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NASA shortens Nemesis/Tyche orbit from 27 million years to "a few million" ! See June 7th post for: UPDATE
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Here is a Q and A from NASA: [link to www.jpl.nasa.gov]
I find their reasoning behind renaming of Nemisis to Tyche (exerpt below) as the most revealing, because it sneaks in 2 major hypothesis. 1. Supposedly there is no cyclical extinction record in Earth's history (news to me). 2. NASA is now announcing a much shorter orbit for Nemesis/Tych of "a few million years." :
Q: Why is the hypothesized object dubbed "Tyche," and why choose a Greek name when the names of other planets derive from Roman mythology?
A: In the 1980s, a different companion to the sun was hypothesized. That object, named for the Greek goddess "Nemesis," was proposed to explain periodic mass extinctions on the Earth. Nemesis would have followed a highly elliptical orbit, perturbing comets in the Oort Cloud roughly every 26 million years and sending a shower of comets toward the inner solar system. Some of these comets would have slammed into Earth, causing catastrophic results to life. Recent scientific analysis no longer supports the idea that extinctions on Earth happen at regular, repeating intervals.[?????] Thus, the Nemesis hypothesis is no longer needed.[????] However, it is still possible that the sun could have a distant, unseen companion in a more circular orbit with a period of a few million years -- one that would not cause devastating effects to terrestrial life.[?????] To distinguish this object from the malevolent "Nemesis," astronomers chose the name of Nemesis's benevolent sister in Greek mythology, "Tyche."
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