Outside our Solar System - planets, planets, planets!

Exoplanet Orbit Database | Exoplanet Data Explorer
The Extrasolar Planets Encyclopaedia
PlanetQuest - The Search for Another Earth
http://www.astro.umd.edu/~miller/teaching/astr380f09/slides19.pdf
Distribution of currently known extrasolar planets
Methods of detecting extrasolar planets - Wikipedia, the free encyclopedia

The ESE's catalog page currently states "Showing 665 planetary systems / 843 planets / 126 multiple planet systems".

Remarkably impressive, and something that I had not expected to see in my lifetime. I remember from my earlier years learning about Peter Van De Kamp's discovery of planets around Barnard's Star, but that was about it. But I recently found out that the Barnard's-Star planets are now discredited; telescope maintenance produced effects comparable in size to the claimed observations. Yet another case of reading too much into borderline observations. It was also was not the first false detection of exoplanets, and not the last one either.

But since the early 1990's, exoplanet detections have been coming in, first slowly, and then faster and faster.


How does one detect them? There are several ways that have been used.

Direct imaging. This is the first method that one might think of, and a few planets have been observed in that manner, like a planet of the star Fomalhaut. It would be very difficult to detect the Earth from outside the Solar System in that way; its reflected visible light is only 10^(-9) of the Sun's visible light. However, stars emit much less of their luminosity in the near infrared, while planet thermal emission peaks there, and a planet of Upsilon Andromedae has been observed to have a mysterious "hot spot" in the infrared.

Transits or mini-eclipses of stars by planets. A sizable number of planets have been discovered in this way, by observing large numbers of stars in the hope that some of the planets will have edge-on orbits and make transits. The Earth has a probability of about 1/200 of being observed in this way, and it makes a dip in the Sun's observed light of about 10^(-4). The Kepler spacecraft is currently observing about 100,000 stars a patch of sky near Vega and Deneb, and its observation analyzers have already discovered a sizable number of planets.


There are less-direct methods of detection, methods that use a planet's gravitational pull on its star or on light traveling nearby. As a star pulls on its planets, the planets will pull on that star, and possibly have some observable effects on that star.

Astrometry A star's moving back and forth can conceivably be detected directly. Peter Van De Kamp's claimed discovery of those Barnard's-Star planets was using this method, and to date, there have been no successful detections with this technique.

Radial velocity A star's moving back and forth will alternately stretch and squeeze the light coming from it, making a "Doppler shift" in its spectral lines. To date, that's the method that the most planets have been detected with. The Earth makes the Sun moves back and forth at about 10 cm/s, making it very difficult to detect, and not surprisingly, most planets detected with this technique have been about as massive as Jupiter.

Example of Doppler Shift using car horn - YouTube
Acela Express Engineer Laaayyyys on the horn! AWESOME Doppler Effect! - YouTube

Timing A star's moving back and forth can make its light arrive alternately early and late. That's too difficult to detect with ordinary stars, but it can be done with pulsars, which keep very good time. Some pulsars have been discovered to have planets.

Gravitational microlensing A massive object will bend light traveling around it, and at a suitable distance, it will act like a lens, focusing the light. There are some planets that have been detected in that way.


It would be good to detect a planet with more than one method, and that's indeed been possible in some cases, like GJ 1214 b, a planet a bit smaller than Neptune.


A big problem with exoplanet detection is a problem in astronomy in general: observational selection. We are limited by what we can detect, and that can provide a poor sample. All the methods work best for larger planets, and that is why large numbers of Neptune-sized and Jupiter-sized planets have been seen, but hardly any Earth-sized ones. Radial velocity works best for planets close to their stars, and combined with the previous bias, has led to the discovery of a sizable number of "hot Jupiters", Jupiter-mass planets that orbit much closer than one might expect. The usual theory is that they formed farther out and then spiraled in by interacting with the still-present protoplanetary nebula. Some of them may run into other ones, and that may explain the highly eccentric orbits that some of them have. The Solar System seems like an oddity: Jupiter and Saturn seem in the right sort of place. But some astronomers have proposed that those two planets spiraled in, then spiraled back out again. This can explain Mars's low mass and the asteroid belt.

Radial velocity works best for planets close to their stars, and combined with the previous bias, has led to the discovery of a sizable number of "hot Jupiters", Jupiter-mass planets that orbit much closer than one might expect. The usual theory is that they formed farther out and then spiraled in by interacting with the still-present protoplanetary nebula. Some of them may run into other ones, and that may explain the highly eccentric orbits that some of them have. The Solar System seems like an oddity: Jupiter and Saturn seem in the right sort of place. But some astronomers have proposed that those two planets spiraled in, then spiraled back out again. This can explain Mars's low mass and the asteroid belt.

I'm a little confused about the italic part. Doesn't the observation bias outlined just before in the text explain why it is that Solar System seems like an oddity? Why do we need a hypothesis to explain why we don't have a "hot Jupiter", if the reason why astronomers find mostly those kinds of extrasolar planets, is only because those are the ones that can be detected?

That's indeed a problem. All the search methods are biased toward planets that are either larger or more massive, and the most successful search methods, radial velocity and transit, are additionally biased toward closer planets.

Now to the back-and-forth hypothesis.

Will inner planets still form if giant planets had spiraled in toward them?
[1004.0971] The Compositional Diversity of Extrasolar Terrestrial Planets: I. In-Situ Simulations -- no migration
[1209.5125] The Compositional Diversity of Extrasolar Terrestrial Planets: II. Migration Simulations

Apparently, they can, at least if not too close to one of these wandering giants. However, their composition tends to be more mixed, because of the giants' mixing up the protoplanetary nebula, and Earthlike planets can get much more water than the Earth has, making much deeper oceans.

But if wandering giant planets are so common, then why is it that the Solar System's ones did not migrate? Or did they?

Some planetary scientists are proposing that Jupiter and Saturn spiraled in, then spiraled out again.

A low mass for Mars from Jupiter/'s early gas-driven migration : Nature : Nature Publishing Group

Jupiter and Saturn formed in a few million years from a gas-dominated protoplanetary disk, and were susceptible to gas-driven migration of their orbits on timescales of only ~100,000 years. Hydrodynamic simulations show that these giant planets can undergo a two-stage, inward-then-outward, migration. The terrestrial planets finished accreting much later, and their characteristics, including Mars' small mass, are best reproduced by starting from a planetesimal disk with an outer edge at about one astronomical unit from the Sun (1 au is the Earth–Sun distance). Here we report simulations of the early Solar System that show how the inward migration of Jupiter to 1.5 au, and its subsequent outward migration, lead to a planetesimal disk truncated at 1 au; the terrestrial planets then form from this disk over the next 30–50 million years, with an Earth/Mars mass ratio consistent with observations. Scattering by Jupiter initially empties but then repopulates the asteroid belt, with inner-belt bodies originating between 1 and 3 au and outer-belt bodies originating between and beyond the giant planets. This explains the significant compositional differences across the asteroid belt. The key aspect missing from previous models of terrestrial planet formation is the substantial radial migration of the giant planets, which suggests that their behaviour is more similar to that inferred for extrasolar planets than previously thought.

Full text here
How Did Jupiter Shape Our Solar System?
NASA - Jupiter's Youthful Travels Redefined Solar System
Here's a nice presentation: The Grand Tack Hypothesis, after a sailing maneuver

In it, Jupiter starts out at 3.5 AU, Saturn at 4.5 AU, Uranus at 6 AU, and Neptune at 8 AU. Jupiter spirals in to about 1.5 AU in 100 thousand years (kyr), Saturn quickly follows at about 100 kyr, while Uranus and Neptune don't move very much.

Along with the giant planets are lots of planetesimals, small asteroid-like objects that condensed out of the solar nebula. From 0.3 to 3 AU are S-type (stony) ones, and from 3.5 to 13 AU are C-type (carbonaceous-chondrite) ones. The C-type ones contain water, from where they formed.

Jupiter and Saturn push the S-type objects together, while mixing up S-type and C-type ones as they go. Some S-type ones end up in the outer Solar System, while some C-type ones end up in the inner Solar System.

Then Jupiter and Saturn get locked in a 3:2 resonance, with Jupiter at 1.5 AU and Saturn at 2 AU, and their interactions with the protoplanetary disk push them outward. As they go outward, they push Uranus and Neptune outward as those planets get into resonances with them. They also leave behind the asteroid belt as they go.

Inside 3.5 AU, it's mostly S-type asteroids, while outside 3.5 AU, it's mostly C-type asteroids.

Mars ends up relatively small, since it does not have as much starting material as the Earth.

The C-type planetesimals supply water to the inner planets, making the Earth's oceans.

It's also a good setup for the Nice model of outer-planet migration. Saturn, Uranus, and Neptune keep going further out, and they scatter lots of planetesimals outward to form the Kuiper Belt. The Nice here is not the English word, but Nice, France, where the model was developed.

The origin of the giant planets is still not very well understood, it must be said.


For some somewhat technical background on planetary-system formation, check out Scott Tremaine's home page

formation of planets and planetoids is interesting also, I bet about as entertaining as watching pain dry.
anyways.
I have an inquiry.
have you seen any article about sugar compounds existing prior to the formation of stars?
I read an article on national geographic but it has since been removed.
my local astronomy club has a 30 inch telescope that weighs about 400 lbs.
I have more interest in stars than planets, and have seen the sun trough a telescope at about 11 am...
sun spots and flares, it was beautiful. boiling...
giving life to that which lives here on Earth.
anyways, I really enjoyed the post above and being able to "connect" with someone who has interest in observables.
I must say viewing the sun and thinking about gets me emotional on some level.

none wrote:have you seen any article about sugar compounds existing prior to the formation of stars?

You ought to try to find a page on that.

But there is a sugarlike molecule that's been detected in interstellar space: glycolaldehyde:
CH2OH - CHO

It's a 2-carbon one instead of the familiar 5-carbon and 6-carbon ones.

A rather sizable number of interstellar molecules has been detected: List of interstellar and circumstellar molecules

lpetrich wrote:

none wrote:have you seen any article about sugar compounds existing prior to the formation of stars?

You ought to try to find a page on that.

But there is a sugarlike molecule that's been detected in interstellar space: glycolaldehyde:
CH2OH - CHO

It's a 2-carbon one instead of the familiar 5-carbon and 6-carbon ones.

A rather sizable number of interstellar molecules has been detected: List of interstellar and circumstellar molecules

sweet
and the article I read on the national geographic website mentioned a specific sugar compound and used the worlds "possible life", but I said the article is gone and there is a 404 error in it's place...
also, I am happy that that the word interstellar is in your post.

I don't believe there was much carbon around to form sugars before there were stars.

I found the link is now operational...
maybe I typed the URL incorrectly...
http://news.nationalgeographic.com/news/2012/08/120829-sugar-space-planets-science-life/

I CoLTed that link: Sugar Found In Space: A Sign of Life?

Yes, that sugar is glycoaldehyde.

Calling it a sign of life is absurd, since that substance was likely produced by prebiotic chemistry. However, numerous molecules have been found near stars and in interstellar space, and many of them are organic. This shows that the successes of the Urey-Miller experiment and numerous similar ones are not something confined to those experiments.

lpetrich wrote:I CoLTed that link: Sugar Found In Space: A Sign of Life?

Yes, that sugar is glycoaldehyde.

Calling it a sign of life is absurd, since that substance was likely produced by prebiotic chemistry. However, numerous molecules have been found near stars and in interstellar space, and many of them are organic. This shows that the successes of the Urey-Miller experiment and numerous similar ones are not something confined to those experiments.

ok, I don't disagree.
Like I said I am more a of a stellar enthusiast, but anything that indicates life will get my attention especially astronomical observations relating to the potential or existence of life.