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David Gerdes, TNOs, and the Quest for Planet Nine

Editor's Note: Gerdes presents "The Coolest Place in the Universe: New Worlds Beyond Neptune" at the 7pm Thursday March 15th GRAAA meeting, Schuler Books, 2660 28th Street SE. Free and open to the public.


David Gerdes is a naturally curious fellow, an important trait for a successful scientist. The University of Michigan professor holds joint appointments in both the departments of Physics and Astronomy. Dr. Gerdes has recently turned greater attention to a subject which for him has become a challenging and fascinating research interest: astronomical bodies orbiting along the fringes of the Solar System termed “trans-Neptunian Objects.”

Could one of those bodies, perhaps lurking in darkness far beyond Pluto, be large enough, and round enough, to be a true planet? If it exits and can one day be found, the Solar System would again have nine planets. The still hypothetical object is thus termed “Planet Nine.”

Most of us know the story about how Pluto, long considered the most distant planet, was demoted by the International Astronomical Union in 2006 amidst considerable controversy. IAU leaders reasoned that Pluto is smaller and different in many ways from the giant, mostly gas planets, Jupiter, Saturn, Uranus and Neptune, which orbit far from the sun.

Pluto fits better as a “dwarf planet” within what is called the Kuiper Belt, a swarm of smaller rock and ice bodies ringing the Solar System. Believed to remnants from the Solar System's formation, they started turning up in increasing numbers during the 1990s, well after Pluto’s 1930 discovery. The Kuiper Belt is named after mid-20th century astronomer Gerard Kuiper, who first predicted its existence.

Pluto and Eris, discovered in 2005 orbiting much further out, remain the largest of the trans-Neptunian Objects, but the inventory continues to grow, and others surely await detection. A few years ago, the idea of using the latest 21st century tools in a quest for more trans-Neptunian Objects, and scientific insight their detection might provide, so enticed Gerdes that he diverted from his then primary research interest to join in on the hunt.

Gerdes’ early career was focused mostly on physics, before his innate curiosity and a background in astronomy that goes back to building and using backyard telescopes as a teenaged amateur brought about an abrupt detour.

For more than a decade, he has been part of a team of researchers from around the world participating in the Dark Energy Survey, a search for the elusive “dark energy” which could be responsible for an observed acceleration in the expansion of the universe. Proof of this theoretical concept would come in part from observing galaxies lying billions of light years away with large telescopes in an effort to learn how dark energy has influenced their growth and evolution.

A group that includes Gerdes and some of his University of Michigan students and colleagues participated in development of a highly sensitive camera that could survey a full three square degrees of sky in one frame. They arranged to attach it to a telescope with a light gathering mirror close to 13 feet across located under dark skies in the Andes Mountains of Chile.

The group then set about repeatedly probing selected areas of the sky rich with distant galaxies. The goal was to take a picture of the same region every six days in an attempt to catch transient supernovas (exploding stars) flaring within them. Measuring the apparent brightness of a supernova in the act of exploding can provide data for determining the host galaxy’s distance.

The search appealed to Gerdes because he likes the challenge of gathering a large amount of data and then mining it for something significant. “I like looking for noisy signals in big data sets,” he said enthusiastically during a recent phone conversation.

From time to time, star-like objects would turn up on photos that clearly could not be galaxies, supernovas, or foreground stars because they shifted position on successive images. They were obviously much closer foreground objects: members of the Solar System shifting position due to a combination of the earth’s orbital motion and their own.

These two images taken about 3 months apart with the large telescope of 7th magnitude star reveal the first TNO discovered by Gerdes' team.

Gerdes’ natural curiosity came into play as he turned his attention to finding more of these intruders, some of them trans-Neptunian Objects, among archived images from the Dark Energy Survey. “Our work with galaxies and dark energy led us to something of interest much closer to home,” he noted.

By accumulating data about known trans-Neptunian Objects, and continually finding new ones, Gerdes and fellow researchers have learned that those objects are not roaming the Kuiper Belt chaotically. There are some orbital quirks. Some are clumped in swarms, and others appear to have reached their current orbits due to gravitational forces not attributable to known planets.

In describing his group's discoveries last year, Gerdes wrote: “Far beyond Pluto is a region surprisingly rich with planetary bodies. Most are quite small but others have sizes that rival Pluto, and some could even be much larger.”

A majority of astronomers involved in a search for Planet Nine think that if it is lurking, it is doing so mostly in darkness, perhaps 30 times more distant from the sun than Neptune and up to 20 times farther out than Pluto. It would have to be either quite a bit larger than earth or highly reflective (ice covered) for astronomers to have any chance of plucking it out of a starry backdrop with today’s optical telescopes.

Even if the visible light reflected by a Planet Nine is not within grasp, astronomers have another tool, radio telescopes, that could assist, if they could only pinpoint a location in the sky toward which to turn them. North of the observatory complex in Chile where Gerdes and his colleague will resume an optical search later this year is an array of sixty-six radio antennas called ALMA. When all are oriented toward a specific spot in the sky, collective energy representing the signature of heat emitted by a potential planetary candidate might be resolved and amplified enough to be detected.

Gerdes admits that his confidence in the existence of Planet Nine waxes and wanes, and notes: “If it were as easy to pick up as some have implied, it would already be here.” Still, that innate sense of curiosity and desire to mine new data for something significant motivates continuing enthusiasm: "It may not be low-hanging fruit, but we're going to keep shaking the tree."

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