Have astronomers found a runaway black hole?

 A runaway black hole weighing 20 million Suns is tearing through space at 3.6 million mph—fast enough to travel from Earth to the Moon in 14 minutes.

An illustration of a supermassive black hole speeding through space, leaving a trail of newly formed stars.

This spectacular object was initially dismissed as a simple camera glitch. In 2023, astronomers analyzing archival images from the Hubble Space Telescope noticed a strange, perfectly straight streak of light. Further investigation revealed it was something unprecedented: a supermassive black hole hurtling through intergalactic space, leaving a 200,000-light-year-long trail of newly born stars in its wake.

Supermassive black holes typically anchor the centers of galaxies. However, when galaxies collide and merge, their central black holes can drift together to form a binary pair. If a third galaxy joins the cosmic pileup, the gravitational dynamics become chaotic and highly unstable. This complex interaction can act like a gravitational slingshot, forcefully ejecting one of the black holes out of the host galaxy entirely.

This specific black hole was discovered by a Yale University team. Because black holes do not emit light, the runaway object is invisible. However, as it plows through the thin gas surrounding its former galaxy, its immense gravity creates a shockwave. This compresses the gas, triggering a massive burst of star formation behind it. The result is a narrow, brilliant contrail of young, hot stars that stretches out into the darkness. The trail points directly back to the center of the original galaxy, providing a clear map of its ejection path.

The linear streak captured by the Hubble Space Telescope, tracing the path of the runaway supermassive black hole.

Smaller, stellar-mass black holes can also go rogue. When a massive star goes supernova, an asymmetrical explosion can give the newly formed black hole a powerful "kick," sending it wandering alone through the Milky Way. In 2022, astronomers confirmed the discovery of an isolated, stellar-mass runaway black hole. It was detected because its intense gravity temporarily magnified the light of a distant background star, a phenomenon known as gravitational microlensing.

While the concept of wandering black holes might sound alarming, space is staggeringly vast. These rogue objects pose zero threat to the solar system and instead serve as remarkable evidence of the extreme forces at work in the universe.

Is Pluto still a planet, or do astronomers call it a dwarf planet?

When it was discovered in 1930, Pluto was considered to be the ninth planet of the Solar System. It’s quite tiny, though: about half the size of our own Moon.

(Pluto is bottom left)

But over time, astronomers began to realise just how many similar objects there are out there beyond Neptune, in what is called the Kuiper Belt (which is kind of similar to the Asteroid Belt between Mars and Jupiter). Dozens, maybe hundreds or even thousands of them.

So if we are calling Pluto a planet, putting in the same category as Earth, Mars, etc., then there’s potentially a vast number more which should be added to the traditional list of planets. That’s not very convenient or clear.

So in 2006 the International Astronomical Union decided to define exactly what we mean by the word ‘planet’. An upshot of that was that Pluto didn’t fully qualify and came to be redefined as a ‘dwarf planet’. That is,

• it’s a round object and it’s orbiting the Sun just like a planet,
• however, it’s very small, more like the size of a moon or asteroid,
• and as such, it’s not big enough (gravitationally) to ever carve out an orbit all to itself like any big planet would.

Pluto is now just one of several dwarf planets that have been identified, including Haumea, Makemake, Eris and Ceres.

A couple of interesting facts about Pluto:

Pluto’s 248-year elliptical orbit around the Sun sometimes brings it closer to the Sun than Neptune’s orbit.

Pluto has a surprisingly big satellite (moon), called Charon. In effect, it’s a binary system.