Thursday, 31 March 2016

Where might Planet Nine be hiding?

On January 20, 2016, the inferred presence of a large planet far beyond Neptune was announced. The finding was a joint collaboration between Mike Brown, who is known for discovering the Kuiper belt objects that helped nix Pluto's planet status, and Konstantin Batgyin who is an expert in orbital simulations. The finding was based on alignments in the orbits of several trans-Neptunian objects, whose alignment can be explained by the presence of a distant planet about ten times the mass of Earth and 30 times the distance of Neptune. It can also be the result of rare events occurring due to  random chance. In March, Brown and Batygin released another paper (initially on ArXiv) trying to figure out where this planet can be found, if it does indeed exist. The paper has since been published  in Astrophysical Journal Letters, a respected journal for short communications about astronomy and astrophysics.

As in many aspects of physics research, even in the absence of positive data one can learn things by asking the right questions about negative results. In this case the question is "Given that we have not yet detected this planet, where could it be?" rather than repeatedly asking "Is it there? No? Ok how about there?" at various points in the sky.

There are a number of telescopes that scan the sky looking for various things (well, that's basically what telescopes do...I'll refer to these as survey telescopes to distinguish them from telescopes like Hubble that look at specific targets when directed). There is one called WISE that is looking for distance planets and nearby brown dwarves (distant from the solar system's perspective and nearby from the galaxy's). So they consider where this planet could be and not be detected by WISE. The same goes for other survey telescopes: one called CRTS, one called Pan-STARRS, one called the Dark Energy Survey. Another group of researchers, in the time since the January announcement, did an analysis of Saturn's orbit given hypothetical perturbations from this planet, and used the fact that Saturn and the asteroids have not been very perturbed to rule out a few more regions.

What remains is a band of sky where Brown and Batygin's analysis shows this planet must be in order to explain the orbital alignments of the trans-Neptunian objects, and various regions of that band are excluded from these various non-detections.

The band of sky where this planet is expected, and all the ruled out regions of that band, leaving only the black areas. This is an image from their paper which I have annotated. I respect Brown and Batygin as scientists but I disapprove of their use of .jpg for a paper figure! And yet here I go using yellow text on white background.
 So, all that remains in the realm of possibility is a medium chunk of sky in the Northern hemisphere, and a tiny one in the Southern hemisphere. Interestingly, Brown gave his first public lecture on the topic at MIT, which I attended. He showed a similar figure, except there was a much larger region in the Southern hemisphere. Since then, it has been ruled out by the Saturn analysis.

Where is this big region on the actual sky? I have approximately traced it out on a sky map, which is shifted 180 degrees compared to the image above. In the Northern hemisphere, the big region encompasses Orion, which is one of the easiest constellations to find!

Very approximate locations. If you think you can do a better job of mapping the graph regions onto the sky map, please show me and I will credit you. Update (July 20 2016): An anonymous commenter has made a better version, with the yellow showing the search regions.
This tells us how to focus our telescope searches to best find this hypothetical planet. In the Northern hemisphere, the best telescope for the job is the Japanese Subaru telescope (named after the Japanese word for the Pleiades, not the car company whose name has the same origin). This may take up to five years, unless they can get more dedicated telescope time, or refine their search with more accurate computations or the discovery of more aligned trans-Neptunian objects.

So, by looking in this region, it could find this new planet, or it could rule out its hypothetical existence. 

Saturday, 19 March 2016

The Robophysics session at the APS March Meeting was delightful

I recently returned from the March Meeting of the American Physical Society (APS) in Baltimore. It is the largest annual physics conference, with about 10,000 attendees and 50 simultaneous lecture sessions laden with technical comic sans. Everyone gets 10 minutes to speak and 2 minutes for questions, and then are affirmatively ushered off to stay on schedule. My talk was on Tuesday. There are about 10 research groups in my field and all of them were there, so it was a very relevant session. On Friday afternoon I had seen everything I needed to see and was pretty exhausted, so decided to go to a session called Robophysics: Physics Meets Robotics. I am glad I did.

This lively fellow made an appearance.
There were about 30 talks spread over two sessions, of which I saw about ten. They generally followed the following format.

1. Here is a video of an animal. Look at how smoothly it moves.
2. Here is our best attempt to make a robot to do that. Look at how terrible it did.
3. Here are several slides of variational calculus to guide us towards a better robot.
4. Here is our new robot. We had an undergrad film it for hours on end to gather data. Enjoy.

This video was played in multiple talks:

There was a variant of the format above, where they filmed animals with high-speed cameras in weird situations to see how they function, for example filming mudskippers pushing themselves up sandy hills. One researcher showed how a centipede rapidly increases its vibration frequency and amplitude when poked, and attempted to make a robot centipede that mimicked this.

The main motivation for the research was to improve the way robots interact with their environment, which currently has to be hard-coded (e.g. for manufacturing robots) and is not at all adaptive, and his holding back the expansion of robotic utility. Much like in nanotechnology, researchers are trying to move forward by drawing inspiration from nature.

I will summarise a few of them. I look a few potato-quality pictures which I might try to supplement by looking up the papers.

One was titled "Is a snake a wave or a particle" and was about the angle that snakes scatter when they slither between two posts. They built a snake robot, had an undergrad gather some data on single-snake diffraction, and found that the snake scattering angle distribution has discrete peaks. Someone in the audience asked how a snake could interact with itself like that, which prompted a brief discussion of the history of quantum mechanics. There were quite a few snake-related talk, including one about dropping flying snakes from the top of a room full of high-speed cameras.

Another was about the propagation of vibrations through a spider web. They showed a high speed video of a spider sensing and attacking a moth in its web, then built a large-scale mechanical spider web full of accelerometers with varying tension to study how the perturbation of the prey reaches the spider. After the talk, somebody asked the question:
"In 30 years when we live in a post-apocalyptic robotic hellscape, and I got caught in a human-sized robotic spider web trap, do you have any recommendations as to how I can try to vibrate the web to avoid detection the longest?" 
 Another talk was inspired by a scene from The Simpsons, where Homer kicks a table in order to vibrate a bowl of dip closer to himself so he could dip his chip without getting up. They managed to invent a mechanism to vibrate a surface to create arbitrary flow fields along the surface and move objects around in precisely controlled ways.

He embiggened the session with his cromulent research.
I only made it to the second session, so I missed the keynote invited lecture in the first. Overall it was a nice coda to week of intensive science exposure.

One of the labs that presented a few talks has a website about this with many videos, which I suggest perusing.

Wednesday, 2 March 2016

5phases1cup: Dry Ice in Pluronic Gel

Today a 0.1 mL vial of DNA that I ordered arrived in the lab. It was shipped in a box 100,000 times its size, full of dry ice to keep it cold. Not wanting to let the dry ice go to waste and make my CO2 emissions productive, I decided to drop it in a vial of pluronic gel that another postdoc in my lab prepared. Let's take a look at what happened, and then I'll explain.

A stabilized version can be found here.

Pluronic is an ABA tri-block co-polymer, meaning that it has three regions of repeating chemical units, and the outer two are the same. When it's above a critical temperature, it collapses into a micelle with the out part forming a protective ball around the inner part. These micelles form a network and behave like a gel, but if the temperature is lowered it reverts back to a liquid, sort of like an re-un-boilable egg.

This image cannibalized from a google image search for pluronic.

When I dropped the dry ice in the pluronic, it landed on top of the gel and quickly cooled the region immediately below it, turning it liquid. This wave of cold slowly propagated downward, liquifying the gel in the process. Gravity gradually brought the ice pellet towards the bottom as it cleared out its own passage downward.

The slow but inexorable pull of gravity brings the dry ice pellet down to the bottom of the vial as it clears its own path via a wave of liquefied gel. This is easier to see in the stabilized video.
The CO2 sublimated into bubbles which floated upwards, leaving a sticky pluronic foam at the top which gradually overflowed out of the vial. Eventually, the liquid around the dry ice got so cold that it started undergoing an actual liquid-solid phase transition, encasing the pellet in H2O ice (possibly with pluronic mixed in?), limiting the region from which the bubbles emerged.

That's what's up.

So during this process, we had gel, liquid, gas, and two kinds of ice. Science is cool!