15.2: Planetary Populations

The Configurations of Other Planetary Systems

Let’s look more closely at the progress in the detection of exoplanets. Figure 14.17 shows the planets that were discovered each year by the two techniques we discussed. In the early years of exoplanet discovery, most of the planets were similar in mass to Jupiter. This is because, as mentioned above, the most massive planets were easiest to detect. In more recent years, planets smaller than Neptune and even close to the size of Earth have been detected.

Figure 14.17 : Masses of Exoplanets Discovered by Year. Horizontal lines are drawn to reference the masses of Jupiter, Saturn, Neptune, and Earth. The gray dots indicate planets discovered by measuring the radial velocity of the star, and the red dots are for planets that transit their stars. In the early years, the only planets that could be detected were similar in mass to Jupiter. Improvements in technology and observing strategies enabled the detection of lower mass planets as time went on, and now even smaller worlds are being found. (Note that this tally ends in 2014, but the trend should be clear.)

We also know that many exoplanets are in multiplanet systems. This is one characteristic that our solar system shares with exosystems. Looking back at Figure 14.15 and seeing how such large disks can give rise to more than one center of condensation, it is not too surprising that multiplanet systems are a typical outcome of planet formation. Astronomers have tried to measure whether multiple planet systems all lie in the same plane using astrometry. This is a difficult measurement to make with current technology, but it is an important measurement that could help us understand the origin and evolution of planetary systems.

Comparison between Theory and Data

Many of the planetary systems discovered so far do not resemble our own solar system. Consequently, we have had to reassess some aspects of the “standard models” for the formation of planetary systems. Science sometimes works in this way, with new data contradicting our expectations. The press often talks about a scientist making experiments to “confirm” a theory. Indeed, it is comforting when new data support a hypothesis or theory and increase our confidence in an earlier result. But the most exciting and productive moments in science often come when new data don’t support existing theories, forcing scientists to rethink their position and develop new and deeper insights into the way nature works.

Nothing about the new planetary systems contradicts the basic idea that planets form from the aggregation (clumping) of material within circumstellar disks. However, the existence of “hot Jupiters”—planets of jovian mass that are closer to their stars than the orbit of Mercury—poses the biggest problem. As far as we know, a giant planet cannot be formed without the condensation of water ice, and water ice is not stable so close to the heat of a star. It seems likely that all the giant planets, “hot” or “normal,” formed at a distance of several astronomical units from the star, but we now see that they did not necessarily stay there. This discovery has led to a revision in our understanding of planet formation that now includes “planet migrations” within the protoplanetary disk, or later gravitational encounters between sibling planets that scatter one of the planets inward.

Many exoplanets have large orbital eccentricity (recall this means the orbits are not circular). High eccentricities were not expected for planets that form in a disk. This discovery provides further support for the scattering of planets when they interact gravitationally. When planets change each other’s motions, their orbits could become much more eccentric than the ones with which they began.

There are several suggestions for ways migration might have occurred. Most involve interactions between the giant planets and the remnant material in the circumstellar disk from which they formed. These interactions would have taken place when the system was very young, while material still remained in the disk. In such cases, the planet travels at a faster velocity than the gas and dust and feels a kind of “headwind” (or friction) that causes it to lose energy and spiral inward. It is still unclear how the spiraling planet stops before it plunges into the star. Our best guess is that this plunge into the star is the fate for many protoplanets; however, clearly some migrating planets can stop their inward motions and escape this destruction, since we find hot Jupiters in many mature planetary systems.

Are Planets Rare?

How rare are these stellar systems? We do not know yet… but many stellar systems are not made up of just one star. Binary Stars are systems in which physically associated star systems are made up of two stars. It appears that about half of all stars are binary star systems. And Multiple Star Systems have more than two stars. We have found and observed numerous multiple star systems. One such system we see in Scorpius – called β Scorpii – has at least 7 stars. Binary and multiple star systems present challenges for intelligent life.

Sample Planets

TrES-4 was discovered when looking for transiting planets. It is about 70 percent bigger than Jupiter but less massive. TrES-4 has the density of approximately 0. 2 grams per cubic centimeter, or that of balsa wood. The Gliese 581 System is a red dwarf star with a mass about 1/3 that of the Sun. It is located 20. 4 light years from Earth and is among the 100 closest stars to Earth. Gliese 581 has at least four planets:

• HD80606b is a strange planet about four times the mass of Jupiter. It comes so close to its star, at its perihelion point HD80606b would have a surface temperature of 1,200oC.

  Then there is the strange planet, Wasp-12b. It takes this planet 1. 1 earth days to go around its star one time; in other words, you would celebrate your birthday every 26. 4 hours. Wasp-12b’s surface temperature is estimated to be 1,500oC, and it is slowly being eaten by its star.

  Stellar System Stars and Life

  Many biologists and astronomers believe that you have to have the right type of star, as well as the right type of planet, and the right distance from its star for intelligent life.

Brown Dwarfs

 

These images show Brown Dwarf Gliese 229B, orbiting its star Gliese 229. Gliese 229B is in the white circle on each image. The left image was the discovery image, taken with the Palomar Telescope in California. The right image was taken with the Hubble Space Telescope. Public Domain | Image courtesy of NASA.

 

The extrasolar planets tell us a lot about our own solar system’s formation and development. Yet, there are some significant issues at this point. First, there appears to be numerous exoplanets much larger than Jupiter. Why is this so? And many of these extrasolar planets orbit much closer to their stars than our Gas Giants, which are much further from the Sun. The thinking here is that these Gas Giants “migrated” closer to their stars during the development of their stellar systems.

Brown Dwarfs are sub-stellar objects with a mass below that necessary to maintain hydrogen nuclear fusion reactions in their cores, as do stars. Sizes of these brown dwarfs – up to 80 Jupiter masses – range from very large gas giant planets to just below the mass necessary to “turn on” as a star. Are brown dwarfs’ planets? Stars? Failed stars? Two extrasolar planets have been discovered orbiting brown dwarfs.

 

 

Attributions

https://phys.libretexts.org/Bookshel...asolar_Planets

https://phys.libretexts.org/Bookshel...ing_Exoplanets

https://phys.libretexts.org/Bookshel...asolar_Planets

https://phys.libretexts.org/Bookshel...netary_Systems