The Humongous Planet Orbiting a Tiny Star

Fagjun | Published 2017-11-03 07:21

An exoplanet that’s far bigger than its host star has left scientists baffled and questioning what they know about how planets form.

Illustration of NGTS-1b and its host star [Image by the University of Warwick]



We know a lot about space--well, more than we used to, anyway. However, more often than not, there are things out in our galaxy that can still surprise and baffle us. The exoplanet NGTS-1b, the largest known planet relative to the size of its star, is one of those things. It’s about the mass of Jupiter, but its host star is definitely not similar in size to our sun.


NGTS-1b lies about 600 light years away from our solar system. While the planet is also a gas planet similar in size to Jupiter, its relatively tiny host star is just half the mass and width of the sun. NGTS-1b is also so close to its star that an entire year on the planet lasts just 2.6 Earth days. The planet’s size, that of its host star, and the proximity between the two are definitely things that scientists aren’t used to seeing.


Illustration of a red star [Image by NASA]



The planet was discovered by the Next-Generation Transit Survey’s telescopes in Chile, which specifically look for undiscovered exoplanets. Astronomers kept an eye out for short and periodic dimming in starlight--events called transits. During transits, a planet passes in front of the star it orbits, resulting in an observable blip in the star’s light. NGTS-1b orbits an old and small red dwarf star, known as an M dwarf star.


Though it may seem that the large size of NGTS-1b should have made the planet conspicuous, it actually made the planet harder for astronomers to spot. The planet was huge, while its host star was small and dim.


According to a new study, the planet is likely to be very old, just like its star. M dwarf stars don’t use up their fuel as quickly as other kinds of stars, which enables them to live for trillions of years.


The manner of the planet’s existence is an oddity. Jupiter-like planets that orbit M dwarf stars are rare, though since M dwarf stars are the most numerous in the Milky Way, there may be more small star-giant planet combos out there.

Changes in Current Thought

Is the discovery of NGTS-1b about to revolutionize the way we look at planetary formation? [Illustration by Mark Garlick/University of Warwick]



Then again, according to existing theories on planetary formation, planets like NGTS-1b should not even exist. "The discovery of NGTS-1b was a complete surprise to us--such massive planets were not thought to exist around such small stars," says Daniel Bayliss, one of the researchers.


As a general rule, large planets are thought to orbit large stars. This is because both are formed by a larger quantity of materials. Smaller stars, meanwhile, are formed by a smaller quantity of materials. Thus, smaller planets also tend to form around smaller stars.


At least, that’s the theory. Reality, however, has turned out to be quite different. Should scientists overhaul current theories on planetary formation altogether, or just make a few tweaks or include an addendum? For now, scientists aren’t sure yet. However, if more large planets are found orbiting small stars, it may be time for a change in scientific thought.

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