pro·​to·​plan·​et ˈprō-tō-ˌpla-nət How to pronounce protoplanet (audio)
: a hypothetical whirling gaseous mass within a giant cloud of gas and dust that rotates around a sun and is believed to give rise to a planet
protoplanetary adjective

Examples of protoplanet in a Sentence

Recent Examples on the Web The most popular theory says that about 4.5 billion years ago, a Mars-size protoplanet slammed into Earth. Kenneth Chang, New York Times, 1 Nov. 2023 But scientists have lacked smoking-gun evidence like a crater or pieces of the protoplanet, named Theia. Kenneth Chang, New York Times, 1 Nov. 2023 Further afield, one leading theory to account for certain quirks of our solar system’s architecture posits that early shifts in the orbits of Jupiter and Saturn sparked brutal cascades of collisions between nascent protoplanets. Lyndie Chiou, Scientific American, 25 Oct. 2023 The new measurements are significant as this is the first time water has been detected in a region where two or more protoplanets are known to exist. Jack Birle, Washington Examiner, 25 July 2023 Scientists think the all-metal asteroid Psyche was once the core of a protoplanet. Nathaniel Scharping, Discover Magazine, 12 June 2017 Vesta is essentially a remnant protoplanet, identical to the myriad small bodies that were incorporated into Earth and the other rocky planets more than 4 billion years ago. Tim Folger, Discover Magazine, 24 Oct. 2012 And when a protoplanet gets massive enough, gravity forces it into a spherical shape, finally cementing its status as a true planet. Jake Parks, Discover Magazine, 11 July 2018 What was once believed to be the largest asteroid has now been revealed as a relict protoplanet that once had a since-frozen over ocean. John Wenz, Discover Magazine, 3 July 2018 See More

These examples are programmatically compiled from various online sources to illustrate current usage of the word 'protoplanet.' Any opinions expressed in the examples do not represent those of Merriam-Webster or its editors. Send us feedback about these examples.

Word History

First Known Use

1949, in the meaning defined above

Time Traveler
The first known use of protoplanet was in 1949

Dictionary Entries Near protoplanet

Cite this Entry

“Protoplanet.” Dictionary, Merriam-Webster, Accessed 27 Feb. 2024.

Kids Definition


pro·​to·​plan·​et ˈprōt-ō-ˌplan-ət How to pronounce protoplanet (audio)
: a whirling mass of gas that rotates around a star and is believed to become a planet
protoplanetary adjective

More from Merriam-Webster on protoplanet

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