chasing the galaxy’s most mysterious worlds

The most famous planets revolve around a star. These planets, including Earth, benefit from the star’s heat and light. And it is the light emitted by these stars that allows us to see them. But there are also “invisible” planets, hidden from our view, which float, abandoned, across the cosmos. These dark, lonely worlds have no orbiting star, no light to bask in, no heat to radiate. These are the “rogue” planets – and astronomers just found a new one, roughly the same size as Earth.

Planets are made from the debris left behind after a star is born. These planets encircle the young star in a thin disc of grains and gas, and expand as these tiny particles stick together and tighten together until they clear their immediate surroundings. Things are chaotic in this world and collisions between planetary embryos, or proto-planets, are common. Stars tend not to form alone, but in groups of hundreds or thousands at a time, and encounters between their nascent planetary systems wreak new havoc.

It is believed that young Earth was hit by a body the size of Mars, removing enough material to form the Moon. But some planets faced a darker future: they were completely knocked out, destined for a life in the immense coldness of space between the stars. These are the free-floating “rogue planets”.

When the planets are still very young, say only a few million years old (Earth is over 4.5 billion years old), they are still warm from their formation and from the energy released by their continuous gravitational contraction and l radio activity of their nuclei. Great examples of such young but floating planets (think a baby Jupiter) have been seen directly in regions where the stars had just formed. But finding smaller rogue planets proved to be nearly impossible until the “lens” was discovered.

Gravitational lenses

Everything that has mass bends space and deflects light 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 the gravitational lens. It was predicted by Einstein’s General Theory of Relativity and was first verified when stars were seen to be displaced from their usual positions when seen near the Sun when 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 amounts of substance between the galaxies and by stars aligned with other stars in the background. One sighting 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 a micro-lens, as they can be so small.

Such a “micro-lens” event has been attributed to the new rogue planet, called OGLE-2016-BLG-1928. Observing the amplification of light from a discrete star in the dense interior regions of the Milky Way galaxy lasted only 42 minutes.

This meant that it had to be a small object and the estimated mass left no doubt that it had to be a planet not much different in size from Earth. The lenticular planet has not been associated with a star. Rogue lens planets have been found before, but this is one of the most compelling cases. As well as being the closest to Earth, OGLE-2016-BLG-1928 is also the smallest thief ever.

Could Earth go rogue?

A large number of rogue planets crisscrossing 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 ice age unlike Earth’s long and varied history? Perhaps they harnessed nuclear power or have they become entirely non-biological?

It may sound like science fiction, but what are the chances that Earth will stumble upon such a planet by chance? This is not inconceivable. It is only in the past two years that rogue asteroids such as Oumuamua and rogue comets such as Borisov have whistled our solar system. A rogue planet is unlikely to pass us so close. But it is not beyond the realm of probability.

Earth has so far escaped banishment from the Sun. But someday, in about 4 billion years, Earth too could go rogue. Because as the Sun ages, swells, and half-propagates through space, the Earth will either be swallowed up by it or driven out. But it is unlikely that it will completely escape its gravitational pull. So, as the dead Sun is degraded into a smoldering white dwarf, Earth will face a similar fate to these other dark and cold worlds. Not entirely alone, but far from the once hot and bright orbit of its star.

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This article is republished from The Conversation under a Creative Commons license. Read the original article.