Earth-like planet’s survival offers clues for our future

Existence of an Earth-like planet around a dead sun offers hope for our planet’s ultimate survival, according to a latest study by University of California – Berkeley. The discovery of an Earth-like planet 4,000 light years away in the Milky Way galaxy provides a preview of one possible fate for our planet billions of years in the future, when the sun has turned into a white dwarf, and a blasted and frozen Earth has migrated beyond the orbit of Mars. The distant planetary system, identified by the university’s astronomers after observations with the Keck 10m telescope in Hawaii, looks very similar to expectations for the sun-Earth system: It consists of a white dwarf about half the mass of the sun and an Earth-size companion in an orbit twice as large as Earth’s today.That is likely to be Earth’s fate. The sun will eventually inflate like a balloon larger than Earth’s orbit today, engulfing Mercury and Venus in the process. As the star expands to become a red giant, its decreasing mass will force planets to migrate to more distant orbits, offering Earth a slim opportunity to survive farther from the sun. Eventually, the outer layers of the red giant will be blown away to leave behind a dense white dwarf no larger than a planet, but with the mass of a star. If Earth has survived by then, it will probably end up in an orbit twice its current size.The discovery, to be published in the journal Nature Astronomy, tells scientists about the evolution of main sequence stars, like the sun, through the red giant phase to a white dwarf, and how it affects the planets around them. Some studies suggest that for the sun, this process could begin in about 1bn years, eventually vaporising Earth’s oceans and doubling Earth’s orbital radius—if the expanding star doesn’t engulf our planet first. Eventually, about 8bn years from now, the sun’s outer layers will have dispersed to leave behind a dense, glowing ball—a white dwarf—that is about half the mass of the sun, but smaller in size than Earth."We do not currently have a consensus on whether Earth could avoid being engulfed by the red giant sun in 6bn years,” said study leader Keming Zhang, a former doctoral student at the University of California, Berkeley, who is now an Eric and Wendy Schmidt AI in Science Postdoctoral fellow at UC San Diego. "In any case, planet Earth will only be habitable for around another billion years, at which point Earth’s oceans would be vaporised by the runaway greenhouse effect—long before the risk of getting swallowed by the red giant.”The planetary system provides one example of a planet that did survive, though it is far outside the habitable zone of the dim white dwarf and unlikely to harbour life. It may have had habitable conditions at some point, when its host was still a sun-like star. "Whether life can survive on Earth through that (red giant) period is unknown. But certainly the most important thing is that Earth isn’t swallowed by the sun when it becomes a red giant,” said Jessica Lu, associate professor and chair of astronomy at UC Berkeley. "This system that Keming’s found is an example of a planet—probably an Earth-like planet originally on a similar orbit to Earth—that survived its host star’s red giant phase.”Zhang noted that even if Earth gets engulfed during the sun’s red giant phase in a billion or so years, humanity may find a refuge in the outer solar system. Several moons of Jupiter, such as Europa, Callisto and Ganymede, and Enceladus around Saturn, appear to have frozen water oceans that will likely thaw as the outer layers of the red giant expand.