When Mike Brown announced the discovery of Eris in 2005, it was initially hailed as the 10th planet. But by the following year, it had been demoted to a dwarf planet, taking Pluto down with it.

Brown, a professor of planetary astronomy at the California Institute of Technology, visited the University of Minnesota last week. A packed autidorium in the Tate Laboratory of Physics listened to his lecture, “How I Killed Pluto and Why It Had It Coming,” on Thursday evening.

The event, the 12th in an ongoing lecture series, marked the inauguration of the University’s Minnesota Institute for Astrophysics, which opened in July and replaced the Department of Astronomy.

Brown was chosen for the event because of the importance and mass appeal of his topic, said Bob Gehrz, the institute’s director.

When Brown began research on the outer solar system in 1998, it was widely believed that nothing comparable in size existed beyond Pluto.

“There wasn’t even an idea that there was something out there to be discovered,” Brown said in an interview.

Because his team was among the first to do this kind of research, Brown said, it worked for nearly a decade with almost no competition. For eight years, they used a telescope at the Palomar Observatory in California to photograph the sky each night.

In 2005, they announced the discovery of Eris — an object on the outskirts of the solar system roughly the size of Pluto.

At the time of Eris’ discovery, Brown said, astronomers were already beginning to think that Pluto couldn’t be classified as a planet because of its irregular orbit and relatively small size. But Brown said he never thought Pluto would be demoted.

“It would be such a sociologically difficult thing” because it’s so ingrained in the culture, he said.

But within a year, the International Astronomical Union announced that both Pluto and Eris were dwarf planets. It also approved a new resolution defining a planet within the solar system as a celestial body orbiting around the sun with enough gravity to assume a nearly round shape and a clear orbit.

The demotion of Pluto caused an uproar.

“I’ve been in denial until today,” said chemistry junior Hanna Erickson, who attended the lecture. Prior to the event, she hadn’t known about the changed definition for planets.

Gehrz said the new classification changed the way astronomers look at the solar system.

“They have to figure out how they’re going to describe the solar system as compared to the way they described it before all this new knowledge,” he said.

Brown and his team have begun work on a new object — Sedna, whose 12,000-year orbit takes it far beyond the reaches of the solar system.

“Sedna is in this region of space where nothing is supposed to be,” Brown said. The region is interesting to Brown because the objects there are remnants from the time of the sun’s birth.

“If you can find more of them and read their record, you’re essentially reading about what it was like as the sun was being born,” he said.

With this knowledge, the race for the next big discovery is on. The competitive aspect of science is something that Brown said is crucial to the process of discovery but is also stressful.

Eris is bright and can be seen with an amateur telescope, he said. It is possible that anyone else could have found it.

“There were probably 10,000 telescopes on the planet that had the capability of seeing this thing,” he said, “if they knew where to look.”