"Grenzgänger" gives up puzzles

Giant Planet or Brown Dwarf has unusually strong magnetic field and polar lights

The object SIMP J01365663 + 0933473 lies almost exactly on the border between brown dwarf and planet - and has in addition to polar lights. © Chuck Carter, NRAO / AUI / NSF
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Brown dwarf or planet? Astronomers have discovered a celestial body just 20 light-years away that is exactly on the boundary between the star and the planet. He is only just bigger than Jupiter, but much heavier. Also puzzling: This object has an unusually strong magnetic field and auroras. How these come about, researchers can only speculate so far.

Brown dwarfs are the frontier crosser among the heavenly bodies. Because they are too big and warm for planets, but too small and cold to become real stars with sustained nuclear fusion. According to astronomers, there could be 100 billion such "failed stars" in our Milky Way alone. Also in our immediate neighborhood there are several brown dwarfs.

Right on the border

But astronomers around Melody Kao of the California Institute of Technology have now discovered a brown dwarf who is riddling in several ways. Observations with the radio telescopes of the Very Large Array (VLA) show that the SIMP J01365663 + 0933473, which is only 20 light-years away, is actually too small and light for a brown dwarf. "This object sits exactly on the border between the planet and the brown dwarf, " explains Kao.

The strange celestial body is about 12.7 Jupiter masses heavy and its radius is "only" 1.22 times as large as that of the gas giant. He sits just below the limit of about 13 Jupiter masses, from which a deuterium nuclear fusion is possible and therefore an object is considered a brown dwarf. In addition: SIMP J01365663 is only 200 million years old and lies in the middle of a group of very young stars. Nevertheless, the celestial body is only 825 degrees hot - that is rather little for a young brown dwarf.

Brown Dwarf or Loner Planet?

Should it be an unusually large planet? If so, then SIMP J01365663 would belong to the so-called loner planet - gas giants, which do not revolve around a star, but alone wander through space. Some of these loners may have been thrown into space by turbulence in their home planetary system. Others were probably formed like stars by the collapse of gas clouds and were thus single from the beginning. display

Brown dwarfs of class T appear dark red in visible light because sodium and potassium absorb much green light in their atmosphere. NASA / R. Hurt

The astronomers still do not know if SIMP J01365663 + 0933473 is a planet or a brown dwarf. For the time being, they have been assigned to the spectral class T2.5. Brown dwarfs of this group show spectral signatures of water, carbon monoxide and methane in their atmospheres, similar to some gas giants. Sodium and potassium are also present in larger quantities.

Unusually strong magnetic field and Aurors

But even more puzzling is another observation: This brown dwarf or planet has a 200 times stronger magnetic field than Jupiter. "The combination of age, mass and temperature of this celestial body can not explain such a strong magnetic field, " say the researchers. They suspect that this brown dwarf could have an unusually fast rotation that helps create its magnetic field.

And not only that: The mysterious nerd also has auroras. Here on Earth, such luminous phenomena arise because the solar wind interacts with the magnetic field of the planet. But SIMP J01365663 + 0933473 is a Einzelg nger, near which there is no star and therefore no solar wind.

But how does his aurora come about? So far astronomers can only speculate about it. One possibility would be that the celestial body has an undiscovered planet or moon. Then interaction of these satellites with the magnetic field of the object could trigger the auroras, similar to Saturn and his moon Enceladus. However, it would also be conceivable that interactions within the atmosphere of SIMP J01365663 produce these aurors, similar to some polar lights of Jupiter.

"This object gives us some surprises, but that's exactly why it could help us better understand the magnetic processes in stars and planets, " says Kao. (Astrophysical Journal, 2018; doi: 10.3847 / 1538-4365 / aac2d5)

(National Radio Astronomy Observatory (NRAO), 03.08.2018 - NPO)