On Planets Far Beyond Us

As many people are looking to the impending holiday and a New Year, we are going to look much further: past our solar system. Today we are looking at exoplanets–planets not in our own solar system.
As of December 17th, 2015, the number of confirmed exoplanets is 1,916, with 5,619 as the number of possible exoplanet discoveries. This of course leads to many questions: what are the properties of these planets? Their mass and size, properties of their orbit, and compositions?
That is where today’s article comes in. “Observed Properties of Extrasolar Planets” by Andrew Howard was published in Science in 2013. ‘Extrasolar planet’ is the more official way of saying ‘exoplanet’. Focusing on smaller planets orbiting within 1 AU of their host and gas giants several AU out, this goes through some of those properties. An AU is an ‘astronomical unit’, the distance between the Earth and the Sun.
There are several ways to find exoplanets, but two are the most common. These are radial velocity and photometry.
Radial velocity uses the Doppler effect. The same phenomena as the pitch of a police siren changing as it approaches and passes you, works for light as well as sound. By looking closely at the change in the frequency of light from a distant star, the presence of a planet becomes observable in the pattern of movement.
Photometry is the measurement of light. If you can see a planet transit across its host star from your point of view, the light from the host star will temporarily dim. This dip in the amount of light, while rather small, is observable. Over 1,000 exoplanets have been found this way using the Kepler space telescope, launched in 2009.
One of the wonderfully amusing posters for PlanetQuest's 'Exoplanet Travel Bureau'. Image courtesy of NASA/JPL-Caltech.
One of the wonderfully amusing posters for PlanetQuest’s ‘Exoplanet Travel Bureau’. Image courtesy of NASA/JPL-Caltech.
So, now that we’ve found the planets, what can we conclude about them? Radial velocity and photometry will yield different information; together, we can find the length of a year for the exoplanet, its mass, radius, and a guess at its density. Taking all that information together, we can find some interesting patterns.
Small planets, measuring either in mass or in radius, are more common than larger ones. Large planets are easy to find through radial velocity, as their gravitational pull is larger, and thus the ‘wobble’ of their star, and its Doppler effect, is greater. Planets close to their stars can be easier to find for this reason (closer objects have a greater gravitational pull than distant ones), and also because they transit more often, making them easier to see with photometry.
Here are some notable occurrences:
  • Planets with a radius between that of Earth and Neptune are fairly common in other solar systems. Around fifteen percent of sun-like stars host a planet between three and thirty Earth masses (Neptune, for reference, is about 17 times that of Earth), and fourteen percent with one to three Earth masses, within 0.25 AU.
  • About half of sun-like stars have at least one planet with their year less than 100 days, regardless of the mass of the planet.
  • A high mass does not necessarily mean a large radius. It depends on the composition of that planet. You can guess at the composition if you have the radius and the mass, using a density calculation. Sometimes more than one explanation of planet composition is possible.
  • “Hot Jupiters” are gas giant planets that orbit close to their star, with their year being on;y around ten days. Often, these are the only planets seen in a solar system. They may have formed farther from their stars, then ‘migrated’ closer in.
  • Exoplanets larger than two Earth radii have more circular orbits (lower eccentricity).
  • The current theory of planet formation is that they accumulate from the dust, ice, and gas circling a young star. Therefore, the planets should be all in the same plane, as the disk is relatively flat. As most multiplanet systems have their planets only slightly tilted in relation to the direction of their host star’s spin, this theory is supported.
There are still some things we need to work through. Current models for solar system formation are good for gas giants, but often the results for low-mass planets not match observed data. Some of these models also show planet-less spaces at distances where planets are often found orbiting. We obviously still have some work to do, but we are well on our way. The new year will bring more exoplanets to our attention, and time will tell what discoveries they will reveal.

 

 

Bibliography
Howard, Andrew W. “Observed properties of extrasolar planets.” Science 340.6132 (2013): 572-576.
Today’s main article.
National Aeronautics and Space Administration. KeplerNASA. Web.
NASA’s page for the Kepler spacecraft mission.
PlanetQuest. Exoplanet Travel Bureau. CalTech, NASA, JPL. Web.
The PlanetQuest page from which I took the image used in this post.
PlanetQuest. New Worlds Atlas. CalTech, NASA, JPL. Web.
Run by CalTech, NASA and JPL, the PlanetQuest website hosts much information about exoplanets and the search for them. This page displays the current count of exoplanets.

 

 

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