Some Planet-like Kuiper Belt Objects Don’t Play “Nice”

Distribution of Kuiper belt objects (green), along with various other outer Solar System bodies, based on data from the Minor Planet Center. [Credit: Minor Planet Center; Murray and Dermott]

The Kuiper belt — the region beyond the orbit of Neptune inhabited by a number of small bodies of rock and ice — hides many clues about the early days of the Solar System. According to the standard picture of Solar System formation, many planetesimals were born in the chaotic region where the giant planets now reside. Some were thrown out beyond the orbit of Neptune, while others stayed put in the form of Trojan asteroids (which orbit in the same trajectory as Jupiter and other planets). This is called the Nice model.

However, not all Kuiper belt objects (KBOs) play nicely with the Nice model.

(I should point out that the model is named named for the city in France and therefore pronounced “neese”.) A new study of large scale surveys of KBOs revealed that those with nearly circular orbits lying roughly in the same plane as the orbits of the major planets don’t fit the Nice model, while those with irregular orbits do. It’s a puzzling anomaly, one with no immediate resolution, but it hints that we need to refine our Solar System formation models.

This new study is described in a recently released paper by Wesley Fraser, Mike Brown, Alessandro Morbidelli, Alex Parker, and Konstantin Baygin (to be published in the Astrophysical Journal, available online). These researchers combined data from seven different surveys of KBOs to determine roughly how many of each size of object are in the Solar System, which in turn is a good gauge of the environment in which they formed.

The difference between this and previous studies is the use of absolute magnitudes — a measure of how bright an object really is — as opposed to their apparent magnitudes, which are simply how bright an object appears. The two types of magnitude are related by the distance an object is from Earth, so the observational challenge comes down to accurate distance measurements. Absolute magnitude is also related to the size of an KBO and its albedo (how much light it reflects), both important physical quantities for understanding formation and composition.

Finding the absolute magnitudes for KBOs is more challenging than apparent magnitudes for obvious reasons: these are small objects, often not resolved as anything other than points of light in a telescope. That means requires measuring the distance to each KBO as accurately as possible. As the authors of the study point out, even small errors in distance measurements can have a large effect on the estimated absolute magnitude.

The bodies in the Kuiper Belt. Credit: Don Dixon
The bodies in the Kuiper Belt. Credit: Don Dixon

In terms of orbits, KBOs fall into two categories: “hot” and “cold”, confusing terms having nothing to do with temperature. The “cold” KBOs are those with nearly circular orbits (low eccentricity, in mathematical terms) and low inclinations, meaning their trajectories lie nearly in the ecliptic plane, where the eight canonical planets also orbit. In other words, these objects have nearly planet-like orbits. The “hot” KBOs have elongated orbits and higher inclinations, behavior more akin to comets.

The authors of the new study found that the hot KBOs have the same distribution of sizes as the Trojan asteroids, meaning there are the same relative number of small, medium, and large KBOs and similarly sized Trojans. That hints at a probable common origin in the early days of the Solar System. This is in line with the Nice model, which predicts that, as they migrated into their current orbits, the giant planets kicked many planetesimals out beyond Neptune.

However, the cold KBOs don’t match that pattern at all: there are fewer large KBOs relative to smaller objects. To make matters more strange, both hot and cold seem to follow the same pattern for the smaller bodies, only deviating at larger masses, which is at odds with expectations if the cold KBOs formed where they orbit today.

To put it another way, the Nice model as it stands could explain the hot KBOs and Trojans, but not the cold. That doesn’t mean all is lost, of course. The Nice model seems to do very well except for a few nagging problems, so it’s unlikely that it’s completely wrong. As we’ve learned from studying exoplanet systems, planet formation models are a work in progress — and astronomers are an ingenious lot.

New Online Classes to Help You Learn More about the Universe

Dark matter in the Bullet Cluster. Otherwise invisible to telescopic views, the dark matter was mapped by observations of gravitational lensing of background galaxies. Credit: X-ray: NASA/CXC/CfA/ M.Markevitch et al.; Lensing Map: NASA/STScI; ESO WFI; Magellan/U.Arizona/ D.Clowe et al. Optical: NASA/STScI; Magellan/U.Arizona/D.Clowe et al.;

Roughly eighty percent of all the mass in the Universe is made of dark matter – a mysterious invisible substance responsible for the structure of galaxies and the patterns of the cosmos on the very largest scales. But how do we know that?

Astronomical images are beautiful, but that’s not their primary purpose from a scientist’s point of view. How can we take those images and infer things about what they are?

We only know of one planet harboring life: Earth. But that doesn’t mean we don’t know anything about the possibility of life elsewhere in the cosmos. How can we infer things about possible alien organisms when we can’t see them (yet)?

If you’re curious about those and other classes, CosmoAcademy — a project from the CosmoQuest educational and citizen-science group — could be for you. We’re offering three new online classes: Introduction to Dark Matter, Introduction to Astronomy via Color Imaging, and Life Beyond Earth: Introduction to Astrobiology.

These classes are short, four-hour courses designed for curious but busy people. All CosmoAcademy classes are offered online through Google+ Hangouts, a type of video chat. Part of the reason we do that is to limit the size of courses to eight students. That allows us to provide individual instruction in a way no other kind of online class is able to do – you aren’t a faceless student, but part of every discussion. In fact, if there’s a topic you want to discuss, there’s a good chance your instructor will take the time to talk about it.

Interested? See our course listings, and please let me know if you have any questions. Here are a few more details:

CQX015: Introduction to Dark Matter

Roughly eighty percent of all the mass in the Universe is made of dark matter – a mysterious invisible substance responsible for the structure of galaxies. But how do we know that? In this course, we’ll examine the evidence in favor of dark matter’s existence, from the rotation of galaxies to the radiation left over from the infancy of the cosmos. After that, we’ll examine what we can infer about the identity of dark matter and sketch out some of the experiments designed to detect it. This class assumes no background except a strong interest in astronomy and cosmology.

Instructor: Matthew Francis
Course structure: Two weeks, four 60-minute meetings
Meeting times: Tuesdays and Thursdays, 9–10 PM US Eastern time (6-7 PM US Pacific time)
Course dates: January 28—February 6, 2014

Enroll today!

CQX021: Introduction to Astronomy Via Color Imaging

When astronomers look at a star, nebula or galaxy for the first time, they see some unreachably distant object acting in some unknown way. What does it have to be made of and how does it have to be acting to look like that? In this class we will be looking at how we use the visual appearance of astronomical objects to figure out what they are. We will examine this problem by making our own color images from the sources provided by observatories from real research projects. From the subtle hues of stars in a distant galaxy to the eerie neon colors of nebulae to the chaotic Sun, by looking at objects in the right light, we can find out what makes them tick.

Instructor: Peter Dove
Course structure: Two weeks, four 60-minute meetings
Meeting times: Tuesdays and Thursdays, 8–9 PM US Eastern time (5-6 PM US Pacific time)
Course dates: Tuesday, February 25—Thursday, March 6

Enroll today!

CQX013 – Astrobiology: Life in the Universe

What will it take to find extraterrestrial life? Frank Drake penned his famous “equation” to determine the instances of life in the Galaxy over 50 years ago. Meant more as a discussion guideline than a rigorous mathematical formula, it will guide our discussion on the science of astronomy, biology, and astrobiology as we consider the possibility of life in the Universe.

Instructor: Nicole Gugliucci
Course structure: Two weeks, four 60-minute meetings
Meeting times: Mondays and Thursdays, 9–10 PM US Eastern time (6-7 PM US Pacific time)
Course dates: Monday, March 17 — Thursday, March 27

Enroll today!

Taking Measure: A ‘New’ Most Distant Galaxy

Galaxy z8_GND_5296 (seen in the inset) is the earliest galaxy that astronomers have measured the distance to accurately. It formed approximately 700 million years after the Big Bang, and is forming stars at an incredibly rapid rate. [Credit: V. Tilvi (Texas A&M), S. Finkelstein (UT Austin), the CANDELS team, and HST/NASA]

“The farthest galaxy yet seen!” Haven’t we heard that one before? (See here and here, for example.) While it’s true that astronomers keep pushing farther back in time with better instruments, there are fundamental challenges both in observing and measuring the distances to the earliest galaxies in the cosmos.

That’s why this new observation of a galaxy that formed about 700 million years after the Big Bang is significant. While scores of galaxies have been identified that formed in that era, astronomers have only measured accurate distances for five of them. This galaxy marks the sixth, and it is the farthest of the bunch. Perhaps even more important than the distance measurement, researchers determined that this galaxy gave birth to new stars at more than 100 times the rate the Milky Way does today. That indicates early galaxies may have been more aggressive with star-formation than previously believed. Continue reading “Taking Measure: A ‘New’ Most Distant Galaxy”