Earth's worst-case comet

💥 This giant Oort Cloud visitor makes extinction asteroids look small.

with Ethan Siegel • June 13, 2026

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Greetings readers,

Every time we look up, we’re treated to not just what we can see with our eyes or even with our instruments and observatories, but our minds feast on all that we can imagine. That includes not only what is and what was, but what might someday be. Out there in the distant recesses of our Solar System are the Kuiper Belt and Oort cloud: full of primordial material that’s been largely unchanged since the formation of the planets and Sun. While most of the objects that will become comets are on the small side, a few of them are huge. The largest, comet Bernardinelli-Bernstein, is over 100 kilometers across, and if it ever struck Earth, it would make the asteroid that wiped out the dinosaurs look like a pebble striking a pond in comparison.

For our Ask Ethan this week, we’ve got a fantastic question about black holes: How can they emit winds, jets, radiation, and more even though nothing can escape once falling into them? Don’t worry; we’ve got the answer for you! We also take on 10 big questions about the search for life beyond Earth, a tour of a wide variety of objects in space on all different size scales, and the scientific successes (and fallacies) that come with the anthropic principle.

And finally, I am pleased to share a new Big Think project that I got to help one of our incredible creative artists (shout-out to Ben Gibson) create: a new poster showcasing the history of our Universe. It’s now available for purchase with museum-quality printing in the Big Think store! As always, the Universe is yours to enjoy and explore, and next week I’ll be at the American Astronomical Society’s big summer meeting to help bring you all the latest!

All the best,
Ethan

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A PLANET KILLER

Move over, giant meteor. Here’s what the largest comet would do to Earth

In 2014, astronomers discovered a new Oort cloud comet: Bernardinelli-Bernstein. Recent Hubble measurements pinned its nucleus at 119 kilometers across, by far the largest comet known to humanity. By comparison, the asteroid that caused Earth’s last mass extinction was only 10 kilometers wide and moving more slowly. Here’s the catastrophe Bernardinelli-Bernstein would cause.

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THE HUNT FOR LIFE

10 big questions about the search for life beyond Earth

Despite how overflowing Earth seems to be with life, every other world we’ve examined, up close or from afar, has yet to show confirmed signs of it. We’re still new to the search, and the three main methods we use to look for life beyond Earth are only now coming into their own. Life may yet prove to be common throughout the cosmos. As our search advances, people keep asking excellent questions about where we stand and what comes next. Here are some of the best answers we have today.

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ASK ETHAN

Ask Ethan: How are black holes active if nothing escapes from them?

Black holes are some of the most puzzling and extreme objects in the Universe, with event horizons forever hiding whatever crossed over to the inside. But black holes are also some of the most active objects of all: emitting radiation, particles, and gravitational waves — all signals that we’ve directly detected. What’s the resolution to this paradox? It turns out that black holes are some of the messiest eaters around, and there’s a big difference between “inside” and “out!”

If you have a burning question about the Universe,
email startswithabang@gmail.com!

EXISTENCE, EXPLAINED

The scientific value, and limits, of the anthropic principle

Since we exist within this Universe, the laws of nature must be consistent with at least the possibility of our existence. This simple idea, known as the weak anthropic principle, can lead to powerful scientific and philosophical insights. But taken too far, it can also lead to conclusions that go well beyond what the evidence supports. The anthropic principle can be a useful tool, but only when used with care.

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EXTREMES OF EXISTENCE

From the smallest to the biggest objects in space

Inside the observable Universe are trillions upon trillions of planets, stars, stellar remnants, galaxies, and even larger cosmic structures. In general, stars are larger than planets, and galaxy clusters are larger than individual galaxies, but there are remarkable exceptions that defy those trends. Here’s a tour of the largest and smallest objects known across the cosmos, where some differences are subtle, and others are truly extreme.

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THE PODCAST

Starts With A Bang podcast #130 – the initial mass function of stars

When you start with a gas cloud that’s massive enough and cool enough, it’ll start to contract under its own gravity: eventually fragmenting, collapsing, and forming new stars. Those new stars come in all different masses, and in order to describe how many stars will form in a given mass range, we need to derive a quantity of immense astronomical importance: the initial mass function. But what, exactly, determines the initial mass function, and how has it evolved over time? Perhaps surprisingly, the answer lies in two places: young, distant galaxies, and in the far corners of our Milky Way. Here’s what we’re learning.

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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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