Astronomers Are Closing In on the Kuiper Belt's Secrets As next-generation telescopes map this outer frontier, astronomers are bracing for discoveries that could reveal hidden planets, strange structures, and clues to the solar system's chaotic youth. Out beyond the orbit of Neptune lies an expansive ring of ancient relics, dynamical enigmas, and possibly a hidden planet--or two. The Kuiper Belt, a region of frozen debris about 30 to 50 times farther from the sun than the Earth is--and perhaps farther, though nobody knows--has been shrouded in mystery since it first came into view in the 1990s. Over the past 30 years, astronomers have cataloged about 4,000 Kuiper Belt objects (KBOs), including a smattering of dwarf worlds, icy comets, and leftover planet parts. But that number is expected to increase tenfold in the coming years as observations from more advanced telescopes pour in.

The seventh asteroid, Justitia, is the most intriguing. About 30 miles wide, Justitia is very reddish, an unusual color for an asteroid. Indeed, it looks more like one of the small icy worlds found in the Kuiper belt, circling the sun beyond the orbit of Neptune. That has led planetary scientists to speculate that Justitia formed in the outer reaches of the solar system and then was scattered inward by the shifting orbits of the giant planets, eventually joining the asteroid belt. If that is true, a visit to Justitia would provide a close-up study of a Kuiper belt object without the long trip to the solar system's distant reaches.

A Measurement of the Kuiper Belt's Mean Plane From Objects Classified By Machine Learning Submitted to AJ ABSTRACT Mean plane measurements of the Kuiper Belt from observational data are of interest for their potential to test dynamical models of the solar system. Recent measurements have yielded inconsistent results. Here we report a measurement of the Kuiper Belt's mean plane with a sample size more than twice as large as in previous measurements. The sample of interest is the non-resonant Kuiper belt objects, which we identify by using machine learning on the observed Kuiper Belt population whose orbits are well-determined. We estimate the measurement error with a Monte Carlo procedure. We find that the overall mean plane of the non-resonant Kuiper Belt (semimajor axis range 35-150 au) and also that of the classical Kuiper Belt (semimajor axis range 42-48 au) are both close to (within 0.7 When binning the sample into smaller semimajor axis bins, we find the measured mean plane mostly consistent with both the invariable plane and the theoretically expected Laplace surface forced by the known planets. Statistically significant discrepancies are found only in the semimajor axis ranges 40.3-42 au and 45-50 au; these ranges are in proximity to the ν These results do not support a previously reported anomalous warp at semimajor axes above 50 au. INTRODUCTION Chiang & Choi (2008) posed the question:"If we could map, at fixed time, the instantaneous locations in threedimensional space of all Kuiper Belt objects [KBOs], on what two-dimensional surface would the density of KBOs be greatest?" The authors demonstrated that this surface, also known as the Laplace surface, is given by the Laplace-Lagrange linear secular theory (Murray & Dermott 1999). This theory is based on the time-variable forcing arising from the planets' secular variations; consequently, the local normal on the Laplace surface varies only slowly with time; secular timescales for KBOs are much longer than 10 The Laplace surface for particles within the Kuiper Belt is not a flat plane because it has warps owing to secular resonances in certain localized regions of semimajor axes within the belt where the rate of orbit pole precession coincides with one of the inclination secular mode frequencies of the planets; at large semimajor axes the Laplace surface converges to the solar system's invariable plane.

There are eight recognized planets in our solar system, four inner rocky planets and four outer gas giants. But beyond the orbit of Neptune, dozens of dwarf planets the size of Pluto or smaller populate a region known as the Kuiper Belt, and new computer models show there may be something even bigger lurking out there – or at least there might have been in the past. In a paper in the Annual Review of Astronomy and Astrophysics this month, Brett Gladman of the University of British Columbia and Kathryn Volk of the University of Arizona argue that new models indicate that the likelihood that a Mars-sized planet orbiting in the Kuiper Belt region is at least 50%, though they also claim that it was ejected from the solar system entirely at some point in the past. This would be a different planet then the theoretical one currently being called Planet Nine, which is believed to be a Neptune-sized gas giant far out beyond the Kuiper Belt. To get a better sense of how the solar system formed, researchers like Gladman and Volk typically use powerful computers to run simulations with different variables to see how changes in any one variable affects the kind of solar system that would result.

There were some tense hours at the operation center for the New Horizons mission when the spacecraft briefly lost contact with Earth on July 4, 2015, just days from its long-awaited flyby of Pluto. It's just one of many gripping moments in a book that Alan Stern, the mission leader and a co-author of "Chasing New Horizons" along with astrobiologist David Grinspoon, describes as a "techno-thriller about how the farthest planet was explored." Dr. Stern recently sat down for an interview in his Boulder, Colo., office, surrounded by photos and mementos from the New Horizons mission – a mission that took decades to convince NASA to get off the ground and another decade to travel 3 billion miles to the last unexplored planet in our solar system. The New Horizons spacecraft continues to explore the vast reaches of the Kuiper Belt, at the outer edge of our solar system. His responses have been edited for clarity and brevity.

In a mere 60 years, we of Earth have gone from launching our first spacecraft, to exploring every planet and major moon in our solar system, to establishing an international, long-lived fleet of robotic spacecraft at the Moon and Mars. What will we do in the next 100 years? With such rapid expansion of capability, it may seem difficult to tell what the next 60 years will bring, much less the next century. But we never do anything in space without first imagining what we could do, so in that spirit, here is an attempt to predict--and nudge us into--the future. So far, almost all of our exploration of worlds beyond Earth has been through the senses of robotic emissaries.