May 02, 2019

Should Pluto be a planet again? Informal vote offers support after experts debate

By Elizabeth Howell

May 02, 2019, 2:44:48 PM EDT

A friendly debate about Pluto's planethood yesterday (April 29) ended in an informal vote that came down in favor of reinstating the dwarf planet's status.

Early in the morning Eastern time, after a livestreamed Philosophical Society of Washington discussion earlier that evening, Alan Stern — principal investigator of the New Horizons mission that flew by Pluto — tweeted that his argument won the vote, which was open to anyone who could access the PSW website, even those who were not members. Results showed 130 people voted in favor of making Pluto a planet and 30 were against. During the debate, Stern argued in favor of using a geophysical definitionto define planethood. Briefly speaking, this suggests that planets must be those bodies massive enough to assume a nearly round shape but not massive enough to have nuclear fusion in the interior (like a star).

Pluto appears in color as New Horizons scientists combined four images from the spacecraft Long Range Reconnaissance Imager (LORRI) with color data from the Ralph instrument to create this enhanced color global view. The spacecraft obtained the images at a distance of 280,000 miles (450,000 kilometers).

But since 2006, the International Astronomical Union, represented in the debate by former IAU president Ron Ekers, has used another definition for planethood, which excludes Pluto. This definition says a planet must orbit the sun, must have a nearly round shape and must have "cleared the neighborhood around its orbit." Historically, it's that third point that has caused the most contention among Pluto planethood advocates, given the number of asteroids orbiting near even the larger planets.

The debate over Pluto's planetary status intensified after the New Horizonsmission flew by the dwarf planet in 2015. New Horizons revealed a world of surprises: large mountains, a possible internal ocean and a tenuous "exosphere" or very thin atmosphere. Given Pluto's complex geology, Stern and some other members of the astronomical community began arguing that Pluto should be designated as a planet once more.

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