with Ethan Siegel • May 9, 2026
Greetings readers,
There are multiple sides to every story, although the merits of each side are rarely equal. Last week, I made the case for why scientists were correct to demote Pluto from its prior “planetary” status to that of a dwarf planet, given all we had learned. This week, I thought it was worth making the counterargument: Pluto, and all dwarf planets, should now be promoted to the status of planet, albeit into a separate category from the “major planets” that make up our current eight. Regardless of how you feel about it, there’s some tremendous science to learn about the issue that’s brand new from the past 20 years, from exoplanets to rogue planets to bodies right here in our Solar System, and I’ll keep doing my best to bring it all to you.
Elsewhere in the Universe, we can find cosmic jets around all kinds of objects, but it’s the jets being launched by supermassive black holes that are the longest, strongest, and most energetic. Scientists at NIST just completed a 10-year quest to make the best-ever measurements of G, the gravitational constant, but the biggest thing they shed light on is the full suite of previously unaccounted-for systematic errors, which could pave the way toward a better understanding of G than ever before. It’s 2026, and despite all the attempts to revive the aether, from scientists and crackpots alike, the truth is that we simply don’t need it, and I’m hoping I can help you understand why. And finally, for Ask Ethan, I’m pleased to take on a complicated question about just how empty the deepest depths of space are. You might not believe how low densities can get across the Universe, and how that compares to the most pristine vacuum systems we’ve ever engineered here on Earth!
Next week, I’ll have a new podcast episode to share with you all, and later this month, I’ll be releasing my first original Starts With A Bang video. I’m excited, and I think you’re going to absolutely love it. Stay tuned, because there’s still plenty more to learn about reality at the deepest levels of all.
All the best,
Ethan
HOLE-Y JETS
Supermassive black holes launch the most powerful cosmic jets
Jet-like features appear around many objects in the Universe, from protoplanetary systems and Herbig-Haro objects to black holes at the centers of galaxies and quasars. The largest and most powerful jets come from black holes, but the black holes themselves don’t launch them. Gravity alone isn’t enough; electromagnetic effects in the surrounding material are what drive these jets. Thanks to observations of black holes near and far, we’re finally figuring out how it works.
JUSTICE FOR PLUTO
The science case for why Pluto should become a planet (again)
Discovered in 1930, Pluto was known as our Solar System’s ninth planet for 76 years. But discoveries in the 1990s and 2000s revealed many additional trans-Neptunian objects, including large ones. Today, we know of more than 5,000 worlds beyond Neptune, including roughly 100 dwarf planets. Even so, many of these worlds behave much like planets, sharing common features and processes. Here’s why maybe the IAU’s definition should change to include Pluto.
ASK ETHAN
Ask Ethan: How empty are the depths of space?
Here on Earth, we can measure the densities of rocks, water, air, and even create vacuums emptier than outer space. But the closer we look, the more we find that “empty space” is never truly empty, containing dust, gas, and plasma even beyond the Solar System and the Milky Way. The sparsest regions lie in vast cosmic voids, far from galaxies and clusters alike, where densities can fall below one proton per cubic meter.
If you have a burning question about the Universe,
email startswithabang@gmail.com!
THE FALL OF THE HOUSE OF AETHER
Why science has abandoned the existence of the aether
It was long assumed that light waves, like sound waves or water waves, required a medium to travel through. Although that medium was never detected, scientists still gave it a name: the luminiferous aether. But experiment after experiment failed to reveal it, and relativity removed the need for it entirely. Is there any evidence that the aether exists?
BIG G ENERGY
A new experiment deepens the physics mystery over “big G”
Originally proposed by Isaac Newton, the universal gravitational constant, G, was not measured until Henry Cavendish succeeded in the late 18th century. Over the 19th and 20th centuries, measurements grew increasingly precise. By the early 2000s, we thought G was well understood. But newer, more precise experiments revealed much larger uncertainties than expected. Now, a new 2026 measurement deepens the puzzle even further.
Ethan Siegel, Ph.D., is an award-winning theoretical astrophysicist who's been writing Starts With a Bang since 2008. You can follow him on Twitter @StartsWithABang.
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Indeed lots of misconceptions about black holes. Here’s a theory I’d like your honest opinion on voidtovoid.com