After all, where astronomers draw the line between planets, dwarf planets, planetoids and moons is essentially arbitrary and makes zero difference to reality. This is a BETA experience. You may opt-out by clicking here. More From Forbes. Nov 11, , am EST. Nov 10, , pm EST. Nov 9, , pm EST.
Nov 9, , am EST. Burney, just 11 years old, knew that the other planets were named after mythical figures, so she proposed the Roman god of the underworld: Pluto. By that point, the definition of a planet had already experienced some shifts. In , astronomers deemed Ceres, a rocky object they had spotted between Mars and Jupiter, a planet, but 50 years later, after further observations, they designated it as an asteroid.
Percival Lowell, the namesake of the Arizona observatory where Pluto was first photographed, had predicted that a ninth planet lurked somewhere deeper in the solar system, and Tombaugh and his colleagues believed they had found it. It was not, as Lowell had imagined, seven times more massive than Earth, but a tiny thing, smaller than the moon. And compared with the other planets, the specifics of its year orbit were weird too.
Still, Pluto stayed a planet. Then, in the s, astronomers started finding icy objects beyond Neptune. Which brings us to that fateful meeting in Prague.
But by the end of the week, the idea was out, and the attendees had settled on a new set of criteria for a planet: It must orbit the sun, be big enough for gravity to have smoothed it into a sphere, and be gravitationally dominant enough to clear other objects out of its orbital zone.
Full Moon Guide: October - November A new paper details how the hydrological cycle of the now-dry lake at Jezero Crater is more complicated than originally thought. This year, the minimum extent of Arctic sea ice dropped to 1. Researchers will use Webb to observe 17 actively forming planetary systems.
The lander cleared enough dust from one solar panel to keep its seismometer on through the summer, allowing scientists to study three big quakes.
Scientists found evidence that an area on Mars called Arabia Terra had thousands of "super eruptions" over a million-year period. Perseverance successfully collected its first pair of rock samples, and scientists already are gaining new insights into the region.
Data received late Sept. The rover will abrade a rock this week, allowing scientists and engineers to decide whether that target would withstand its powerful drill. Drought is a complicated problem that requires lots of data.
Satellites from NASA and its partners help collect that data. Drought Makes its Home on the Range. Gene Roddenberry would have been years old on Aug.
The images show Venus approaching from the left while the Sun is off-camera to the upper right. Researchers had to ask themselves these questions: If Pluto was a planet, then did that mean Eris was one as well?
What about all those other icy objects out in the Kuiper belt? Where exactly was the cut-off line for classifying a body as a planet? A word that had seemed straightforward and simple was suddenly shown to be oddly slippery. Intense debate followed, with many new proposals for the definition of planet being offered.
A year later, astronomers were no closer to a resolution, and the dilemma hung like a dark cloud over the IAU General Assembly meeting in Prague in At the conference, researchers endured eight days of contentious arguments, with four different proposals being offered. One controversial suggestion would have brought the total number of planets in the solar system up to 12 , including Ceres, the largest asteroid, and Pluto's moon Charon.
The suggestion was "a complete mess," astronomer Mike Brown of Caltech, Eris' discoverer, told Space. Near the end of the Prague conference, the astronomers who remained voted to create three new categories for objects in the solar system.
From then on, only the worlds Mercury through Neptune would be considered planets.
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