OSSOS XV: PROBING THE DISTANT SOLAR SYSTEM WITH OBSERVED SCATTERING TNOS.

Most known trans-Neptunian objects (TNOs) gravitationally scattering off the giant planets have orbital inclinations consistent with an origin from the classical Kuiper belt, but a small fraction of these "scattering TNOs" have inclinations that are far too large (i > 45°) for this origin. These scattering outliers have previously been proposed to be interlopers from the Oort cloud or evidence of an undiscovered planet. Here we test these hypotheses using N-body simulations and the 69 centaurs and scattering TNOs detected in the Outer Solar Systems Origins Survey and its predecessors. We confirm that observed scattering objects cannot solely originate from the classical Kuiper belt, and we show that both the Oort cloud and a distant planet generate observable highly inclined scatterers. Although the number of highly inclined scatterers from the Oort Cloud is ~3 times less than observed, Oort cloud enrichment from the Sun's galactic migration or birth cluster could resolve this. Meanwhile, a distant, low-eccentricity 5 M⊕ planet replicates the observed fraction of highly inclined scatterers, but the overall inclination distribution is more excited than observed. Furthermore, the distant planet generates a longitudinal asymmetry among detached TNOs that is less extreme than often presumed, and its direction reverses across the perihelion range spanned by known TNOs. More complete models that explore the dynamical origins of the planet are necessary to further study these features. With observational biases well-characterized, our work shows that the orbital distribution of detected scattering bodies is a powerful constraint on the unobserved distant solar system.

A Uranian Trojan and the frequency of temporary giant-planet co-orbitals.

Trojan objects share a planet's orbit, never straying far from the triangular Lagrangian points, 60° ahead of (L4) or behind (L5) the planet. We report the detection of a Uranian Trojan; in our numerical integrations, 2011 QF99 oscillates around the Uranian L4 Lagrange point for >70,000 years and remains co-orbital for ~1 million years before becoming a Centaur. We constructed a Centaur model, supplied from the transneptunian region, to estimate temporary co-orbital capture frequency and duration (to a factor of 2 accuracy), finding that at any time 0.4 and 2.8% of the population will be Uranian and Neptunian co-orbitals, respectively. The co-orbital fraction (~2.4%) among Centaurs in the International Astronomical Union Minor Planet Centre database is thus as expected under transneptunian supply.

Impact seeding and reseeding in the inner solar system.

Assuming that asteroidal and cometary impacts onto Earth can liberate material containing viable microorganisms, we studied the subsequent distribution of the escaping impact ejecta throughout the inner Solar System on time scales of 30,000 years. Our calculations of the delivery rates of this terrestrial material to Mars and Venus, as well as back to Earth, indicate that transport to great heliocentric distances may occur in just a few years and that the departure speed is significant. This material would have been efficiently and quickly dispersed throughout the Solar System. Our study considers the fate of all the ejected mass (not just the slowly moving material), and tabulates impact rates onto Venus and Mars in addition to Earth itself. Expressed as a fraction of the ejected particles, roughly 0.1% and 0.001% of the ejecta particles would have reached Venus and Mars, respectively, in 30,000 years, making the biological seeding of those planets viable if the target planet supported a receptive environment at the time. In terms of possibly safeguarding terrestrial life by allowing its survival in space while our planet cools after a major killing thermal pulse, we show via our 30,000- year integrations that efficient return to Earth continues for this duration. Our calculations indicate that roughly 1% of the launched mass returns to Earth after a major impact regardless of the impactor speed; although a larger mass is ejected following impacts at higher speeds, a smaller fraction of these ejecta is returned. Early bacterial life on Earth could have been safeguarded from any purported impact-induced extinction by temporary refuge in space.

The Kuiper belt and the solar system's comet disk.

Our planetary system is embedded in a small-body disk of asteroids and comets, vestigial remnants of the original planetesimal population that formed the planets. Once formed, those planets dispersed most of the remaining small bodies. Outside of Neptune, this process has left our Kuiper belt and built the Oort cloud, as well as emplacing comets into several other identifiable structures. The orbits in these structures indicate that our outer solar system's comet disk was shaped by a variety of different physical processes, which teach us about how the giant planets formed. Recent work has shown that the scattered disk is the most likely source of short-period comets. Moreover, a growing body of evidence indicates that the sculpting of the Kuiper belt region may have involved large-scale planetary migration, the presence of other rogue planetary objects in the disk, and/or the close passage of other stars in the Sun's birth cluster.

Discovery of five irregular moons of Neptune.

Each giant planet of the Solar System has two main types of moons. 'Regular' moons are typically larger satellites with prograde, nearly circular orbits in the equatorial plane of their host planets at distances of several to tens of planetary radii. The 'irregular' satellites (which are typically smaller) have larger orbits with significant eccentricities and inclinations. Despite these common features, Neptune's irregular satellite system, hitherto thought to consist of Triton and Nereid, has appeared unusual. Triton is as large as Pluto and is postulated to have been captured from heliocentric orbit; it traces a circular but retrograde orbit at 14 planetary radii from Neptune. Nereid, which exhibits one of the largest satellite eccentricities, is believed to have been scattered from a regular satellite orbit to its present orbit during Triton's capture. Here we report the discovery of five irregular moons of Neptune, two with prograde and three with retrograde orbits. These exceedingly faint (apparent red magnitude m(R) = 24.2-25.4) moons, with diameters of 30 to 50 km, were presumably captured by Neptune.