Eris, Pluto, and the dwarf planets


In January 2005, a team of astronomers at Palomar Observatory led by Mike Brown from Caltech looked at three images of the same star field taken over three hours.

All the stars remained in their expected positions. However, one tiny point of light moved almost imperceptibly.

Further calculation indicated that its orbit around the sun took 559 years. It was 8.5 trillion miles away. Although its diameter was slightly smaller than Pluto’s, it was denser and more massive.

Here then was the 10th planet, as the news media initially described it. Pluto, first spotted in 1930 by Clyde Tombaugh at Lowell Observatory, was the ninth.

Eris, as the International Astronomical Union (IAU) decided to call it, orbits in the Edgeworth-Kuiper Belt (EKB), a ring of material out past the orbit of Neptune.

Since Pluto inhabits the EKB, the region became the hottest place to look for new planets. An embarrassment of riches ensued. Many celestial objects similar to Pluto and Eris orbit the sun.

Their discovery put astronomers into an awkward position. Do they designate potentially hundreds of small bodies as planets, or do they reduce Pluto’s status by creating a new category of solar-system inhabitants?

The problem originated in 1950 when astronomers were trying to determine the origin of comets.

Comets are mixtures of ice and rock that travel around the sun in long, thin orbits that stretch from the deep space past Neptune and swing from there close to the sun and back outward again. As they approach the sun, they develop a tail of glowing gas.

Comets come in two flavors. Short-period comets orbit in less than 200 years. They always orbit along the plane on which the planets lie.

Comets that take over 200 years to orbit the sun are called long-period comets. They move into the inner solar system from all directions.

In 1950, Dutch astronomer Jan Oort proposed a loose, spherical cloud of cometary material surrounding the solar system like a shell. The Oort Cloud, as astronomers now call it, consists of trillions of icy-dusty objects.

It stretches from 2,000 to 200,000 astronomical units (au) away from the sun. In miles, the Oort cloud starts at 190 billion miles and ends at approximately 20 trillion miles.

By comparison, astronomers define Earth’s distance from the sun as 1.0 au, about 93 million miles. Neptune, the farthest planet, is about 30 au or about 2.7 billion miles.

The Oort cloud’s outer extremities thus extend 3/4’s of the way to Proxima Centauri, the nearest star to the sun.

Astronomers estimate that trillions of comets inhabit the Oort cloud. However, those trillions inhabit an enormous volume of space and rarely interact with each other.

When two of them occasionally come close enough, their gravitational encounter nudges one of them into the inner solar system.

The distance to Oort’s cometary cloud explains the orbits of the long-period comets quite well. But the short-period variety requires a closer cloud.

In 1943, Irish astronomer Kenneth Edgeworth suggested the existence of a ring of comets and other cosmic debris out past the orbit of Neptune and along the plane of the solar system. It must have been present in the early stages of our solar system before planets formed.

The comets didn’t clump into a planet because they were too far apart. However, a close gravitational encounter occasionally occurs, and one of the participants reroutes into the inner solar system.

Sadly, most astronomers were unaware of Edgeworth’s proposal.

In 1951, American astronomer Gerald Kuiper suggested that the cometary ring had existed in the solar system’s early days but that it had since dispersed because of the disruptive force of Pluto’s gravity.

In 1980, Uruguayan astronomer Julio Fernandez argued that Kuiper’s belt must still exist to explain the sheer number of short-period comets.

The hunt for objects in the Edgeworth-Kuiper Belt (EKB) was on.

The EKB is a ring of primarily rocky/icy bodies that begins at about the orbit of Neptune. The primary belt ends at approximately 50 au. However, a thinner region, called the scattered disk, extends to 1,000 au. It thus lies between Neptune and the Oort Cloud.

Since 1980, astronomers have discovered around 2,000 EKB objects. They estimate that the EKB may contain hundreds of thousands of objects 60 miles or greater in diameter. Current telescope technology prevents astronomers from seeing smaller, fainter objects at such distances.

Some of those objects were large enough to form planet-like spheres and undergo a process called differentiation.

Pluto and Eris are such objects. Since they have considerable gravitational oomph, the heavier metallic elements sink to the center. Ice and rock float to the surface. Any gases remain as an atmospheric shell if the object has enough gravity to support an atmosphere.

The process occurs when planets form. It also happened in planetoids, the smaller spherical objects that crashed together to form the more massive planets.

The first EKB object discovered was Pluto. No wonder. It inhabits the inner portion of the EKB and is the largest known at 1,477 miles wide.

Eris is nearly as large at 1,445 miles in diameter. However, it is a whopping 96 au from the sun, so Brown’s team didn’t discover it until 2005. And it wasn’t the first EKB planetoid found.

That honor goes to Haumea, which Brown’s team discovered in late 2004. Astronomers estimate that it is approximately Pluto’s density. Like many differentiated objects in the EKB, it is primarily a ball of rock with a thin layer of ice on its surface.

Of all the larger EKB bodies discovered, Haumea is undoubtedly the weirdest. Although it probably originated in the EKB, it now orbits 28 degrees above the solar system’s plane.

At some point in its life, Haumea had a gravitational encounter with another EKB object that drove it out of the solar system’s plane. At its closest point, it passes 35 AU from the sun. At the farthest end of its orbit, it reaches 50 AU.

Stunningly, Haumea rotates once every 4.3 hours. Early in its life, its rapid rotation stretched it out to a watermelon-shaped object 1,430 miles long and 619 miles wide.

Its unusual shape suggests that it is rotating end to end along its long axis. If it had been spinning any faster, it would have looked more like a dumbbell or would have split apart entirely.

Finally, Haumea has a faint ring, which astronomers discovered when Haumea passed in front of a distant star.

The discovery of Eris and Makemake, a spherical EKBer 816 miles wide, soon followed.

Here we return to astronomers’ thorny problem. Were the newly discovered objects planets?

EKB-inhabitant Pluto was already classified as a planet. Ceres in the Asteroid Belt possessed many similar qualities. Should they add Ceres, Haumea, Makemake, and Eris to the mix to maintain Pluto as a planet?

As telescope technology improved, many more large EKB “planets” would probably be added. Hundreds of thousands may exist that are currently undetectable.

Many grade-schoolers might (and did!) balk at the “demotion” of Pluto. They would surely revolt if they had to learn the names of scores of planets.

Besides, astronomers suspected that the objects weren’t fully developed planets at all. They believe that they are planetoids, protoplanetary precursors to full-fledged planets. Such planetoids clumped together billions of years ago to form the eight planets we see today.

And thus it was that astronomers from the International Astronomical Union convened in January 2006. They created a new category of objects orbiting the sun — the dwarf planet.

The IAU initially put Pluto, Ceres, and Eris in that category. In 2008, they added Haumea and Makemake to the official list.

Several others wait in the wings. They include Gonggong, Quaoar, Orcus, Salacia, and Sedna. For example, NASA recognizes Gonggong as a dwarf planet but not the IAU.

Dwarf planets share many qualities of planets. They possess enough gravity to produce a spherical shape.

They have differentiated. The heavier elements have fallen to the center and the lighter elements have floated to the surface.

The critical difference is whether the planetary body shares its orbit with other bodies. Planets have “cleared their orbits” of similar-sized objects. In the process, they have accumulated a lot of material and made the transition from a planetoid to a planet. Earth and Jupiter have no similarly-sized worlds left to absorb.

Objects like Eris and Pluto have not cleared their orbits. In the fullness of time, they may absorb significantly more material but probably not.

They will most likely remain planetoids, leftovers from the creation of planets billions of years ago. They are therefore vitally important if we want to understand the formation of the planets.

Many people condemned Pluto’s reclassification, but its “demotion” was the inevitable consequence of a decades-long scientific inquiry.

It was a recognition that Pluto survived planetary formation and is one of the senior residents of our solar system. Pluto and its elderly contemporaries have many stories to tell about our early days.

By Tom Burns


Tom Burns is the former director of the Perkins Observatory in Delaware.

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