Observing with Street Lights

Observing with Street Lights
Dark sky sites not always necessary to see the Milky Way (This image was taken ouside of a B&B in Julian, CA)

Saturday, May 16, 2015

Working with Astronomer Assistant Danny and looking at one paper about planet formation

Darn, the weather is just not cooperating this week.  We've had rain and both daytime solar and nighttime observing has been pretty much covered over.  The other resident astronomer, Peggy, has been out of town in San Diego this week babysitting 4 year old great nephew Jack.  I guess that means I'm in charge of the observatory?

Oops, I remember now my clear directions.  It's not that I'm now in charge of the observatory, but that I'm in charge of looking after Astronomer Helper, Danny.  Yes, that's right my first and most important assignment is to
look after Danny and the rest of the astronomical equipment and such and any observing will and must come second.  Yes, now I'm clear on my assignment.  And since watching after Astronomer Assistant Danny is so "exhausting", I feel I've deserved and earned quite a few Martini credits which I will shortly redeem.

There were several nights when I would take Danny out for his last nightly inspection of the observatory grounds at about 10:00, that it seemed as if the sky was clearing up and I might be able to do some observing.  Maybe just a bit, but it was getting quite late to just begin setting up in hopes of fewer clouds in an hour or so.

 So, what to do other than just do more astrophysics homework?  Right, isn't what you would do too?  And I knew just what to do since as I mentioned last time, I enjoyed the dynamic astronomy conference in Pasadena, where the topic of planetary migration came up, but still left me with some missing understanding.  At first, I had no sense as to how planets can migrate and had a hard time keeping up with the presentations.  What force can cause the early planets to move closer and then further away from the sun?  I just couldn't see it until I started reading more deeply and checking some of the authors published papers.

Now I can see and understand that planetary migration is a key necessity to explain the birth and growth not only of our solar system, but solar systems in general.  Scientists recognized after the many recent discoveries of planets, so call exoplanets, around other stars that migration was a key and necessary concept.  Many discoveries, mainly due to the fact that large objects are easier to find, were of large Jupiter size planets.  Jupiter size planets are made of gas and can only form at distances of 4-5 times further away from the sun than the Earth is, else the gas in the solar system disk would be too hot to become gravitationally bound to the growing Jupiter.  Astronomers use the distance from the Earth to the Sun as a fundamental unit called one astronomical unit (AU).  So how could it be that many of these large exoplanets were observed at about 2 AU when they must form further out at about 4-5 AU?  Currently our Jupiter is about 5.2 AU or such away from their sun.

With the discovery of how migration could occur, the theories and observation started to finally fall into place and started to agree more and more.  So, this week's astro photo is of a couple of figures that I found in the literature and these two figures helped me see what could be going on and you can check it out for yourselves.

Oh, oh, I see once the photo is pasted in here that it is not as clear as it was originally.  Sorry about that.  Maybe I can talk my way through what is not that readable.  Figure 1 shows a computer simulation of a solar system similar to our own and the timeframe is just shortly after some giant Jupiter has gobbled up a lot of gas, while the four inner rocky planets of the solar system are still orbiting piles of rubble.  Then the interesting part about migration begins when the gravitational forces on Jupiter from the gas slightly located on an inner orbit generates slightly more torque, called Linblad torques, than the gas in outer orbits can provide, so the net result is inward force and migration of the giant Jupiter.  This is all consistent with conservation of energy and angular momentum, even though I still have a hard time calculating everything.

The simulations show that once the Jupiter has migrated down to about 1.5 AU that the simulated Saturn has been growing all this time and it now too begins to migrate closer to the sun and since it is smaller it migrates even faster.  Once this Saturn gets close enough to Jupiter to be in want is called 2:3 resonance the inward migration stops and is reversed and the two planets begin to migrate outward.  The resonance effect occurs because of the different orbital speeds of the two planets that they will just happen to be close to each other that the gravitation attraction and torques is such that they stay locked together as they migrate out.  The 3:2 resonance, or any other ratio, means that for example as that Jupiter has made three trips around the sun, the Saturn being further out make just two trips around the sun and then they lined up again and are both back being at the closest approach again.  This effect and the resulting torques build up over thousands of orbits to move the planets out.

So, Figure 1 (courtesy xxx) shows the mass of the simulated planets and the AU distance changes during this in and out migration period of about 600,000 years.


Figure 2 shows the actual details of the gravitational simulation of the growing accumulation of planets and locations from which the summary in Figure 1 is derived.  The individual panels show the state of the simulation, including the motion of the simulated Jupiter and Saturn and other simulated planets.  The bottom panel shows the simulated solar system about 100 million years later.  The four rocky planets are shown within about 1.5 AU of the sun, with a loosely grouped bunch of planetesimals at about 2-3 AU, and the bigger planets out at locations about where they really are today.  The body of planetesimals in the 2-3 AU are just where the asteroid belt is located.  Notice too that the simulated asteroids are at somewhat higher inclination than the orbit plane of the rest of the planets, which is just what is observed today.

 Now this simulation should not be considered as proof as to the exact how and why our very own solar system formed, but it is indicative in a Monte Carlo simulation type of proof that it is entirely feasible for the solar system to form in this fashion.

This is neat stuff and it will take me much more time to develop a better understanding of the details and to understand what the scientists and astronomers still worry about in these models and what effects still can't be explained.  There is still more work to be done, but with more and more exoplanet systems available for study it seems that there will be a good understanding of how solar systems and their planets and even Kuiper Belt Objects, like Pluto, come to be where they are located.

> Until next time,

No comments:

Post a Comment