protoplanet
noun
pro·to·plan·et ˈprō-tō-ˌpla-nət 
: 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
ˌprō-tō-ˈpla-nə-ˌter-ē
adjective 
Example Sentences
Recent Examples on the Web There are two main possibilities: that they were delivered on comets and asteroids that crashed into our protoplanet, or that Earth quite literally sucked them into its atmosphere from the nebula of gas and dust that surrounded our young sun. 
WIRED, 19 Aug. 2022 For years, Lindy Elkins-Tanton had been looking forward to blasting a probe to the metallic asteroid Psyche, which is orbiting about 230 million miles away and may be the remains of a protoplanet whose surface was smashed to bits eons ago. Wired, 29 July 2022 The protoplanet's star is estimated to be a relatively young 2 million years old, the same age at which our now 4.6 billion-year-old solar system saw planet formation. Wyatte Grantham-philips, USA TODAY, 5 Apr. 2022 In September, Morbidelli and his colleagues published an article in Nature Astronomy based on studies of the protoplanet Vesta that suggested how planetesimals would explain the current configuration of the solar system. Quanta Magazine, 3 Nov. 2021 Scientists have long agreed that the Moon formed when a protoplanet, called Theia, struck Earth in its infancy some 4.5 billion years ago. Paul Voosen, Science | AAAS, 23 Mar. 2021 Such a dry, large protoplanet would have separated into layers with an iron-depleted core and an iron-rich mantle, Desch says, some 2% to 3.5% denser than present-day Earth. Paul Voosen, Science | AAAS, 23 Mar. 2021 Such an event would have seen Jupiter absorb the protoplanet, which would have been 10 times Earth’s mass, causing their two dense cores to combine and diffuse after only 10 hours, as seen in the simulation above. Paul Voosen, Science | AAAS, 14 Aug. 2019 Scientific consensus about how the moon formed is that billions of years ago, a protoplanet crashed into Earth and the pieces of it revolved around the Earth to form the moon. Kristen Rogers, CNN, 22 Nov. 2019 See More
WIRED, 19 Aug. 2022 For years, Lindy Elkins-Tanton had been looking forward to blasting a probe to the metallic asteroid Psyche, which is orbiting about 230 million miles away and may be the remains of a protoplanet whose surface was smashed to bits eons ago. Wired, 29 July 2022 The protoplanet's star is estimated to be a relatively young 2 million years old, the same age at which our now 4.6 billion-year-old solar system saw planet formation. Wyatte Grantham-philips, USA TODAY, 5 Apr. 2022 In September, Morbidelli and his colleagues published an article in Nature Astronomy based on studies of the protoplanet Vesta that suggested how planetesimals would explain the current configuration of the solar system. Quanta Magazine, 3 Nov. 2021 Scientists have long agreed that the Moon formed when a protoplanet, called Theia, struck Earth in its infancy some 4.5 billion years ago. Paul Voosen, Science | AAAS, 23 Mar. 2021 Such a dry, large protoplanet would have separated into layers with an iron-depleted core and an iron-rich mantle, Desch says, some 2% to 3.5% denser than present-day Earth. Paul Voosen, Science | AAAS, 23 Mar. 2021 Such an event would have seen Jupiter absorb the protoplanet, which would have been 10 times Earth’s mass, causing their two dense cores to combine and diffuse after only 10 hours, as seen in the simulation above. Paul Voosen, Science | AAAS, 14 Aug. 2019 Scientific consensus about how the moon formed is that billions of years ago, a protoplanet crashed into Earth and the pieces of it revolved around the Earth to form the moon. Kristen Rogers, CNN, 22 Nov. 2019 See More Word History
First Known Use
1949, in the meaning defined above