9/01790) and the European Research Council (Award No. 802699). The content of this article does not necessarily reflect the views of these organizations.

More information about The PanSTARRS project can be found at the PanSTARRS project website http://panstarrs.ifa.hawaii.edu

The Panoramic Survey Telescope and Rapid Response System (Pan-STARRS) is a wide-field survey observatory operated by the University of Hawaiʻi Institute for Astronomy. The Minor Planet Center is hosted by the Harvard-Smithsonian Center for Astrophysics and is a sub-node of the Planetary Data System Small Bodies Node at the University of Maryland (http://www.minorplanetcenter.net ). JPL hosts the Center for Near-Earth Object Studies (CNEOS). All are projects of NASA's Near-Earth Object Observations Program, and elements of the agency's Planetary Defense Coordination Office within NASA's Science Mission Directorate.

Founded in 1967, the Institute for Astronomy at the University of Hawaii at Manoa conducts research into galaxies, cosmology, stars, planets, and the sun. Its faculty and staff are also involved in astronomy education, deep space missions, and in the development and management of the observatories on Haleakalā and Maunakea. The Institute operates facilities on the islands of Oahu, Maui, and Hawaii.

## Contents

Oort was born in Franeker, a small town in the Dutch province of Friesland, on April 28, 1900. He was the second son of Abraham Hermanus Oort, [9] a physician, who died on May 12, 1941, and Ruth Hannah Faber, who was the daughter of Jan Faber and Henrietta Sophia Susanna Schaaii, and who died on November 20, 1957. Both of his parents came from families of clergymen, with his paternal grandfather, a Protestant clergyman with liberal ideas, who "was one of the founders of the more liberal Church in Holland" [10] and who "was one of the three people who made a new translation of the Bible into Dutch." [10] The reference is to Henricus Oort (1836–1927), who was the grandson of a famous Rotterdam preacher and, through his mother, Dina Maria Blom, the grandson of theologian Abraham Hermanus Blom, a "pioneer of modern biblical research". [10] Several of Oort's uncles were pastors, as was his maternal grandfather. "My mother kept up her interests in that, at least in the early years of her marriage", he recalled. "But my father was less interested in Church matters." [10]

In 1903 Oort's parents moved to Oegstgeest, near Leiden, where his father took charge of the Endegeest Psychiatric Clinic. [3] Oort's father, "was a medical director in a sanitorium for nervous illnesses. We lived in the director's house of the sanitorium, in a small forest which was very nice for the children, of course, to grow up in." Oort's younger brother, John, became a professor of plant diseases at the University of Wageningen. In addition to John, Oort had two younger sisters and an elder brother who died of diabetes when he was a student. [3]

Oort attended primary school in Oegstgeest and secondary school in Leiden, and in 1917 went to Groningen University to study physics. He later said that he had become interested in science and astronomy during his high-school years, and conjectured that his interest was stimulated by reading Jules Verne. [3] His one hesitation about studying pure science was the concern that it "might alienate one a bit from people in general", as a result of which "one might not develop the human factor sufficiently." But he overcame this concern and ended up discovering that his later academic positions, which involved considerable administrative responsibilities, afforded a good deal of opportunity for social contact.

Oort chose Groningen partly because a well known astronomer, Jacobus Cornelius Kapteyn, was teaching there, although Oort was unsure whether he wanted to specialize in physics or astronomy. After studying with Kapteyn, Oort decided on astronomy. "It was the personality of Professor Kapteyn which decided me entirely", he later recalled. "He was quite an inspiring teacher and especially his elementary astronomy lectures were fascinating." [10] Oort began working on research with Kapteyn early in his third year. According to Oort one professor at Groningen who had considerable influence on his education was physicist Frits Zernike.

After taking his final exam in 1921, Oort was appointed assistant at Groningen, but in September 1922, he went to the United States to do graduate work at Yale and to serve as an assistant to Frank Schlesinger of the Yale Observatory. [4]

At Yale, Oort was responsible for making observations with the Observatory's zenith telescope. "I worked on the problem of latitude variation", he later recalled, "which is quite far away from the subjects I had so far been studying." He later considered his experience at Yale useful as he became interested in "problems of fundamental astronomy that [he] felt was capitalized on later, and which certainly influenced [his] future lectures in Leiden." Personally, he "felt somewhat lonesome in Yale", but also said that "some of my very best friends were made in these years in New Haven." [10]

### Early discoveries Edit

In 1924, Oort returned to the Netherlands to work at Leiden University, where he served as a research assistant, becoming Conservator in 1926, Lecturer in 1930, and Professor Extraordinary in 1935. [4] In 1926, he received his doctorate from Groningen with a thesis on the properties of high-velocity stars. The next year, Swedish astronomer Bertil Lindblad proposed that the rate of rotation of stars in the outer part of the galaxy decreased with distance from the galactic core, and Oort, who later said that he believed it was his colleague Willem de Sitter who had first drawn his attention to Lindblad's work, realized that Lindblad was correct and that the truth of his proposition could be demonstrated observationally. Oort provided two formulae that described galactic rotation the two constants that figured in these formulae are now known as "Oort's constants". [4] Oort "argued that just as the outer planets appear to us to be overtaken and passed by the less distant ones in the solar system, so too with the stars if the Galaxy really rotated", according to the Oxford Dictionary of Scientists. [11] He "was finally able to calculate, on the basis of the various stellar motions, that the Sun was some 30,000 light-years from the center of the Galaxy and took about 225 million years to complete its orbit. He also showed that stars lying in the outer regions of the galactic disk rotated more slowly than those nearer the center. The Galaxy does not therefore rotate as a uniform whole but exhibits what is known as 'differential rotation'." [12]

These early discoveries by Oort about the Milky Way overthrew the Kapteyn system, named after his mentor, which had envisioned a galaxy that was symmetrical around the Sun. As Oort later noted, "Kapteyn and his co-workers had not realized that the absorption in the galactic plane was as bad as it turned out to be." [10] Until Oort began his work, he later recalled, "the Leiden Observatory had been concentrating entirely on positional astronomy, meridian circle work and some proper motion work. But no astrophysics or anything that looked like that. No structure of the galaxy, no dynamics of the galaxy. There was no one else in Leiden who was interested in these problems in which I was principally interested, so the first years I worked more or less by myself in these projects. De Sitter was interested, but his main line of research was celestial mechanics at that time the expanding universe had moved away from his direct interest." [10] As the European Space Agency states, Oort "sh[ook] the scientific world by demonstrating that the Milky Way rotates like a giant 'Catherine Wheel'." He showed that all the stars in the galaxy were "travelling independently through space, with those nearer the center rotating much faster than those further away." [5]

This breakthrough made Oort famous in the world of astronomy. In the early 1930s he received job offers from Harvard and Columbia University, but chose to stay at Leiden, although he did spend half of 1932 at the Perkins Observatory, in Delaware, Ohio. [4]

In 1934, Oort became assistant to the director of Leiden Observatory the next year he became General Secretary of the International Astronomical Union (IAU), a post he held until 1948 in 1937 he was elected to the Royal Academy. In 1939, he spent half a year in the U.S., and became interested in the Crab Nebula, concluding in a paper, written with American astronomer Nicholas Mayall, that it was the result of a supernova explosion. [4]

### Nazi invasion of Netherlands Edit

In 1940, Nazi Germany invaded the Netherlands. Soon after, the occupying regime dismissed all Jewish professors from Leiden University and other universities. "Among the professors who were dismissed", Oort later recalled, "was a very famous … professor of law by the name of Meyers. On the day when he got the letter from the authorities that he could no longer teach his classes, the dean of the faculty of law went into his class … and delivered a speech in which he started by saying, 'I won't talk about his dismissal and I shall leave the people who did this, below us, but will concentrate on the greatness of the man dismissed by our aggressors.'" [10]

This speech (26 November 1940) made such an impression on all his students that on leaving the auditorium they defiantly sang the anthem of the Netherlands and went on strike. Oort was present for the lecture and was greatly impressed. This occasion formed the beginning of the active resistance in Holland. The speech by Rudolph Cleveringa, the dean of the faculty of Law and former graduate student of professor Meijers, was widely circulated during the rest of the war by the resistance groups. Oort was in a little group of professors in Leiden who came together regularly and discussed the problems the university faced in view of the German occupation. Most of the members of this group were put in hostage camps soon after the speech by Cleveringa. Oort refused to collaborate with the occupiers, "and so we went down to live in the country for the rest of the war." Resigning from the Royal Academy, from his professorial post at Leiden, and from his position at the Observatory, Oort took his family to Hulshorst, a quiet village in the province of Gelderland, where they sat out the war. In Hulshorst, he began writing a book on stellar dynamics. [4] [10]

### Oort's radio astronomy Edit

Before the war was over, he initiated, in collaboration with a Utrecht University student, Hendrik van de Hulst, a project that eventually succeeded, in 1951, in detecting the 21-centimeter radio emission from interstellar hydrogen spectral line at radio frequencies. Oort and his colleagues also made the first investigation of the central region of the Galaxy, and discovered that “the 21-centimeter radio emission passed un-absorbed through the gas clouds that had hidden the center from optical observation. They found a huge concentration of mass there, later identified as mainly stars, and also discovered that much of the gas in the region was moving rapidly outward away from the center.” [12] In June 1945, after the end of the war, Oort returned to Leiden, took over as director of the Observatory, and became Full Professor of Astronomy. [4] During this immediate postwar period, he led the Dutch group that built radio telescopes at Radio Kootwijk, Dwingeloo, and Westerbork and used the 21-centimeter line to map the Milky Way, including the large-scale spiral structure, the galactic center, and gas cloud motions. Oort was helped in this project by the Dutch telecommunications company, PTT, which, he later explained, “had under their care all the radar equipment that was left behind by the Germans on the coast of Holland. This radar equipment consisted in part of reflecting telescopes of 7 1/2 meter aperture. Our radio astronomy was really started with the aid of one of these instruments… it was in Kootwijk that the first map of the Galaxy was made.” [10] For a brief period, before the completion of the Jodrell Bank telescope, the Dwingeloo instrument was the largest of its kind on earth.

It has been written that “Oort was probably the first astronomer to realize the importance” of radio astronomy. [4] “In the days before radio telescopes,” one source notes, “Oort was one of the few scientists to realise the potential significance of using radio waves to search the heavens. His theoretical research suggested that vast clouds of hydrogen lingered in the spiral arms of the Galaxy. These molecular clouds, he predicted, were the birthplaces of stars.” [5] These predictions were confirmed by measurements made at the new radio observatories at Dwingeloo and Westerbork. Oort later said that “it was Grote Reber's work which first impressed me and convinced me of the unique importance of radio observations for surveying the galaxy.” [10] Just before the war, Reber had published a study of galactic radio emissions. Oort later commented, “The work of Grote Reber made it quite clear [radio astronomy] would be a very important tool for investigating the Galaxy, just because it could investigate the whole disc of the galactic system unimpeded by absorption.” [10] Oort's work in radio astronomy is credited by colleagues with putting the Netherlands in the forefront of postwar astronomy. [4] Oort also investigated the source of the light from the Crab Nebula, finding that it was polarized, and probably produced by synchrotron radiation, confirming a hypothesis by Iosif Shklovsky. [13]

### Comet studies Edit

Oort went on to study comets, which he formulated a number of revolutionary hypotheses. He hypothesized that the Solar System is surrounded by a massive cloud consisting of billions of comets, many of them “long-period” comets that originate in a cloud far beyond the orbits of Neptune and Pluto. This cloud is now known as the Oort Cloud. He also realized that these external comets, from beyond Pluto, can “become trapped into tighter orbits by Jupiter, and become periodic comets, like Halley's comet.” According to one source, “Oort was one of the few people to have seen Comet Halley on two separate apparitions. At the age of 10, he was with his father on the shore at Noordwijk, Netherlands, when he first saw the comet. In 1986, 76 years later, he went up in a plane and was able to see the famous comet once more.” [12]

In 1951 Oort and his wife spent several months in Princeton and Pasadena, an interlude that led to a paper by Oort and Lyman Spitzer on the acceleration of interstellar clouds by O-type stars. He went on to study high-velocity clouds. Oort served as director of the Leiden Observatory until 1970. After his retirement, he wrote comprehensive articles on the galactic center and on superclusters and published several papers on the quasar absorption lines, supporting Yakov Zel’dovich's pancake model of the universe. He also continued researching the Milky Way and other galaxies and their distribution until shortly before his death at 92. [4]

One of Oort's strengths, according to one source, was his ability to “translate abstruse mathematical papers into physical terms,” as exemplified by his translation of the difficult mathematical terms of Lindblad's theory of differential galactic rotation into a physical model. Similarly, he “derived the existence of the comet cloud on the outskirts of the Solar System from the observations, using the mathematics needed in dynamics, but then deduced the origin of this cloud using general physical arguments and a minimum of mathematics.” [4] [12]

In 1927, Oort married Johanna Maria (Mieke) Graadt van Roggen (1906–1993). They had met at a university celebration at Utrecht, where Oort's brother was studying biology at the time. Oort and his wife had two sons, Coenraad (Coen) and Abraham, and a daughter, Marijke. Abraham became a professor of climatology at Princeton University.

According to the website of Leiden University, Oort was very interested in and knowledgeable about art. "[W]hen visiting another country he would always try to take some time off to visit the local museums and exhibitions…and in the fifties served for some years as chairman of the pictorial arts committee of the Leiden Academical Arts Centre, which had among other things the task of organizing expositions". [14]

"Colleagues remembered him as a tall, lean and courtly man with a genial manner," reported his New York Times obituary. [4]

## A fifth fundamental force could really exist, but we haven't found it yet

Particles and interaction bosons of the standard model. Credit: Particle Data Group

The universe is governed by four fundamental forces: gravity, electromagnetism and the strong and weak nuclear forces. These forces drive the motion and behavior of everything we see around us. At least, that's what we think. But over the past several years, there's been increasing evidence of a fifth fundamental force. New research hasn't discovered this fifth force, but it does show that we still don't fully understand these cosmic forces.

The fundamental forces are a part of the Standard Model of particle physics. This model describes all the quantum particles, including electrons, protons, antimatter and others. Quarks, neutrinos and the Higgs boson are all part of the model.

The term "force" in the model is a bit of a misnomer. In the Standard Model, each force is the result of a type of carrier boson. Photons are the carrier boson for electromagnetism. Gluons are the carrier bosons for the strong interaction, and bosons known as W and Z are for the weak interaction. Gravity isn't technically part of the Standard Model, but it's assumed that quantum gravity has a boson known as the graviton. We still don't fully understand quantum gravity, but one idea is that gravity can be united with the Standard Model to produce a grand unified theory (GUT).

Every particle we've ever discovered is a part of the Standard Model. The behavior of these particles matches the model extremely accurately. Scientists have looked for particles beyond the Standard Model, but so far, they have never found any. The Standard Model is a triumph of scientific understanding. It is the pinnacle of quantum physics.

But we've started to learn it has some serious problems.

Observations of galaxies show the distribution of dark matter. Credit: X-ray: NASA/CXC/Ecole Polytechnique Federale de Lausanne, Switzerland/D.Harvey & NASA/CXC/Durham Univ/R.Massey Optical & Lensing Map: NASA, ESA, D. Harvey (Ecole Polytechnique Federale de Lausanne, Switzerland) and R. Massey (Durham University, UK)

To begin with, we now know the Standard Model can't combine with gravity in the way that we thought. In the Standard Model, the fundamental forces "unify" at higher energy levels. Electromagnetism and the weak combine into the electroweak, and the electroweak unifies with the strong to become the electronuclear force. At extremely high energies the electronuclear and gravitational forces should unify. Experiments in particle physics have shown that the unification energies don't match up.

More problematic is the issue of dark matter. Dark matter was first proposed to explain why stars and gas on the outer edge of a galaxy move faster than predicted by gravity. Either our theory of gravity is somehow wrong, or there must be some invisible (dark) mass in galaxies. Over the past 50 years, the evidence for dark matter has become really strong. We've observed how dark matter clusters galaxies together, how it is distributed within particular galaxies, and how it behaves. We know it doesn't interact strongly with regular matter or itself, and it makes up the majority of mass in most galaxies.

But there is no particle in the Standard Model that could make up dark matter. It's possible that dark matter could be made of something such as small black holes, but astronomical data doesn't really support that idea. Dark matter is most likely made of some as-yet undiscovered particle, one the Standard Model doesn't predict.

Then there is dark energy. Detailed observations of distant galaxies show that the universe is expanding at an ever-increasing rate. There seems to be some kind of energy driving this process, and we don't understand how. It could be that this acceleration is the result of the structure of space and time, a kind of cosmological constant that causes the universe to expand. It could be that this is driven by some new force yet to be discovered. Whatever dark energy is, it makes up more than two-thirds of the universe.

All of this points to the fact that the Standard Model is, at best, incomplete. There are things we are fundamentally missing in the way the universe works. Lots of ideas have been proposed to fix the Standard Model, from supersymmetry to yet undiscovered quarks, but one idea is that there is a fifth fundamental force. This force would have its own carrier boson(s) as well as new particles beyond the ones we've discovered.

We don’t understand most of the universe. Credit: Chandra X-ray Observatory

This fifth force would also interact with the particles we have observed in subtle ways that contradict the Standard Model. This brings us to a new paper claiming to have evidence of such an interaction.

The paper looks at an anomaly in the decay of helium-4 nuclei, and it builds off an earlier study of beryllium-8 decays. Beryllium-8 has an unstable nucleus that decays into two nuclei of helium-4. In 2016, the team found that the decay of beryllium-8 seems to violate the Standard Model slightly. When the nuclei are in an excited state, it can emit an electron-positron pair as it decays. The number of pairs observed at larger angles is higher than the Standard Model predicts, and is known as the Atomki anomaly.

There are lots of possible explanations for the anomaly, including experiment error, but one explanation is that it's caused by boson the team named X17. It would be the carrier boson for a (yet unknown) fifth fundamental force, with a mass of 17 MeV. In the new paper, the team found a similar discrepancy in the decay of helium-4. The X17 particle could also explain this anomaly.

While this sounds exciting, there's reason to be cautious. When you look at the details of the new paper, there's a bit of odd data tweaking. Basically, the team assumes X17 is accurate and shows that the data can be made to fit with their model. Showing that a model can explain the anomalies isn't the same as proving your model does explain the anomalies. Other explanations are possible. If X17 does exist, we should have also seen it in other particle experiments, and we haven't. The evidence for this "fifth force" is still weak.

The fifth force could exist, but we haven't found it yet. What we do know is that the Standard Model doesn't entirely add up, and that means some very interesting discoveries are waiting to be found.

## Interstellar Object ‘Oumuamua Has Purely Natural Origin, Say Astronomers

A fast moving, cigar-shaped object of extrasolar origin was discovered on October 19, 2017 by the Pan-STARRS 1 telescope in Hawai’i. Named 1I/2017 U1 (‘Oumuamua), the object was first spotted three days after its closest approach to Earth, well after it had passed closest to the Sun on September 9, 2017. Its discovery provoked intense and continuing interest from the scientific community and the general public. Reports of its unusual characteristics even led to speculation that ‘Oumuamua might be an interstellar spacecraft. Now, a review of all the available evidence strongly suggests that the object has a purely natural origin.

This artist’s impression shows the first interstellar asteroid — 1I/2017 U1 (’Oumuamua). Image credit: M. Kornmesser / ESO.

“While ‘Oumuamua’s interstellar origin makes it unique, many of its other properties are perfectly consistent with objects in our own Solar System,” said Dr. Robert Jedicke, an astronomer at the University of Hawaii’s Institute for Astronomy and a member of an international group of asteroid and comet experts called the ‘Oumuamua ISSI Team.

“We have never seen anything like ‘Oumuamua in our Solar System. It’s really a mystery still,” said team member Dr. Matthew Knight, a researcher at the University of Maryland.

“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.”

According to the authors, ‘Oumuamua is red in color, similar to many small objects observed in our Solar System. But that’s where the familiarity ends.

The interstellar object likely has an elongated, cigarlike shape and an odd spin pattern — much like a soda bottle laying on the ground, spinning on its side.

“Its motion through the Solar System is particularly puzzling. While it appeared to accelerate along its trajectory — a typical feature of comets — we could find no evidence of the gaseous emissions that typically create this acceleration,” Dr. Knight said.

“The motion of ‘Oumuamua didn’t simply follow gravity along a parabolic orbit as we would expect from an asteroid. 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.”

Montage of potential formation scenarios of ‘Oumuamua as a natural planetesimal. Image credit: NASA / Hubble Heritage Team / STScI / AURA / C.R. O’Dell, Vanderbilt University / ESA / A. Feild, STScI / JAXA / Susanne Pfalzner / ESO / L. Calçada / JPL / D. Seal / CfA / Mark A. Garlick.

The team 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.

“We tend to assume that the physical processes we observe here, close to home, are universal,” Dr. Knight said.

“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 team’s paper was published online in the journal Nature Astronomy.

Michele T. Bannister et al (The ‘Oumuamua ISSI Team). The natural history of ‘Oumuamua. Nature Astronomy, published online July 1, 2019 doi: 10.1038/s41550-019-0816-x

## Is Humanity Ignoring Our First Chance For A Mission To An Oort Cloud Object?

In 2003, scientists discovered an object beyond Neptune that was unlike any other: Sedna. While there were larger dwarf planets beyond Neptune, and comets that would travel farther from the Sun, Sedna was unique for how far it always remained from the Sun. It always remained more than twice as distant from the Sun as Neptune was, and would achieve a maximum distance nearly 1,000 times as far as the Earth-Sun distance. And despite all that, it’s extremely large: perhaps 1,000 kilometers in diameter. It’s the first object we’ve ever found that might have originated from the Oort cloud. And we’ll only get two chances if we want to send a mission there: in 2033 and 2046. Right now, there isn’t even a proposed NASA mission looking at the possibility. If we do nothing, the opportunity will simply pass us by.

As we journey farther away from the Sun, out past the rocky planets, asteroid belt, and the gas giants, the Solar System doesn’t simply come to an end. There’s the Kuiper belt, home to innumerable icy bodies, ranging in size from dwarf planets like Eris and Pluto down to comet-sized objects and even smaller. Beyond that lies the scattered disk: bodies which once came close to Neptune and were flung into more eccentric orbits, often taking them hundreds of astronomical units (where 1 A.U. is the Earth-Sun distance) from the Sun. Going even farther out are detached objects: bodies which never come close to any of the major planets and which have even larger perihelia than anything from the Kuiper belt or scattered disk. But most distant of all would be objects originating from the Oort cloud: thousands of A.U. away and representative of the edge of our Solar System.

The Oort cloud has not yet been demonstrated to exist, although there are compelling theoretical and indirect observational reasons (such as the ultra-long-period or hyperbolically-orbiting comets we’ve found) to think it’s real. In theory, a spherically-distributed set of bodies that formed very early on, at the same time the Solar System did, should exist from around

1000 A.U. away all the way to perhaps a light year or two. In 2003, the team of Mike Brown, Chad Trujillo, and David Rabinowitz discovered the first candidate Oort cloud object: Sedna. Sedna has an aphelion (its farthest distance from the Sun) of around 900 A.U., one of the most distant aphelia known. But its closest distance to the Sun (perihelion) is a very large 76 A.U. Sedna never gets close enough to any of the major planets for a gravitational interaction to have scattered it.

There’s rampant speculation, therefore, that Sedna is the first object we’ve ever discovered originating from the Oort cloud. In the 15 years that have passed since its discovery, only one more Sedna-like object has been discovered: 2012 VP113, with a perihelion of 80 A.U. But the biggest difference is size-wise: Sedna is enormous, with a diameter of 1000 km, making it slightly larger than the dwarf planet Ceres. We were only able to discover Sedna because of how large and bright and reflective it is to date, it is the only detached object (or beyond) that was discovered by direct obseration. And even at that, we only saw it because it happened to be quite close to perihelion, rather than aphelion, at the time of its discovery.

Sedna takes approximately 11,000 years to complete an orbit around the Sun, and is approximately about 85 A.U. away as of today. It is moving closer to the Sun, and will reach perihelion in 2075. Owing to Sedna’s size, orbital characteristics, and its origin, it is often considered to be the most scientifically important trans-Neptunian object ever discovered. And if we choose to, we can send a mission to the outer Solar System to reach it as it nears its perihelion. But due to the orbital particulars of all the planets in our Solar System, we really only get two chances. They’re both coming up fast: 2033 and 2046, if we want to truly learn about this fascinating relic from our Solar System’s formation.

The reasons are simple. Sedna’s impending close approach means we will not get the opportunity to study it this close to the Sun for many millennia again. At present, there are no missions even under consideration by NASA to explore Sedna. In order to get to Sedna, however, the most energy-efficient path would be to use a gravity assist from Jupiter, and there are only two windows where Earth, Jupiter, and Sedna are properly aligned to make such a launch: May of 2033 and June of 2046. If we choose one of these windows, we could arrive at Sedna after a journey of 24.5 years in space. If we chose the 2033 launch, that would correspond to an arrival in late 2057, when Sedna will be 77.27 A.U. from the Sun. The 2046 window would get there in December of 2070, at a slightly closer 76.43 A.U.

Think about everything we learned from the New Horizons mission. We know what Pluto looks like, what its geology is, what its atmosphere is made of, about its various ices, compositions, the weather it experiences, the full extent of its lunar system, its topography, and much, much more. We now understand more about how our Solar System formed and the young objects that formed in its outskirts than ever before. And we did it with instruments that were designed and built in the early 2000s.

Now, imagine learning those same things about a whole new class of objects: bodies that originated from far beyond where our Solar System’s protoplanetary disk formed. Imagine what we instruments we could design and build, and what scientific questions we could answer, if we constructed a mission during the 2020s or 2030s. This is our best chance to explore what’s possibly the most unique and serendipitous object that will pass close to our Sun for thousands of years, and if we ever believed in the spirit of space exploration, this is our golden opportunity.

Does the Oort cloud exist? Is Sedna, in terms of composition and its geophysical properties, distinctly different from the objects that formed in the Kuiper belt? Does it have its origin in the Oort cloud? At its extreme size, what are its planetary science properties? Does it have satellites or an atmosphere? Does it rotate or tumble, and are there ingredients for life on it? These are questions that, if we’re curious about them, we could design and build a mission that gives us the answers. Sedna won’t be back for over 10,000 years, and it may be the largest, most distant object that we’ll have the chance to have a close encounter with until its return. Missions take a very long time to design, plan, and execute, particularly the most ambitious ones. If we want to go in 2033, the time to start planning is now.

## How is it possible that we haven't discovered anything in the Oort cloud yet? - Astronomy

How will we be remembered in 200 years? I happen to live in a little town, Princeton, in New Jersey, which every year celebrates the great event in Princeton history: the Battle of Princeton, which was, in fact, a very important battle. It was the first battle that George Washington won, in fact, and was pretty much of a turning point in the war of independence. It happened 225 years ago. It was actually a terrible disaster for Princeton. The town was burned down it was in the middle of winter, and it was a very, very severe winter. And about a quarter of all the people in Princeton died that winter from hunger and cold, but nobody remembers that. What they remember is, of course, the great triumph, that the Brits were beaten, and we won, and that the country was born. And so I agree very emphatically that the pain of childbirth is not remembered. It's the child that's remembered. And that's what we're going through at this time.

I wanted to just talk for one minute about the future of biotechnology, because I think I know very little about that — I'm not a biologist — so everything I know about it can be said in one minute. (Laughter) What I'm saying is that we should follow the model that has been so successful with the electronic industry, that what really turned computers into a great success, in the world as a whole, is toys. As soon as computers became toys, when kids could come home and play with them, then the industry really took off. And that has to happen with biotech.

There's a huge — (Laughter) (Applause) — there's a huge community of people in the world who are practical biologists, who are dog breeders, pigeon breeders, orchid breeders, rose breeders, people who handle biology with their hands, and who are dedicated to producing beautiful things, beautiful creatures, plants, animals, pets. These people will be empowered with biotech, and that will be an enormous positive step to acceptance of biotechnology. That will blow away a lot of the opposition. When people have this technology in their hands, you have a do-it-yourself biotech kit, grow your own — grow your dog, grow your own cat. (Laughter) (Applause) Just buy the software, you design it. I won't say anymore, you can take it on from there. It's going to happen, and I think it has to happen before the technology becomes natural, becomes part of the human condition, something that everybody's familiar with and everybody accepts.

So, let's leave that aside. I want to talk about something quite different, which is what I know about, and that is astronomy. And I'm interested in searching for life in the universe. And it's open to us to introduce a new way of doing that, and that's what I'll talk about for 10 minutes, or whatever the time remains. The important fact is, that most of the real estate that's accessible to us — I'm not talking about the stars, I'm talking about the solar system, the stuff that's within reach for spacecraft and within reach of our earthbound telescopes — most of the real estate is very cold and very far from the Sun.

If you look at the solar system, as we know it today, it has a few planets close to the Sun. That's where we live. It has a fairly substantial number of asteroids between the orbit of the Earth out through — to the orbit of Jupiter. The asteroids are a substantial amount of real estate, but not very large. And it's not very promising for life, since most of it consists of rock and metal, mostly rock. It's not only cold, but very dry. So the asteroids we don't have much hope for.

There stand some interesting places a little further out: the moons of Jupiter and Saturn. Particularly, there's a place called Europa, which is — Europa is one of the moons of Jupiter, where we see a very level ice surface, which looks as if it's floating on top of an ocean. So, we believe that on Europa there is, in fact, a deep ocean. And that makes it extraordinarily interesting as a place to explore. Ocean — probably the most likely place for life to originate, just as it originated on the Earth. So we would love to explore Europa, to go down through the ice, find out who is swimming around in the ocean, whether there are fish or seaweed or sea monsters — whatever there may be that's exciting —- or cephalopods. But that's hard to do. Unfortunately, the ice is thick. We don't know just how thick it is, probably miles thick, so it's very expensive and very difficult to go down there — send down your submarine or whatever it is — and explore. That's something we don't yet know how to do. There are plans to do it, but it's hard.

Go out a bit further, you'll find that beyond the orbit of Neptune, way out, far from the Sun, that's where the real estate really begins. You'll find millions or trillions or billions of objects which, in what we call the Kuiper Belt or the Oort Cloud — these are clouds of small objects which appear as comets when they fall close to the Sun. Mostly, they just live out there in the cold of the outer solar system, but they are biologically very interesting indeed, because they consist primarily of ice with other minerals, which are just the right ones for developing life. So if life could be established out there, it would have all the essentials — chemistry and sunlight — everything that's needed.

So, what I'm proposing is that there is where we should be looking for life, rather than on Mars, although Mars is, of course, also a very promising and interesting place. But we can look outside, very cheaply and in a simple fashion. And that's what I'm going to talk about. There is a — imagine that life originated on Europa, and it was sitting in the ocean for billions of years. It's quite likely that it would move out of the ocean onto the surface, just as it did on the Earth. Staying in the ocean and evolving in the ocean for 2 billion years, finally came out onto the land. And then of course it had great — much greater freedom, and a much greater variety of creatures developed on the land than had ever been possible in the ocean. And the step from the ocean to the land was not easy, but it happened.

Now, if life had originated on Europa in the ocean, it could also have moved out onto the surface. There wouldn't have been any air there — it's a vacuum. It is out in the cold, but it still could have come. You can imagine that the plants growing up like kelp through cracks in the ice, growing on the surface. What would they need in order to grow on the surface? They'd need, first of all, to have a thick skin to protect themselves from losing water through the skin. So they would have to have something like a reptilian skin. But better — what is more important is that they would have to concentrate sunlight. The sunlight in Jupiter, on the satellites of Jupiter, is 25 times fainter than it is here, since Jupiter is five times as far from the Sun. So they would have to have — these creatures, which I call sunflowers, which I imagine living on the surface of Europa, would have to have either lenses or mirrors to concentrate sunlight, so they could keep themselves warm on the surface. Otherwise, they would be at a temperature of minus 150, which is certainly not favorable for developing life, at least of the kind we know. But if they just simply could grow, like leaves, little lenses and mirrors to concentrate sunlight, then they could keep warm on the surface. They could enjoy all the benefits of the sunlight and have roots going down into the ocean life then could flourish much more. So, why not look? Of course, it's not very likely that there's life on the surface of Europa. None of these things is likely, but my, my philosophy is, look for what's detectable, not for what's probable.

There's a long history in astronomy of unlikely things turning out to be there. And I mean, the finest example of that was radio astronomy as a whole. This was — originally, when radio astronomy began, Mr. Jansky, at the Bell labs, detected radio waves coming from the sky. And the regular astronomers were scornful about this. They said, "It's all right, you can detect radio waves from the Sun, but the Sun is the only object in the universe that's close enough and bright enough actually to be detectable. You can easily calculate that radio waves from the Sun are fairly faint, and everything else in the universe is millions of times further away, so it certainly will not be detectable. So there's no point in looking." And that, of course, that set back the progress of radio astronomy by about 20 years. Since there was nothing there, you might as well not look. Well, of course, as soon as anybody did look, which was after about 20 years, when radio astronomy really took off. Because it turned out the universe is absolutely full of all kinds of wonderful things radiating in the radio spectrum, much brighter than the Sun. So, the same thing could be true for this kind of life, which I'm talking about, on cold objects: that it could in fact be very abundant all over the universe, and it's not been detected just because we haven't taken the trouble to look.

So, the last thing I want to talk about is how to detect it. There is something called pit lamping. That's the phrase which I learned from my son George, who is there in the audience. You take — that's a Canadian expression. If you happen to want to hunt animals at night, you take a miner's lamp, which is a pit lamp. You strap it onto your forehead, so you can see the reflection in the eyes of the animal. So, if you go out at night, you shine a flashlight, the animals are bright. You see the red glow in their eyes, which is the reflection of the flashlight. And then, if you're one of these unsporting characters, you shoot the animals and take them home. And of course, that spoils the game for the other hunters who hunt in the daytime, so in Canada that's illegal. In New Zealand, it's legal, because the New Zealand farmers use this as a way of getting rid of rabbits, because the rabbits compete with the sheep in New Zealand. So, the farmers go out at night with heavily armed jeeps, and shine the headlights, and anything that doesn't look like a sheep, you shoot. (Laughter)

So I have proposed to apply the same trick to looking for life in the universe. That if these creatures who are living on cold surfaces — either on Europa, or further out, anywhere where you can live on a cold surface — those creatures must be provided with reflectors. In order to concentrate sunlight, they have to have lenses and mirrors — in order to keep themselves warm. And then, when you shine sunlight at them, the sunlight will be reflected back, just as it is in the eyes of an animal. So these creatures will be bright against the cold surroundings. And the further out you go in this, away from the Sun, the more powerful this reflection will be. So actually, this method of hunting for life gets stronger and stronger as you go further away, because the optical reflectors have to be more powerful so the reflected light shines out even more in contrast against the dark background. So as you go further away from the Sun, this becomes more and more powerful. So, in fact, you can look for these creatures with telescopes from the Earth. Why aren't we doing it? Simply because nobody thought of it yet.

But I hope that we shall look, and with any — we probably won't find anything, none of these speculations may have any basis in fact. But still, it's a good chance. And of course, if it happens, it will transform our view of life altogether. Because it means that — the way life can live out there, it has enormous advantages as compared with living on a planet. It's extremely hard to move from one planet to another. We're having great difficulties at the moment and any creatures that live on a planet are pretty well stuck. Especially if you breathe air, it's very hard to get from planet A to planet B, because there's no air in between. But if you breathe air — (Laughter) — you're dead — (Laughter) — as soon as you're off the planet, unless you have a spaceship.

But if you live in a vacuum, if you live on the surface of one of these objects, say, in the Kuiper Belt, this — an object like Pluto, or one of the smaller objects in the neighborhood of Pluto, and you happened — if you're living on the surface there, and you get knocked off the surface by a collision, then it doesn't change anything all that much. You still are on a piece of ice, you can still have sunlight and you can still survive while you're traveling from one place to another. And then if you run into another object, you can stay there and colonize the other object. So life will spread, then, from one object to another. So if it exists at all in the Kuiper Belt, it's likely to be very widespread. And you will have then a great competition amongst species — Darwinian evolution — so there'll be a huge advantage to the species which is able to jump from one place to another without having to wait for a collision. And there'll be advantages for spreading out long, sort of kelp-like forest of vegetation. I call these creatures sunflowers. They look like, maybe like sunflowers. They have to be all the time pointing toward the Sun, and they will be able to spread out in space, because gravity on these objects is weak. So they can collect sunlight from a big area. So they will, in fact, be quite easy for us to detect.

So, I hope in the next 10 years, we'll find these creatures, and then, of course, our whole view of life in the universe will change. If we don't find them, then we can create them ourselves. (Laughter) That's another wonderful opportunity that's opening. We can — as soon as we have a little bit more understanding of genetic engineering, one of the things you can do with your take-it-home, do-it-yourself genetic engineering kit — (Laughter) — is to design a creature that can live on a cold satellite, a place like Europa, so we could colonize Europa with our own creatures. That would be a fun thing to do. (Laughter) In the long run, of course, it would also make it possible for us to move out there. What's going to happen in the end, it's not going to be just humans colonizing space, it's going to be life moving out from the Earth, moving it into its kingdom. And the kingdom of life, of course, is going to be the universe. And if life is already there, it makes it much more exciting, in the short run. But in the long run, if there's no life there, we create it ourselves. We transform the universe into something much more rich and beautiful than it is today. So again, we have a big and wonderful future to look forward. Thank you. (Applause)

## Space object with orbit stretching into the Oort cloud discovered

Astronomers Pedro Bernardinelli and Gary Bernstein discovered a space object recently that has an orbit around the sun and also stretches into the Oort cloud—they have named it 2014 UN271. The researchers made the discovery while studying archival images collected for the Dark Energy Survey over the years 2014 to 2018. Since its discovery, entities such as the MMPL forum, the Minor Planet Center and JPL Solar System Dynamics have been tracking the object and have found that it will make its closest approach to Earth in 2031.

Measurements of the object put it between the size of a very small planet and a comet—it is believed to have a diameter of 100 to 370 km. If it turns out to be on the larger end of that spectrum, it would mark the largest Oort cloud object discovered to date. But it is the path of the object that has drawn the attention of astronomers—its orbit is nearly perpendicular to the plane created by the nine inner planets and takes it deep into the solar system and into the Oort cloud. One trip around the sun has been calculated to take 612,190 years. It is currently moving deeper into the solar system, which means astronomers will have an opportunity to observe it 10 years from now.

Sam Deen, an amateur astronomer posting on the MMPL forum described the find as “radically exceptional.” Study of 2014 UN271 as it draws closer will allow researchers to analyze an object that sometimes passes through the Oort cloud at distances as close as 10.9 AU from the sun—near the orbit of Saturn. As it draws nearer to the sun, it is likely to develop a comet-like tail as frozen material on its surface is vaporized. It is not clear just yet, however, how bright 2014 UN271 will appear in the night sky here on Earth—but it is likely that its brightness will fall somewhere between that of Pluto or its moon Charon enough for amateurs and professionals alike to get a good view of it using strong telescopes.

Space object with orbit stretching into the Oort cloud discovered

Are we alone in the universe?

It is one of the most profound questions posed in modern astronomy. But although our understanding of the cosmos has grown significantly, the question remains unanswered. We know that Earth-like planets are common, as are the building blocks necessary for terrestrial life, and yet we still haven’t found definitive evidence for life beyond Earth. Perhaps part of our problem is that we are mostly looking for life similar to our own. It is possible that alien life is so radically different from that of Earth it goes unnoticed.

## 12 Possible Reasons We Haven't Found Aliens

In 1950, a learned lunchtime conversation set the stage for decades of astronomical exploration. Physicist Enrico Fermi submitted to his colleagues around the table a couple contentions, summarized as 1) The galaxy is very old and very large, with hundreds of billions of stars and likely even more habitable planets. 2) That means there should be more than enough time for advanced civilizations to develop and flourish across the galaxy.

So where the heck are they?

This simple, yet powerful argument became known as the Fermi Paradox, and it still boggles many sage minds today. Aliens should be common, yet there is no convincing evidence that they exist.

Here are twelve possible reasons why this is so.

1. There aren't any aliens to find. As unlikely as it seems in a galaxy with hundreds of billions of stars and as many as 40 billion Earth-size planets in habitable zones, we could be alone.

2. There is no intelligent life besides us. (This assumes, of course, that humans count as intelligent.) Life may exist, but it could simply take the form of miniscule microbes or other cosmically "quiet" animals.

3. Intelligent species lack advanced technology. Currently, astronomers utilize radio telescopes to listen intently to the night sky. So if alien species aren't broadcasting any signals, we'd never know they existed.

4. Intelligent life self-destructs. Whether via weapons of mass destruction, planetary pollution, or manufactured virulent disease, it may be the nature of intelligent species to commit suicide, existing for only a short time before winking out of existence.

5. The universe is a deadly place. On cosmic timescales – think billions of years – life may be fleeting. All it takes is a single asteroid, supernova, gamma ray burst, or solar flare to render a life-harboring planet lifeless.

6. Space is big. The Milky Way alone is 100,000 light years across, so it's conceivable that the focused signals of intelligent aliens, which are limited to the speed of light, simply haven't reached us yet.

7. We haven't been looking long enough. Eighty years. That's the amount of time that radio telescopes, which allow us to detect alien signals, have been around. And we've been actively searching for aliens for maybe sixty years. That's not very long at all.

8. We're not looking in the correct place. As previously mentioned, space is big, so there are tons of regions to listen for alien signals. If we're not listening precisely in the direction from which a signal is originating, we'd never hear it. As Andrew Fain explained at Universe Today, it's like trying to speak with your friend on a 250,000,000,000-channel CB radio, without any knowledge of the frequency on which they are transmitting. You'll probably be channel flipping for a long time.

9. Alien technology may be too advanced. Radio technology may be commonplace here on Earth, but on far-flung worlds, alien societies may have graduated to more advanced communication technologies, like neutrino signals. We can't decipher those just yet.

10. Nobody is transmitting. Instead, everybody may be listening. That's basically how it is here on Earth. Apart from a few paltry efforts to broadcast strong signals over a narrow frequency band towards the stars above, we've barely made our presence known in the universe. In fact, if aliens have radio telescopes similar to what we have on Earth, our television and radio broadcasts would only be detectable up to 0.3 light-years away. That distance doesn't even transcend the farthest reaches of our solar system.

11. Earth is deliberately not being contacted. On Earth, we have policies about contacting indigenous peoples it's possible that the same thing could be happening with us. Just like in Star Trek, advanced alien societies may enforce rules that limit contact only to species that attain a lofty degree of technological or cultural evolution.

12. Aliens are already here and we just don't realize it. Conspiracy theorists love this unlikely explanation. While the chances are remote, it's not impossible that government agencies are concealing the presence of aliens. Although it's more likely that aliens are already amongst us, observing humanity in the clever and ironic guise of lab mice.

## Are Mass Extinctions Periodic? And Are We Due For One?

“That which can be asserted without evidence, can be dismissed without evidence.” -Christopher Hitchens

65 million years ago, a massive asteroid, perhaps five to ten kilometers across, struck the Earth at speeds in excess of 20,000 miles per hour. In the aftermath of this catastrophic collision, the giant behemoths known as the dinosaurs, which had dominated the Earth’s surface for over 100 million years, were exterminated. In fact, around 30% of all species currently existing on Earth at the time were wiped out. This wasn’t the first time Earth had been struck by such a catastrophic object, and given what’s out there, it likely won’t be the last. An idea that’s been considered for some time is that these events are actually periodic, caused by the Sun’s motion through the galaxy. If that’s the case, we should be able to predict when the next one is coming, and whether we’re living in a time of severely increased risk.

There’s always a danger of a mass extinction, but the key is quantifying that danger accurately. Extinction threats in our Solar System — from cosmic bombardment — generally come from two sources: the asteroid belt in between Mars and Jupiter, and the Kuiper belt and Oort cloud out beyond the orbit of Neptune. For the asteroid belt, the suspected (but not the certain) origin of the dinosaur-killer, our odds of getting hit by a large object significantly decrease over time. There’s a good reason for this: the amount of material in between Mars and Jupiter gets depleted over time, with no mechanism for replenishing it. We can understand this by looking at a few things: young Solar Systems, early models of our own Solar System, and most airless worlds without particularly active geologies: the Moon, Mercury and most moons of Jupiter and Saturn.

The history of impacts in our Solar System is literally written on the faces of worlds like the Moon. Where the lunar highlands are — the lighter spots — we can see a longstanding history of heavy cratering, dating all the way back to the earliest days in the Solar System: more than 4 billion years ago. There are a great many large craters with smaller and smaller craters inside: evidence that there was an incredibly high level of impact activity early on. However, if you look at the dark regions (the lunar maria), you can see far fewer craters inside. Radiometric dating shows that most of these areas are between 3 and 3.5 billion years old, and even that is different enough that the amount of cratering is far less. The youngest regions, found in Oceanus Procellarum (the largest mare on the moon), are only 1.2 billion years old and are the least cratered.

From this evidence, we can infer that the asteroid belt is getting sparser and sparser over time, as the cratering rate drops. The leading school of thought is that we haven’t reached it yet, but at some point over the next few billion years, the Earth should experience its very final large asteroid strike, and if there’s still life on the world, the last mass extinction event arising from such a catastrophe. The asteroid belt poses less of a danger, today than it ever has in the past.

But the Oort cloud and the Kuiper belt are different stories.

Out beyond Neptune in the outer Solar System, there’s tremendous potential for a catastrophe. Hundreds of thousands — if not millions — of large ice-and-rock chunks wait in a tenuous orbit around our Sun, where a passing mass (like Neptune, another Kuiper belt/Oort cloud object, or a passing star/planet) has the potential to gravitationally disrupt it. The disruption could have any number of outcomes, but one of them is to hurl it towards the inner Solar System, where it could arrive as a brilliant comet, but where it could also collide with our world.

The interactions with Neptune or other objects in the Kuiper belt/Oort cloud are random and independent of anything else going on in our galaxy, but it’s possible that passing through a star-rich region — such as the galactic disk or one of our spiral arms — could enhance the odds of a comet storm, and the chance of a comet strike on Earth. As the Sun moves through the Milky Way, there’s an interesting quirk of its orbit: approximately once every 31 million years or so, it passes through the galactic plane. This is just orbital mechanics, as the Sun and all the stars follow elliptical paths around the galactic center. But some people have claimed that there’s evidence for periodic extinctions on that same timescale, which might suggest that these extinctions are triggered by a comet storm every 31 million years.

Is that plausible? The answer can be found in the data. We can look at the major extinction events on Earth as evidenced by the fossil record. The method we can use is to count the number of genera (one step more generic than “species” in how we classify living beings for human beings, the “homo” in homo sapiens is our genus) in existence at any given time. We can do this going back more than 500 million years in time, thanks to the evidence found in sedimentary rock, allowing us to see what percent both existed and also died off in any given interval.

We can then look for patterns in these extinction events. The easiest way to do it, quantitatively, is to take the Fourier transform of these cycles and see where (if anywhere) patterns emerge. If we saw mass extinction events every 100 million years, for example, where there was a big drop in the number of genera with that exact period every time, then the Fourier transform would show a huge spike at a frequency of 1/(100 million years). So let’s get right to it: what does the extinction data show?

There is some relatively weak evidence for a spike with a frequency of 140 million years, and another, slightly stronger spike at 62 million years. Where the orange arrow is, you can see where a 31 million year periodicity would occur. These two spikes look huge, but that’s only relative to the other spikes, which are totally insignificant. How strong, objectively, are these two spikes, which are our evidence for periodicity?

500 million years, you can only fit three possible 140 million year mass extinctions in there, and only about 8 possible 62 million year events. What we see doesn’t fit with an event happening every 140 million or every 62 million years, but rather if we see an event in the past, there’s an increased chance of having another event either 62 or 140 million years in the past or future. But, as you can clearly see, there’s no evidence for a 26–30 million year periodicity in these extinctions.

If we start looking at the craters we find on Earth and the geological composition of the sedimentary rock, however, the idea falls apart completely. Of all the impacts that occur on Earth, less than one-quarter of them come from objects originating from the Oort cloud. Even worse, of the boundaries between geological timescales (Triassic/Jurassic, Jurassic/Cretaceous, or the Cretaceous/Paleogene boundary), and the geological records that correspond to extinction events, only the event from 65 million years ago shows the characteristic ash-and-dust layer that we associate with a major impact.

The idea that mass extinctions are periodic is an interesting and compelling one, but the evidence simply isn’t there for it. The idea that the Sun’s passage through the galactic plane causes periodic impacts tells a great story, too, but again, there’s no evidence. In fact, we know that stars come within reach of the Oort cloud every half-million years or so, but we’re certainly well-spaced between those events at present. For the foreseeable future, the Earth isn’t at increased risk of a natural disaster coming from the Universe. Instead, it looks like our greatest danger is posed by the one place we all dread to look: at ourselves.