May 26, 2020

Happy 85th to Pluto!

An artist’s conception of New Horizons

passing Pluto and Charon.

Credit: NASA/JHU APL/SwRI/Steve Gribben

Where were you in 1930?

If you were American astronomer Clyde Tombaugh, you were announcing the discovery of a curious object caught moving through the constellation Gemini just the month prior.

The discovery position of Pluto in 1930. Created using Starry Night Education software.

Hailed initially as the end of the hunt for Planet X, the announcement of the discovery of Pluto on February 18th, 1930 85 years ago today marked the start of one of the most fascinating tales in modern astronomy.

And 2015 is a watershed year for the planet/turned dwarf planet/turned Plutoid, as NASA’s New Horizons spacecraft is set to make a historic flyby of Pluto and its moons this coming July.

Pluto and Charon (to the lower right) on the Maine Solar System model. Photo by author.

Through an Earth-based telescope, Pluto has never appeared as more than a dim +14th magnitude dot. It was true feat in visual athletics that Tombaugh spotted Pluto at all, as he carefully viewed plates taken with the Lowell Observatory’s 13” astrograph telescope through a machine known as a blink comparator. In the modern internet age, such a task might’ve been crowd sourced in the Zooniverse tradition similar to projects like Galaxy Zoo, but back in the 1930s, Tombaugh carried out this arduous task alone.

Tombaugh’s observing log noting the discovery of Pluto, on display at the Lowell Observatory.

Photo by author.

But beyond just being the King of the Kuiper belt, there’s a human side to the tale of Pluto as well. The distant world was named by 6-year old Venetia Burney and the name was officially adopted on the same year as Pluto’s discovery on March 24th. Miss Burney just passed away in 2009.

A year on Pluto. Credit: NASA/APL.

You can still find 20th century text books that list Pluto as a planet, and the 2006 demotion by the International Astronomical Union raised a minor outcry from 5th graders and fans of the maligned 9th planet around the world. Here’s a small bet: the “is Pluto a planet?” debate will become topical once again this summer as we get our first good looks of the distant world. Already, views from New Horizons are rivaling that of the Hubble Space Telescope.

Hopes for the discovery of the legendary Planet X were soon dashed through the middle of the 20th century as astronomers realized just how tiny Pluto is. The running joke among astronomers as the estimations of the mass and size of Pluto continued to plummet was that by the 1980s, Pluto would disappear entirely!

The discovery of Pluto’s massive moon Charon in 1978 by James Christy cinched the issue: Pluto is now known to be 2,368 kilometres in diameter and 18% as massive as our Moon, while Charon is 1,207 kilometres in diameter and 12% as Pluto.

Curiously, this also puts the mutual barycenter of the Pluto-Charon system just outside the surface of Pluto. This is a singular rarity in our solar system, as even the Earth-Moon barycenter lies 1,670 kilometres below the surface of our fair planet…

A single orbit of Charon as seen from New Horizons…

note that you can now discern the ‘barycenter swing!’ Credit: NASA/APL/SW Research institute.

Is Pluto and Charon the first true binary planet?

Come the 86th anniversary of discovery next February, we’ll have thousands of new pictures of Pluto courtesy of New Horizons. Launched on January 19th 2006, New Horizons passed our Moon just nine hours later and is headed on a trajectory out of our solar system to join the twin Voyager and Pioneer spacecraft in a lazy orbit about the galaxy.

Happy 85th, Pluto — or at least, it’s been 85 orbits of our home world since curious bipeds such as ourselves knew of your presence — we’re just getting to know thee.

And you’ve got lots to teach us about the solar system!



  1. John says:

    I have been into Star’s since I was a kid’ now I am up in my 70′s wow I know’ but when I don’t know about something or want to fine out about something … I have always tried to you to someone who knows more then I do about it. this is why I am here now to ask you or any one who want to came back to me on this matter at hand. I know that most won’t tell any thing if it’s real … yet I would like to know if the so called “Planet X” is real and if it’s going to start changing the world as we know it in the coming year ‘ and will kill off nearly every one on the planet’ … or is it just a mif’ of story’s trying to make a name for them self’s with a ly’
    An why aren’t there any up dates on a day to day kind of way telling us who watch our space’ so we will know the truth of things that might come about before it’s over with… Pray tell what they say about Sept 2017 & Oct. 2017 will still be here to see another day when the sun comes up….

    thank you for your time’



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