Big Discovery from NASA’s New Horizons; Pluto is Biggest Kuiper Belt Body

Plutophiles everywhere rejoice. On the eve of history’s first ever up close flyby of mysterious Pluto on Tuesday morning July 14 making the first detailed scientific observations, NASA’s New Horizons has made a big discovery about one of the most basic questions regarding distant Pluto. How big is it?

Measurements by New Horizons gathered just in the past few days as the spacecraft barrels towards the Pluto planetary system now confirm that Pluto is indeed the biggest object in the vast region beyond the orbit of Neptune known as the Kuiper Belt.

Pluto is thus the undisputed King of the Kuiper Belt!

Pluto measures 1,473 miles (2,370 kilometers) in diameter, which is at the higher end of the range of previous estimates.

The big news was announced today, by New Horizons principal investigator Alan Stern of the Southwest Research Institute, Boulder, Colorado, during a live media briefing at Pluto mission control at the Johns Hopkins University Applied Physics Laboratory (APL) in Laurel, Maryland.

“This settles the debate about the largest object in the Kuiper Belt,” Stern noted.

New Horizons will swoop to within about 12,500 kilometers (nearly 7,750 miles) of Pluto’s surface and about 17,900 miles (28,800 kilometers) from Charon during closest approach at approximately 7:49 a.m. EDT (11:49 UTC) on July 14.

The new and definitive measurement of Pluto’s size is based on images taken by the high resolution Long Range Reconnaissance Imager (LORRI) to make this determination.

“The size of Pluto has been debated since its discovery in 1930. We are excited to finally lay this question to rest,” said mission scientist Bill McKinnon, Washington University, St. Louis.

Pluto was the first planet discovered by an American, Clyde Tombaugh.

Pluto’s “Heart” is seen in this new image from New Horizons’ Long Range Reconnaissance Imager (LORRI) received on July 8, 2015 after normal science operations resumed following the scary July 4 safe mode anomaly that briefing shut down all science operations. The LORRI image has been combined with lower-resolution color information from the Ralph instrument. Credits: NASA-JHUAPL-SWRI

Pluto is bigger than Eris, another big Kuiper Belt object discovered in 2005 by Mike Brown of Caltech, which is much further out from the Sun than Pluto. The discovery of Eris further fueled the controversial debate about the status of Pluto’s planethood.

Eris comes in second in size in the Kuiper Belt at only 1,445 miles (2,326 km) in diameter.

On July 11, 2015, New Horizons captured a world that is growing more fascinating by the day. For the first time on Pluto, this view reveals linear features that may be cliffs, as well as a circular feature that could be an impact crater. Rotating into view is the bright heart-shaped feature that will be seen in more detail during New Horizons’ closest approach on July 14. The annotated version includes a diagram indicating Pluto’s north pole, equator, and central meridian.

Stern also noted that because Pluto is slight bigger than the average of previous estimates, its density is slightly lower than previously thought. Therefore the fraction of ice in its interior is slightly higher and the fraction of rock is slightly lower. But further data is required to pin the density down more precisely.

The uncertainty in Pluto’s size has persisted for decades and was due to the fact that Pluto has a very tenuous atmosphere composed of nitrogen.

Furthermore Pluto’s lowest atmospheric layer called the troposphere, is shallower than previously believed.

On the other hand, its largest moon Charon with which it forms a double planet, lacks a substantial atmosphere and its size was known with near certainty based on ground-based telescopic observation.

New Horizons LORRI imagery has confirmed that Charon measures 751 miles (1208 km) kilometers) across.

Stern also confirmed that frigid Pluto also has a polar cap composed of methane and nitrogen ices based on measurements from the Alice instrument.

LORRI has also zoomed in on two of Pluto’s smaller moons, Nix and Hydra.

“We knew from the time we designed our flyby that we would only be able to study the small moons in detail for just a few days before closest approach,” said Stern. “Now, deep inside Pluto’s sphere of influence, that time has come.”

The approximate sizes of Pluto’s moons Nix and Hydra compared to Denver, Colorado. While Nix and Hydra are illustrated as circles in this diagram, mission scientists anticipate that future observations by New Horizons will show that they are irregular in shape. Credits: JHUAPL/Google

But because they are so small, accurate measurement with LORRI could only be made in the final week prior to the July 14 flyby.

Nix is estimated to be about 20 miles (about 35 kilometers) across, while Hydra is roughly 30 miles (roughly 45 kilometers) across. These sizes lead mission scientists to conclude that their surfaces are quite bright, possibly due to the presence of ice.

Determinations about Pluto’s two smallest moons, Kerberos and Styx, will be made later at some point during the 16-month long playback of data after the July 14 encounter.

It has been three decades since we last visited planetary bodies at the outer reaches of our solar system when Voyager 2 flew past Uranus and Neptune in 1986 and 1989.

New Horizons’ last look at Pluto’s Charon-facing hemisphere reveals the highest resolution view of four intriguing darks spots for decades to come. This image, taken early the morning of July 11, 2015, shows newly-resolved linear features above the equatorial region that intersect, suggestive of polygonal shapes. This image was captured when the spacecraft was 2.5 million miles (4 million kilometers) from Pluto. Credit: NASA/JHUAPL/SWRI

New Horizons is closing in fast on its quarry at a whopping 31,000 mph (49,600 kph) after a nine year interplanetary voyage and is now less than half a million miles away, in the final hours before closest approach.

The New Frontiers spacecraft was built by a team led by Stern and included researchers from SwRI and the Johns Hopkins University Applied Physics Laboratory (APL) in Laurel, Maryland. APL also operates the New Horizons spacecraft and manages the mission.

Watch for Ken’s continuing onsite coverage of the Pluto flyby on July 14 from the Johns Hopkins University Applied Physics Laboratory (APL).

Stay tuned here for Ken’s continuing Earth and planetary science and human spaceflight news.

Ken Kremer

Ken Kremer

Dr. Ken Kremer is a speaker, research scientist, freelance science journalist (KSC area,FL) and photographer whose articles, space exploration images and Mars mosaics have appeared in magazines, books, websites and calendars including Astronomy Picture of the Day, NBC, FOX, BBC,, Spaceflight Now, Science and the covers of Aviation Week & Space Technology, Spaceflight and the Explorers Club magazines. Ken has presented at numerous educational institutions, civic & religious organizations, museums and astronomy clubs. Ken has reported first hand from the Kennedy Space Center, Cape Canaveral, NASA Wallops, NASA Michoud/Stennis/Langley and on over 80 launches including 8 shuttle launches. He lectures on both Human and Robotic spaceflight - Follow Ken on Facebook and Twitter

View Comments

  • If we underestimated the size of Pluto, does it stand to reason that we also underestimated Eris? Did we use the same method, e.g. establishing the lower limit?

    • I think the uncertainty about Pluto's size had to do with its thin exo/atmosphere. Eris is about three times more distant and should have much less exosphere. But it is unknown terrain, we'll see.

  • So, can we now end the dwarf planet nonsense about Pluto and make it a planet again? But then I suppose that 44 Km difference of the diameter is not enough for reconsideration.

    • I like the school of thought some astronomers share; calling them all (moons included) "Worlds" as they are all unique in their own special ways ...

    • Make all those same scientists walk 44 Km and then revote. And yes, a Dwarf Planet is still a Planet.

  • So you have a "Pluto planetary system" but you don`t have a planet ? A whole hysteria provoked by a senile man (scientist wannabe), while Pluto is a hundred times more of a beautiful planet (my favorite now) than Mercury ...

    • Yes...Pluto is quite the celestial sight...and for me will always be a Planet, regardless of others myopic opinions,

  • Do not throw your hats in the air yet, the Kuiper belt is largely unknown! So far the space has always had surprises coming...

    • It'll be cool if it turns out to be plenty of planetesimals out there. Well, whatever is in the Kuiper Belt is certainly "cool". Maybe the concept Kuiper Belt and Oort Cloud will change? They were pretty speculative hypothesis to begin with. Needlestick New Horizon is a great complement to the great telescopes coming up which can map that distant region.

    • Ok, I now read that " some scientific circles the term "Kuiper belt object" has become synonymous with any icy minor planet native to the outer Solar System believed to have been part of that initial class..." and "A consensus among astronomers as to the precise definition of the Kuiper belt has yet to be reached, and this issue remains unresolved." (
      So, it seems like some definitions of the Kuiper belt also includes the scattered-disk objects.

Recent Posts

Astronomers Challenge Recent Findings About Venus. “No Statistically Significant Detection of Phosphine”

In September, a team of scientists reported finding phosphine in the upper atmosphere of Venus.…

8 hours ago

Scientists Think They Know What Caused the Deadliest Mass Extinction in the History of the Earth

Humanity can have a love/hate relationship with itself, but there's no denying that we're the…

10 hours ago

Weekly Space Hangout: October 21, 2020, Dr. Jill Tarter and the Search for Technosignatures This week we are excited (and honored) to welcome Dr. Jill Tarter to the…

12 hours ago

Tales of Two Fall Comets: 88P Howell and M3 ATLAS

Two more comets worth scouting the sky for into November 2020.

15 hours ago

What Would a Realistic Space Battle Look Like?

Science fiction space movies can do a poor job of educating people about space. In…

1 day ago

The Crew of the ISS has Found the Source of the Station’s Air Leak

The ISS crew has found the source of the elusive leak using (wait for it!)…

1 day ago