This Is Everything That’s Wrong With Our Definition Of ‘Planet’
“There are many people who would love to see Pluto regain its planetary status, and there’s a part of me that grew up with planetary Pluto that’s extraordinarily sympathetic to that perspective. But including Pluto as a planet necessarily results in a Solar System with far more than nine planets. Pluto is only the 8th largest non-planet in our Solar System, and is clearly a larger-than-average but otherwise typical member of the Kuiper belt. It will never be the 9th planet again.
But that’s not necessarily a bad thing. We may be headed towards a world where astronomers and planetary scientists work with very different definitions of what attains planethood, but we all study the same objects in the same Universe. Whatever we call objects — however we choose to classify them — makes them no less interesting or worthy of study. The cosmos simply exists as it is. It’s up to the very human endeavor of science to make sense of it all.”
Next month will mark 13 years since the International Astronomical Union (IAU) officially defined the term planet and ‘Plutoed’ our Solar System’s (up-until-that-point) 9th planet. With an additional 13 years of knowledge, understanding, data, and discoveries, though, did they get the decision right?
Certainly, there were aspects that needed to be revised, but the IAU’s definition comes along with some major gaps and mistakes. We can do better! Come learn how.
What Was It Like When Venus And Mars Became Uninhabitable Planets?
“But the changes that Mars endured were rapid and sweeping. Planets are born with a fixed amount of internal heat, which radiates away over their lifetime. A planet like Mars, with half the diameter of Earth, is born with only about 10-15% the amount of internal heat as our world, and will therefore see a greater percentage of it radiate away much faster than Earth will.
Approximately 3 billion years ago, the core of Mars became cool enough that it stopped producing that protective magnetic dynamo, and the solar wind began striking the Martian atmosphere. In short order, which is to say in just tens of millions of years, the atmosphere was knocked off into interplanetary space. As a result, the oceans were unable to remain in liquid form, and either froze beneath the surface or sublimated away.”
When our Solar System first formed, it wasn’t just Earth that looked promising for life, but also Venus and Mars. All three of these planets had large, liquid water oceans, substantial atmospheres, and the ingredients for complex biochemistry and even life. Over on Venus, its close proximity to the Sun and the large presence of atmospheric water vapor led to a runaway greenhouse effect, boiling the oceans after just ~200 million years. But Mars, despite being small and distant, maintained Earth-like conditions for 1.5 billion years. Considering that life arose on Earth after just one-sixth of that duration, perhaps Mars once had life, too?
Come get the story of the Solar System’s closest version of a failed version of Earth, Mars, and learn how it ultimately lost its chance at habitability.
What Was It Like When Planet Earth Took Shape?
“There was almost certainly a high-energy collision with a foreign, out-of-orbit object that struck our young Earth in the early stages of the Solar System, and that collision was required to give rise to our Moon. But it was very likely much smaller than Mars-sized, and it was almost certainly a sturdy strike, rather than a glancing collision. Instead of a cloud of rock fragments, the structure that formed was a new type of extended, vaporized disk known as a synestia. And over time, it settled down to form our Earth and Moon as we know them today.
At the end of the early stages of our Solar System, it was as promising as it could be for life. With a central star, three atmosphere-rich rocky worlds, the raw ingredients for life, and with gas giants only existing much further beyond, all the pieces were in place. We know we got lucky for humans to arise. But with this new understanding, we also think the possibility for life like us has happened millions of times before all throughout the Milky Way.”
One of the deepest existential questions we can ask about the Universe is how, after more than 9 billion years, all the phenomena in our cosmic history led to the creation of planet Earth. Going from an environment where stars were actively forming to one where the Sun, Earth, and all the other planets were in place is a daunting task for people who create scientific simulations of our early environment, and involves gravitational interactions, planetary migrations and ejections, and even enormously energetic collisions between planets and proto-planets.
Yet somehow, it all came together, and gave rise to us. From what we’re learning, we might not even be all that rare. Come check out the current story.
What Was It Like When Our Solar System First Formed?
“Over the past few years, we’ve finally been able to observe solar systems in these very early stages of formation, finding central stars and proto-stars shrouded by gas, dust, and protoplanetary disks with gaps in them. These are the seeds of what will become giant and rocky planets, leading to full-on solar systems like our own. Although most of the stars that form — including, very likely our own — will have formed amidst thousands of others in massive star clusters, there are a few outliers that form in relative isolation.
Although the history of the Universe may subsequently separate us from all of our stellar and planetary siblings from the nebula that they formed in billions of years ago, scattering them across the galaxy, our shared history remains. Whenever we find a star with approximately the same age and abundance of heavy elements as our Sun, we cannot help but wonder: is this one of our long-lost siblings? The galaxy is likely full of them.”
It took a whopping 9.2 billion years of cosmic evolution for the Universe to give rise to the very beginning of our Solar System; our Sun and planets didn’t form until 2/3rds of the time since the Big Bang had passed. In order to get there, we needed to form the right ingredients for life, rocky planets, and the chemistry we need. But when it happened to us, we weren’t alone. It likely happened exactly the same way for thousands of other stars at once, and continues to happen even up through the present day.
Are we alone in the Universe? The cosmic story that brought us to existence seems to be a story that’s universal. Here’s a key step in how we got here.
A Billion Years In Interstellar Space: What We Know Today About ‘Oumuamua
“The incredible conclusion isn’t just that ‘Oumuamua came from outside
of our Solar System, but that this was both rare and common. For an
individual object, like ‘Oumuamua, it will likely never come this close
to another Solar System again. Only once every 100 trillion years — some
10,000 times the current age of the Universe — will it pass so close to
a star. As scientist Gregory Laughlin put it, “this was the time of
But for our Solar System, because of the sheer
number of objects like this flying through the galaxy, we probably
experience a close encounter like this around a few times per year. 2017
marked the first time we saw such an object, but we’ve likely gotten
billions of them over the course of our Solar System’s lifetime. Some of
them, if nature was kind, may have even collided with Earth.
There may be as many as ~1025
of objects like this flying through our galaxy, and every so often,
we’ll get lucky enough to encounter one of them. For the first time,
we’ve actually seen one of them for ourselves.”
In 2017, our Solar System received a visit like never before: from an object originating from interstellar space. Likely ejected more than a billion years ago from a foreign solar system, it happened to pass within even the orbit of Mercury, only becoming visible to our telescopes when it came within 60 lunar distances of the Earth.
But we found it, observed it, and learned everything we could about it. What do we know, today? Spoiler: it’s not from aliens.
Triton, Not Pluto or Eris, Is The Kuiper Belt’s Largest World
“The result, today, is that the largest and most massive body ever to form in the Kuiper belt — 20% larger than Pluto; 29% more massive than Eris — is now Neptune’s largest moon: Triton. Today, Triton makes up 99.5% of the mass orbiting Neptune, an enormous departure from all the other giant planet systems we know of. The only explanation for its properties, especially its bizarre and unique orbit, is that Triton is a captured Kuiper belt object.
We often talk about icy moons with subsurface oceans as candidate worlds for life. We imagine large, distant, icy bodies as planets or dwarf planets in their own right. Triton was born not as a moon of Neptune, but as the largest and most massive Kuiper belt object to survive. You don’t cease to exist when you move locations, and neither did Triton. It’s the original king of the Kuiper belt, and its true origin story is a cosmic mystery that deserves to be solved.”
In October of 1846, just months after Neptune was discovered, a large moon was discovered around it: Triton. Today, Triton is a supremely unusual moon for a number of reasons, but the largest is that it rotates in the wrong direction. While Neptune orbits the Sun counterclockwise and spins counterclockwise on its tilted axis, Triton orbits in the opposite direction. The only way this could have happened is if it were a captured object. And that’s exactly what it looks like: a captured object from the Kuiper belt!
We know what it’s like and where it came from; the biggest mystery, now, is reconstructing how it came to be there. Come get the story on the true king of the Kuiper belt: Triton!
This Is Why Scientists Think Planet Nine Doesn’t Exist
“Of course, this study isn’t enough to rule out Planet Nine; it still could be out there. As a counterpoint, Mike Brown has contended that a different survey strategy could have been definitive, and OSSOS simply isn’t a good survey for indicating yea or nay on Planet Nine. But remember, the old saying goes, “where there’s smoke, there’s fire,” indicating that if you observe an effect, it likely has a cause.
If you all of a sudden discover that what you thought was smoke was a figment of your imagination, it doesn’t mean there wasn’t a fire, but it sure does make the hypothesis that there ever was a fire a lot less compelling. The OSSOS study doesn’t rule out Planet Nine, but it does cast doubt on the idea that the Solar System needs one. Unless a deeper, better survey indicates otherwise, or Planet Nine serendipitously turns up, the default position should be its non-existence.”
Is there another massive planet in the Solar System? Do we have a super-Earth after all, between the masses and sizes of Earth and Neptune? And has it only gone undiscovered until now owing to our telescopic limitations, and the fact that it’s so much more distant than the presently known planets?
It’s possible. That’s the radical idea behind Planet Nine, proposed nearly three years ago by Konstantin Batygin and Mike Brown. They looked at the unusual orbits of a number of Kuiper Belt objects, and conjectures that a ninth planet, located hundreds of times as distant as Earth is from the Sun, could be the culprit. But on closer inspection, the evidence that they’re looking at might just be biased, and there may be no Planet Nine at all.
There may not even be a puzzle to solve. Come get the scientific story on Planet Nine that you haven’t heard today.
Our Motion Through Space Isn’t A Vortex, But Something Far More Interesting
“We know exactly how the Earth moves through the Universe, and it’s both beautiful and simple. Our planet and all the planets orbit the Sun in a plane, and the entire plane moves in an elliptical orbit through the galaxy. Since every star in the galaxy also moves in an ellipse, we see ourselves appear to pass in-and-out of the galactic plane periodically, on timescales of tens of millions of years, while it takes around 200-250 million years to complete one orbit around the Milky Way. The other cosmic motions all contribute, too: the Milky Way within the Local Group, the Local Group in our Supercluster, and all of it with respect to the rest-frame of the Universe.
The Solar System isn’t a vortex, but rather the sum of all our great cosmic motions. Thanks to the incredible science of astronomy and astrophysics, we at last understand, to tremendous precision, exactly what that is.”
There are images, GIFs, and videos that claim to show our motion through the galaxy. A great many of them are incredibly visually appealing; stunning, even. But are they scientifically accurate visualizations, or are they simply pseudoscience gussied up in the language of science, along with pretty-but-misleading visuals? Rather than nitpick the work of what someone else has created, let’s just go all the way and learn about what our actual cosmic motion through the Universe is, and get it as accurate as modern-day science actually knows!
Our motion through space isn’t a vortex, but something far more fascinating. As of 2018, we can accurately describe it better than ever before.
Which Worlds Will Survive When The Sun Dies?
“But the Sun will be so hot and so bright that much of the outer Solar System will be absolutely destroyed. Each of the gas giants has a ringed system; although Saturn’s is the most famous, all four of them have rings. These rings are mostly made of various ices, such as water ice, methane ice, and carbon dioxide. With the extreme energies given off by the Sun, not only will these ices melt/boil away, but the individual molecules will be so energetic that they will be ejected from the Solar System.”
When the Sun becomes a red giant, lots of changes are going to happen. Mercury and Venus will surely become engulfed; Earth and Mars will lose their atmospheres and oceans, becoming barren and charred. But even beyond that, the outer worlds and structures in the Solar System will melt and lose their volatiles. Asteroids will lose mass and become rocky/metallic cores; moons like Europa and Enceladus will melt away; the rings around the gas giants will disappear; even Pluto and the other large Kuiper Belt objects will lose their atmospheres and top layers, melting away until they’re only a rock-and-metal core.
Who will survive, who will transform, and who will be annihilated when the Sun dies? The carnage is severe, but not complete. Get all the details here.
Ask Ethan: How Do We Know The Age Of The Solar System?
“How do we know the age of our solar system? […] I have a loose grasp on the concept of dating the time elapsed since a rock was liquid, but 4.5 Billion years is roughly how long ago Theia hit proto-Earth liquefying a massive amount of everything. […] How do we know we’re actually dating the solar system and not just finding dozens of ways to date the Theia collision?”
You’ve probably heard the estimates before: that the Earth, the Sun, and the rest of the Solar System are all about 4.5 or 4.6 billion years old. But why be so imprecise? We don’t have to be! In fact, we know that there are slight variations, and based on the fact that we think that the Earth-Moon system formed from a giant impact tens of millions of years after the rest of the Solar System did, we shouldn’t get the same answer for everything! It turns out that we’ve now advanced to the point where we can actually give answers that are extremely accurate: the Earth-Moon system should be 4.51 billion years old; the oldest meteorites show an age for the rest of the Solar System of 4.568 billion years, and the Sun may be a little older at 4.6 billion years.
How do we know? The science of radioactive decay holds the answer, and it’s a lot more complex, but a lot more well-understood, than you might think!