'Failed stars' may have a cosmic speed limit to how fast they can spin — otherwise they'll break apart

Representative of brown dwarfNASA
  • Brown dwarfs are also called ‘almost stars’ or ‘failed stars’ because they’re bigger than most planets but not heavy enough to set themselves on fire.
  • They’re born spinning and can rotate upwards of 300,000 kilometres per hour.
  • However, scientists at the National Aeronautics and Space Administration (NASA) believe that brown dwarfs are subject to a speed limit — going any faster would mean falling apart.
Brown dwarfs, also called ‘almost stars’ or ‘failed stars’, can spin as fast as 300,000 kilometres per hour (kmph) or more. But the National Aeronautics and Space Administration (NASA) believes there may be a limit to how fast they can go. Going any faster could spell doom for them.

This artist's concept shows a brown dwarf with bands of clouds, thought to resemble those seen at Neptune and the other outer planetsNASA

There are billions of brown dwarfs across the Milky Way and each of them spins at a different pace. But, three of the most rapidly rotating ultra-cool brown dwarfs observed to date — T7, L3.5 and L8 — all take just over an hour to complete one rotation.

According to NASA’s study published in Astronomical Journal, this is more than just a coincidence. Spinning more rapidly could mean that the planets would break apart.

Brown dwarfTime taken to complete one rotation
T71.08 hours
L3.51.14 hours
L81.23 hours

It’s not an ‘age thing’

Brown dwarfs are already spinning on the day that they are born. As they cool down and contract, they start to speed up.

So, one could assume that the faster a brown dwarf is spinning, the closer it is to death. However, the three brown dwarfs observed in the study aren’t the same age or at the same point in their development cycle.

One is a warm brown dwarf, the other is cold, while the third falls somewhere in between. This implies that brown dwarfs are not speeding up as they inch closer to their end but subject to a speed limit, according to NASA.

Cosmic breaking mechanisms may be holding Saturn together

Anything that rotates, generates a centripetal force. The faster the object spins, the higher the centripetal force. It’s the reason why rides at the amusement park have seat belts — to keep riders from getting thrown away from their seats.

When it comes to stars and planets, spinning too fast can mean getting torn apart. However, before that point comes, the cosmic entity is likely to start bulging around its midsection as it succumbs to the pressure. This is what scientists call oblation.

In our own system, Saturn is already showing signs of oblation. Its speed is one rotation every 10.2 hours — similar to how fast Jupiter takes a turn despite being only 0.3 times Jupiter’s mass.

Brown dwarfs are more massive than planets but not quite as massive as stars. Generally speaking, they have between 13 and 80 times the mass of Jupiter. A brown dwarf becomes a star if its core pressure gets high enough to start nuclear fusion.NASA

This doesn’t necessarily mean that Saturn’s ready to fall apart — but NASA isn’t sure why. The scientists have not yet been able to pinpoint the exact cosmic forces that may be acting as natural braking mechanisms, but scientists know that they are there.

“It would be pretty spectacular to find a brown dwarf rotating so fast it is tossing its atmosphere out into space,” said Megan Tannock, lead author of the study. “But so far, we haven’t found such a thing. I think that must mean that either something is slowing the brown dwarfs down before they hit that extreme or that they can’t get that fast in the first place.”

The full picture is yet to reveal itself. Further research may reveal whether there’s some braking mechanism that stops brown dwarfs from self-destruction. Or, perhaps, there are brown dwarfs spinning even faster in the darkness just waiting to be found.

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