What Makes Something A Planet, According To An Astrophysicist?
“A dolphin may look like a fish, but it’s really a mammal. Similarly, the composition of an object is not the only factor in classifying it: its evolutionary history is inextricably related to its properties. Scientists will likely continue to argue over how to best classify all of these worlds, but it’s not just astronomers and planetary scientists who have a stake in this. In the quest to make organizational sense of the Universe, we have to confront it with the full suite of our knowledge.
Although many will disagree, moons, asteroids, Kuiper belt and Oort cloud objects are fascinating objects just as worthy of study as modern-day planets are. They may even be better candidates for life than many of the true planets are. But each object’s properties are inextricably related to the entirety of its formation history. When we try to classify the full suite of what we’re finding, we must not be misled by appearances alone.”
You’ve heard about the IAU’s definition, where in order to be a planet, you must pull yourself into hydrostatic equilibrium, orbit the Sun and nothing else, and gravitationally clear your orbit. You’ve also heard about the controversial new definition from geophysical/planetary science arguments, that planets should be based on their ability to pull themselves into a spheroidal shape alone.
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?
You Won’t Like The Consequences Of Making Pluto A Planet Again
“There are some out there who are desperate to save Pluto’s planetary status, and would be willing to open the floodgates and bestow planethood on every moon, asteroid, and ice ball out there that’s massive enough to be round. There are others who spend 100% of their time looking down at their feet on whatever world they’re considering when it comes to planethood, and to them, everything with enough mass will be a planet. But for the rest of us, where you are in the Universe is an inseparable part of what you are. Nothing in the Universe exists in a vacuum, and where you are determines a huge number of properties of you, regardless of whether you’re a planet, moon, asteroid, centaur, comet, Kuiper belt object, or Oort cloud object. If you want to ignore all of that — and proclaim, “round means planet” — then more power to you. But in planethood, as in most things, the full scientific story is far more interesting.”
When you say, “Pluto should be a planet,” what I hear is, “let’s ignore all of astronomy.” When you say, “we’re using a geophysical definition of a planet,” I hear, “we don’t believe in looking up.” And when you say, “we call ourselves planetary scientists, and so we get to decide what a planet is,” I hear, “we don’t care about the full suite of scientific evidence.” There is a long and interesting history to planets and planethood, and yes, the IAU definition is flawed. But does that mean, as Alan Stern and David Grinspoon contend, that we should call every object that can pull itself into a round shape a planet?
NASA Kepler’s Scientists Are Doing What Seems Impossible: Turning Pixels Into Planets
“It isn’t the image itself that gives you this information, but rather how the light from image changes over time, both relative to all the other stars and relative to itself. The other stars out there in our galaxy have sunspots, planets, and rich solar systems all their own. As Kepler heads towards its final retirement and prepares to be replaced by TESS, take a moment to reflect on just how it’s revolutionized our view of the Universe. Never before has such a small amount of information taught us so much.”
When you think about exoplanets, or planets around stars other than the Sun, you probably visualize them like we do our own Solar System. Yet direct images of these worlds are exceedingly rare, with less than 1% of the detected exoplanets having any sort of visual confirmation. The way most planets have been found has been from the Kepler spacecraft, which gives you the very, very unimpressive image of the star you see featured at the top. Yet just by watching that star, the light coming from it, and the rest of the field-of-view over time, we can infer the existence of sunspots, flares, and periodic “dips” in brightness that correspond to the presence of a planet. In fact, we can figure out the radius, orbital period, and sometimes even the mass of the planet, too, all from this single point of light.
This Is What Sun-Like Stars Making Planets Look Like
“Owing to a new instrument on a remarkable telescope, the ESO’s Very Large Telescope, we can now image protoplanetary disks directly. The SPHERE instrument, optimized for infrared exoplanet research, includes the IRDIS imager, designed for high-resolution viewing. When it looked at T Tauri stars, very young stars of 2 solar masses or less, here’s what it saw. Regardless of age or mass, symmetric and well-defined rings, disks, and gaps exist around every one.”
What did our Solar System look like when it was just forming? How did we go from a single, central mass with a disk around it to a full-fledged planetary system with well-defined bodies and boundaries? Since the turn of the century, we’ve managed to image protoplanetary disks to unprecedented resolution, finding some with symmetric, ring-like features in them, and others with large, sweeping spiral shapes. Both classes should have planets, but which one was our Sun? In a new study that focused on low-mass protostellar systems, eight T Tauri stars, of two solar masses or less, were studied. Between the ages of 1 and 15 million years, they all exhibited the symmetric structures we saw hints of earlier, without any spiral perturbations at all.
Sorry, Super-Earth Fans, There Are Only Three Classes Of Planet
“What’s really interesting is how the mass/radius relationship changes for these three different classes of world. Up to about double the Earth’s mass, or a size just ~25% larger than Earth’s radius, you have an opportunity to be Earth-like, with thriving life on the surface. Beyond that, you’ll have an enormous hydrogen/helium envelope, and be much more akin to Neptune, Uranus or Saturn. In other words, what we’ve been classifying as “super-Earths” aren’t anything like Earth at all, but are instead gas giant worlds, expected to be wholly inhospitable to life on their surfaces.”
Thanks to NASA’s Kepler mission, we’ve discovered literally thousands of worlds that lie beyond our own Solar System. Surprisingly, the majority of them aren’t like anything we have in our own backyard, but are somewhere in between Earth and Neptune in terms of size and mass. These worlds, usually broken into categories like “super-Earths” and/or “mini-Neptunes,” have often been viewed as new categories of planets, along with the “super-Jupiters” that we don’t see here, either. Yet these classifications are purely arbitrary, based on what we’ve seen and how we classified planets in our own neighborhood. What would we get if we classified them based on the properties that they actually possess, like mass and radius? We’d find, quite surprisingly, that there are only three classes of planet: Terran, Neptunian, and Jovian-like worlds.
Ask Ethan: Why don’t comets orbit the same way planets do?
“Why [do] comets orbit the Sun in a parabolic path, unlike planets which orbit in an elliptical one? Where do comets get the energy to travel such a long distance, from the Oort cloud to the Sun & back? Also, how could interstellar comets/asteroids come out of their parent star [system] and visit other ones?”
When we see comets in our Solar System, they can be either periodic, passing near the Sun and then extending very far away, to return many years later, or they could be a one-shot deal. But comets are driven by the same gravitational laws that drive the planets, which simply make fast-moving, nearly-circular ellipses around the Sun. So what makes these orbits so different, particularly if they’re obeying the same laws? Believe it or not, most of the would-be comets out there are moving in exactly the same nearly-circular paths, only they’re far more tenuously held by the Sun. Gravitational interactions might make small changes in their orbits, but if you’re already moving very slowly, a small change can have a very big effect!
Further analysis showed that these are driven by liquid water, not avalanches.
As the seasons change, water condenses and dissolves martian salts.
These then flow down the crater, as before-and-after images demonstrate.
If you think of Mars as a boring, reddish, desert world with dunes, sands, and craters, you’ve never seen it with the proper eyes. The Mars Reconnaissance Orbiter, equipped with its HiRISE camera, was launched more than a decade ago, and has covered the entire surface of the red planet, taking more than 50,000 images in enhanced color. It’s revealed countless features, shown us the insides of crater walls, viewed the martian bedrock, found impact craters, discovered evidence for liquid, flowing water, and even helped determine the origin of Mars’ moons. Yet to fully appreciate what we’ve learned, you simply have to see it for yourself.
Ask Ethan: How Many Planets Did NASA’s Kepler Miss?
“Since Kepler uses the transit method to detect exoplanets, how many are we missing due to non-ecliptic alignment?”
With a field-of-view encompassing 150,000 stars, NASA’s Kepler mission delivered an overwhelming prize when it came to hunting worlds beyond our own Solar System: thousands of new exoplanets. The majority of them, however, were different from what we have at home. They were larger, more massive, closer to their parent stars, and orbiting more quickly than what we find in our own neighborhood. In other words, we found the worlds that were easiest to find. But NASA’s Kepler wasn’t sensitive to all the worlds that were out there. Sure, to observe a transit, where a planet passes in between its parent star and ourselves, requires a fortuitous alignment, and we can certainly extrapolate how many more exoplanets like the ones we’ve already seen are out there. But to know how many planets NASA’s Kepler mission truly missed requires a whole slew of other information, much of which the Universe hasn’t yet revealed to us given our current technology.