In a study published in the Astrophysical Journal (arXiv.org preprint), a team of astronomers at Cornell University has modeled the locations of the habitable zones for stars of spectral classes F to M and how long exoplanets can stay in them.
This graphic shows where a planet can be habitable and warm around our Sun, as it ages over billions of years. Image credit: Cornell University.
The habitable zone (HZ) is the region where water could be liquid on the surface of terrestrial (rocky) planets.
“When a star ages and brightens, the HZ moves outward and you’re basically giving a second wind to a planetary system,” said Dr. Ramses Ramirez, lead author on the study.
“Currently objects in these outer regions are frozen in our own Solar System, and Europa and Enceladus are icy for now.”
Dependent upon the mass of the original star, planets and their moons loiter in this red giant HZ up to 9 billion years.
Earth, for example, has been in our Sun’s HZ so far for about 4.5 billion years, and it has teemed with changing iterations of life.
However, in a few billion years our Sun will become a red giant, engulfing Mercury and Venus, turning Earth and Mars into sizzling terrestrial planets, and warming gas giants like Jupiter, Saturn and Neptune – and their moons – in a newly-established red giant HZ.
“Long after our own plain yellow Sun expands to become a red giant star and turns Earth into a sizzling hot wasteland, there are still regions in our Solar System – and other solar systems as well – where life might thrive,” said co-author Dr. Lisa Kaltenegger.
“For stars that are like our Sun, but older, such thawed planets could stay warm up to half a billion years in the red giant HZ.”
“That’s no small amount of time,” Dr. Ramirez added.
“In the far future, such worlds could become habitable around small red suns for billions of years, maybe even starting life, just like Earth,” Dr. Kaltenegger said.
“That makes me very optimistic for the chances for life in the long run.”
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Ramses M. Ramirez & Lisa Kaltenegger. 2016. The Habitable Zones of Pre-Main-Sequence Stars. ApJ 823, 6; doi: 10.3847/0004-637X/823/1/6
