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Brian Koberlein
The Universe is amazing, let me tell you.
The Universe is amazing, let me tell you.

Brian's posts

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They're On Dantooine

A rocky asteroid belt found in a binary system has been referred to as a system out of Star Wars, but the most interesting aspect is what it implies for planets orbiting two stars.

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All In The Family

When you think of a family tree, you probably think of human ancestry, and how we can trace our ancestors back to different geographical regions. All living things have a common family tree, which can be seen in our genetic code. While stars aren’t living things, they have a similar family tree, and we’re starting to gather enough data to piece it together.

We’ve long known that stars can be categorized into generations depending on their origin. The very first generation of stars formed from the hydrogen and helium formed in the big bang. When the largest of those stars died in supernovae, the remnant gas and dust collapsed to form a new generation of stars. The largest second-generation stars later exploded, a third generation of stars formed, and so on. Since heavier elements such as carbon and iron forms within stars, later generations of stars tend to have a higher metallicity. Our Sun, for example, has a relatively high metallicity, and is therefore a third or fourth generation star.

As our observations of stars has increased, there have been efforts to find stellar siblings of our Sun. Stars don’t form on their own, but rather form with other stars in large nebulae known as stellar nurseries. Stars formed in the same stellar nursery would have similar ages and similar chemical compositions. The more similar the composition and age, the more likely the stars are to be related.

With sky survey satellites such as Gaia, we are finally gathering this kind of information on millions of stars, so in principle we should be able to study the connections between stars and groups of stars. A new paper in MNRAS shows how this can be done. By piecing together a stellar family tree, we can better understand the dynamics of stellar evolution within our galaxy.

Paper: Paula Jofré et al. Cosmic phylogeny: reconstructing the chemical history of the solar neighbourhood with an evolutionary tree. MNRAS 467 (1): 1140-1153. (2017)

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Sci-Fi Sky

The newly discovered planetary system is perfect for science fiction. Seven rocky worlds, three potentially habitable. But since the planets all orbit close to their star, the apparent sizes of these planets would rival that of the Moon. I put together a graphic showing what TRAPPIST-1 and its planets would look like as seen from TRAPPIST-1f, which is an Earth-sized planet in the center of the habitable zone. If you lived there, the sky would be amazing.

Image credits:
Planets - NASA/JPL (public domain)
TRAPPIST-1: NASA/Walt Feimer (public domain)
Moon - Gregory H. Revera (CC BY-SA 3.0)
Sun - NASA/Goddard (public domain)

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Sun With A Hundred Worlds

Ah, Pluto, for such a small and distant world you are able to stir controversy.

In 2006 the International Astronomical Union (IAU) formally defined a planet as an object which 1) orbits the Sun, 2) is massive enough to be in hydrostatic equilibrium (basically that means it’s round), and 3) it has cleared the neighborhood around its orbit. Since Pluto didn’t satisfy the third criteria, it was tossed out of the official list of planets, much to the outcry of the general public. Pluto is still considered a dwarf planet, along with Ceres, Eris, Haumea, and Makemake, but that was small consolation. Then when New Horizons flew past Pluto in 2015, it found that Pluto was a rich geological world, with mountains and thin blue skies. The images captured by the probe showed that Pluto was very planet-like, and there were new calls to redefine Pluto as a planet.

Even some astronomers have issues with the current definition of a planet. To begin with, a planet must orbit the Sun, so the thousands of exoplanets orbiting other stars are not planets. That’s fine if you want to keep planets and exoplanets separate, but most people would figure that an Earth-like body orbiting any star would be a planet. Then there is the third criteria, the whole “cleared it’s neighborhood” business. If it weren’t for that, Pluto would still be a planet. The problem with this criteria is that the more distant a planet is, the more difficult it is to clear an orbit. Earth is a planet under the current definition, but if Earth were beyond Pluto in the Kuiper belt, it wouldn’t be a planet. That seems rather arbitrary.

So Alan Stern (principle investigator for the New Horizons mission) and others have proposed a new definition: A planet is a sub-stellar mass body that has never undergone nuclear fusion and that has sufficient self-gravitation to assume a spheroidal shape adequately described by a triaxial ellipsoid regardless of its orbital parameters. In other words, if a body is basically round but too small to be a star, then it’s a planet.

This would broaden the definition of planet significantly. If this definition were adopted by the IAU Pluto would officially be a planet once once again. So would all the exoplanets we’ve discovered so far, and so would rogue planets that wander cold deep space all alone. Anything with a diameter larger than about 500 km, all the way up to bodies 15 times more massive than Jupiter would be considered planets.

But such a definition might too broad. Not only would Pluto be a planet, but so would its largest moon Charon. So would our Moon, making Earth a double planet. So would the largest moons of Jupiter and Saturn. The definition would shift our solar system from 8 planets to more than a hundred. Sure, Pluto would be a planet, but who cares at that point?

Personally, I think the proposed definition is too general. You could argue that Pluto should be a planet (I disagree), but Saturn’s small moon Enceladus shouldn’t be a planet. Even the common definition of a moon is that it orbits a larger body. In science fiction we have no problem with moons being large and Earth-like, even habitable. They are still moons. I think there is also something to be said for some kind of orbit-clearing condition. It is quite likely that there are thousands of objects larger than Enceladus in the outer edge of our solar system, and they aren’t the same as closer objects like Mars or Mercury.

All of this is worth discussing. As we learn more about both our own solar system and others our definition of what a planet actually is will have to adapt. Whether we end up with hundreds of planets or only eight will depend on what we want the word “planet” to mean.

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Seven In One Blow

NASA announces discovery of seven Earth-sized planets orbiting a single star.

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Hologram Codex

A new study tries to determine whether the holographic principle applies to our universe.

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Life In A Black Hole

Your spacecraft is failing, and ahead looms the dark majesty of a black hole. As its gravity pulls you ever closer, you cross its event horizon and your fate is sealed. You are trapped forever. What happens next is the subject of numerous movies. Do you travel through a wormhole and enter another universe? Do you confront the intersection of reason and faith? Do you travel back in time to communicate with your daughter? The scientific answer is much more mundane. You die, crushed by the the tidal forces of the black hole interior as you are pulled inevitably to its singularity. But perhaps there is an alternative where you are trapped but could continue to live a full life.

There is no question that if you enter a black hole you can never leave. The event horizon of a black hole ensures that not even light can escape a black hole. To find out what happens next, you have to calculate your orbital path within the black hole, what is known as a geodesic. The simplest geodesic to calculate is for a simple mass dropped into a non-rotating black hole. What is tells us is that you are fated to be crushed by the black hole singularity in a relatively short time. In a supermassive black hole you would have a bit more time, but your days are numbered. More general calculations support this idea. In relativity, the harder you try to escape, the more energy you would need, and the gravitational pull of that energy would work against you. Entering a non-rotating black hole is a death sentence.

But real black holes rotate, and this is where things get more complicated. When a black hole rotates space and time are twisted through an effect known as frame dragging. Fall into a rotating black hole, and its rotation will cause you to spiral around the black hole as you’re entering it. Calculating the geodesics for a rotating black hole interior are more complex, and have to be done numerically. But given the way black holes work it was generally thought that rotation wouldn’t save you. You might orbit the singularity a few times before reaching it, but you would still be doomed. Recently, however, it was found that this might not be the case.

Calculations of orbital paths within rotating and charged black holes shows that it is possible to have stable orbits within a black hole’s event horizon. This means it would be possible to enter a black hole and find a stable orbit around the singularity. You’d still be trapped inside the black hole forever, but you wouldn’t be doomed. In principle, you could survive inside a black hole.

The calculations are only for the ideal case of orbits in a vacuum. Any radiation in the black hole, or other infalling matter would degrade your orbit and make it unstable, so you would still be doomed inside a real black hole. But perhaps with a bit of ingenuity and luck you might be able to create a somewhat stable orbit in order to live our your life before you reach a singularity grave. It’s all pretty speculative, but at the very least it gives science fiction an answer to “what happens next?” that is a bit closer to reality.

Paper: Vyacheslav I. Dokuchaev. Is there life inside black holes? Class. Quantum Grav. 28 235015 (2011)

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Doing It Wrong

Lots of models kinda fit observational data. Good science focuses on the weaknesses of a model and strives to improve them. Bad science claims a rough fit overcomes any theoretical contradictions.

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Limited Time Offer

The deadline for applications is Feb 19, so if you're interested in participating, time is running out.

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On The Air

This afternoon I was on the radio with +Ethan Siegel talking about meteors and funding for NASA. If you listen closely you can also catch a caller claiming the Earth is flat!

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