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The best known planets orbit around a star. These planets, including the Earth, benefit from the heat and light of the star. And it is the light emitted by these stars that allows us to see them. But there are also “invisible” planets, hidden from our gaze, floating, abandoned, in the cosmos. These dark and lonely worlds have no stars in orbit, no light to bask in, no heat to be radiated from. They are the “rogue” planets – and astronomers have just found a new one, roughly the same size as Earth.
Planets are made from debris left behind after a star is born. These planets surround the young star in a thin disk of grains and gases and grow as these tiny particles attach and join each other until they release their surroundings. Things are chaotic in this world and collisions between planetary embryos, or proto-planets, are common. Stars tend not to form on their own, but in groups of hundreds or thousands at the same time, and the encounters between their nascent planetary systems cause further damage.
The young Earth is thought to have been hit by a body the size of Mars, which cleared out enough material to form the Moon. But some planets have faced a darker future: they have been eliminated altogether, destined for a life in the vast coldness of space between the stars. These are the “rogue planets” that float freely.
When the planets are still very young, say a few million years (the Earth is over 4.5 billion years old) they are still hot due to their formation and the energy released by their continuous gravitational contraction and radio activity. in their cores. Great examples of such young but floating planets (think a small Jupiter) were seen directly in the regions where stars had just formed. But finding smaller rogue planets proved nearly impossible until “lensing” was discovered.
Gravitational lensing
Anything with mass bends space and causes light to deflect from a straight path. The result is that an object with mass focuses light from a source behind it, amplifying it like a huge magnifying glass. This is called a gravitational lens. It was predicted by Einstein’s theory of general relativity and was first verified when the stars were seen to have shifted from their usual positions when viewed near the Sun while it was perfectly eclipsed by the Moon in 1919.
The gravitational lensing effect has been observed in galaxies made up of trillions of stars, caused by the large amount of material between the galaxies and the stars lining up with other stars in the background. One observation was caused by a black hole in a massive “nearby” galaxy called Messier 87, in 2019. So even an “invisible” rogue planet could act as a gravitational lens – or micro-lens, as they can be so small.
One such “micro-lens” event has been attributed to the new rogue planet, named OGLE-2016-BLG-1928. The sighting of light amplification from an inconspicuous star in the dense inner regions of the Milky Way lasted only 42 minutes.
This meant it had to be a small object, and the estimated mass left no doubt that it had to be a planet not very different in size from Earth. The target planet was not found to be associated with a star. Lensing’s rogue planets have been found earlier, but this is one of the more compelling cases. In addition to being the most Earth-like, OGLE-2016-BLG-1928 is also the smallest thief ever found.
Could the Earth become a rogue?
A large number of rogue planets traversing our galaxy raise intriguing questions. Could life have formed and survived, or settled on such worlds? Perhaps technologically advanced civilizations could overcome the drawbacks of eternal darkness and an unparalleled ice age in Earth’s long and varied history? Have they harnessed nuclear energy or have they gone completely non-biological?
It may sound like science fiction, but what are the chances that Earth will accidentally come across such a planet? This is not inconceivable. In the past two years alone, rogue asteroids like Oumuamua and rogue comets like Borisov have darted through our solar system. A rogue planet is unlikely to pass us that closely. But it’s not beyond the realms of probability.
So far the Earth has escaped exile from the Sun. But one day, in about 4 billion years, even the Earth could become a rogue. Because as the Sun ages, swells and blows half of itself into space, the Earth will either be swallowed up or pushed away. But it is unlikely that it will completely escape its gravitational pull. So, as the dead Sun is degraded to a smoking white dwarf, the Earth will face a fate similar to those other dark and cold worlds. Not entirely alone, but far from the once hot and bright orbit of its star.
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