Podcaster: Avivah Yamani
Title: Why Does The Solar System Have No Super-Earths?
Organization: 365 Days Of Astronomy ; langitselatan
Link : http://cosmoquest.org/x/365daysofastronomy; http://langitselatan.com
Source for this podcast:
New Research Suggests Solar System May Have Once Harbored Super-Earths http://www.caltech.edu/news/new-research-suggests-solar-system-may-have-once-harbored-super-earths-46017
Why Does Earth Have No Super-Earth Cousins? http://www.astrobio.net/news-exclusive/why-does-earth-have-no-super-earth-cousins/
Mengapa Tidak Ada Bumi Super di Tata Surya? http://langitselatan.com/2015/05/10/mengapa-tidak-ada-bumi-super-di-tata-surya/
Description: Today’s question Why Does The Solar System Have No Super-Earths?
Bio: Avivah is a project director of 365 Days Of Astronomy and astronomy communicator from Indonesia.
Today’s sponsor: This episode of “365 Days of Astronomy” is sponsored by — no one. We still need sponsors for many days in 2015, so please consider sponsoring a day or two. Just click on the “Donate” button on the lower left side of this webpage, or contact us at signup@365daysofastronomy.org.
Transcript:
Hi! Welcome to the Q&A of 365 Days of Astronomy. I’m Avivah your host today. Today’s question Why Does The Solar System Have No Super-Earths?
Let’s start with this. Up until 20 years ago, in 1995, our Solar system was the only example of a planetary system we knew about. There was no example we could use as a comparison. So… as the only model, astronomers then thought that if there was another planetary system or planet orbiting other stars then it would be similar to the Solar System. Which means, it would have rocky planets near the star and gas giant and ice giant planets located farther out from the parent star.
But the universe has its own mysteries… Well it’s a mystery because we don’t know the answer yet. It’s not a mystery that we can’t solve. All we need is to look for the answers using our current and ultimately future technologies.
In 1995, for the first time, a planet orbiting a solar-type star was discovered. And since then, our solar system is not the only system we know of and the Sun is not the only star we know that is harbouring planets. We also discovered that multiple stars can harbour planets as well. The extrasolar planets era has begun.
The first exoplanet was actually discovered in 1992 when a system of terrestrial-mass planets was announced to be present around the millisecond pulsar PSR B1257+12. Planets orbiting a pulsar! But the extrasolar planet era began in earnest when the first planet orbiting around a solar-type star was discovered in 1995. The discovery gave a new perspective to astronomers. The planet is located really close to its star, closer than Mercury to the Sun. Unlike the solar system, the planet was not a terrestrial planet but a hot gas giant! And as astronomers discovered more and more exoplanets, hot gas giants, later known as hot Jupiter planets became common. After that astronomers also discovered earth-size and possibly earth-like planets orbiting other stars.
One planet type is the so-called Super-Earth. The term stands for a planet with a mass greater than Earth’s, but substantially below the mass of the Solar System’s ice giants Uranus and Neptune, which are 15 and 17 Earth masses respectively. The term super-Earth refers only to the mass of the planet, and does not imply anything about the surface conditions such as temperature, composition, orbital properties, or habitability. The upper limit for super-Earths is 10 Earth masses.
Another definition was made by the Kepler Mission was to define Super-Earth planets as bigger than Earth-like planets (from 0.8 Earth-radii up to 1.25), but smaller than mini-Neptunes (from 2 Earth-radii up to 4).
Since the first super-Earth discovered around a main sequence star in 2005, astronomers found that around 30 to 50 percent of stars similar to the Sun have hot super-Earths. In addition to hot super-Earths, there are also ice super-Earths, a rocky planet with a thick atmosphere that formed far away from the star and which later migrated inward.
The question is, why is there no super-Earth in the solar system? Why the Earth has no hot super-Earth cousin is still debatable. But astronomers are trying to find the answer.
One big unknown is how hot super-Earths form in the first place, a mystery that has spawned at least two competing theories among astronomers.
The first theory suggests hot super-Earths grew very fast near their parent star because they had enough mass from huge disks of rocky debris. The scenario requires a very large amount of mass close to the star — something that conflicts with observations about dusty debris disks surrounding other stars, and the current understanding of disk physics.
The second theory suggests that super-Earths first formed farther out from their parent stars. That means the planets originally formed in a colder region beyond the so-called “frost line,” the point where water and other chemical compounds that contain hydrogen will condense into solid ice. By comparison, Earth probably formed inside the frost line and later received its water in the form of asteroids or other debris from beyond the frost line.
So again, how about super-Earths in the Solar System? We know there is no super-Earth today in our beloved solar system but how about in the past? It is not easy to traceback time but astronomers did run a simulation to find the answer.
There are 2 similar models to show why there is no super-Earth in Solar System and both theories include Jupiter. A Jupiter that migrated inward to the Sun.
The first model suggests that Jupiter and Saturn formed far faster than Earth even though they are hundreds of times more massive. The gas giants first formed 5 to 10 Earth mass cores and then gas gravitationally accreted on top. Exactly how the cores grew is not known. According to the grand tack model, Jupiter once migrated inward toward the Sun as far as the current orbit of Mars, before retreating back to the Outer Solar System.
Jupiter’s movement would have acted like a gravitational shovel in the early Solar System by pushing about half the dust and rocky material in front of it and scattering the rest behind it.
Jupiter’s quirky journey would help explain why Mars ended up with less planet-making material in its region and eventually ended up smaller than Earth or Venus. As Jupiter retreated back into the outer Solar System beyond the frost line, its “shovel” could have also tossed icy asteroids and other debris over its shoulder into the inner Solar System—perhaps delivering water to Earth and other planets in the process.
Jupiter’s visit to the Inner Solar System could also explain why Earth has no super-Earth cousins in its neighborhood. The fast-forming gas giants such as Jupiter can block super-Earths from migrating inward toward their parent stars. In the Solar System’s case, the planetary cores of Uranus, Neptune, and even Saturn might have become super-Earths if not for Jupiter.
The second model suggests that long before Mercury, Venus, Earth, and Mars formed, the inner solar system may have harbored a number of super-Earths — planets larger than Earth but smaller than Neptune. If so, those planets are long gone —broken up and fallen into the Sun billions of years ago largely due to a great inward-and-then-outward journey that Jupiter made early in the solar system’s history.
Jupiter’s inward-outward migration could have destroyed a first generation of planets and set the stage for the formation of the mass-depleted terrestrial planets that our solar system has today. This model also incorporates something known as the Grand Tack scenario. That scenario says that during the first few million years of the solar system’s lifetime, when planetary bodies were still embedded in a disk of gas and dust around a relatively young Sun, Jupiter became so massive and gravitationally influential that it was able to clear a gap in the disk. And as the Sun pulled the disk’s gas in toward itself, Jupiter also began drifting inward, as though carried on a giant conveyor belt.
Jupiter would have continued on that belt, eventually being dumped onto the Sun if not for Saturn. Saturn formed after Jupiter but got pulled toward the Sun at a faster rate, allowing it to catch up. Once the two massive planets got close enough, they locked into a special kind of relationship called an orbital resonance which allowed the two planets to open up a mutual gap in the disk, and they started playing this game where they traded angular momentum and energy with one another, almost to a beat. Eventually, that back and forth would have caused all of the gas between the two worlds to be pushed out, a situation that would have reversed the planets’ migration direction and sent them back outward in the solar system.
The simulation shows that as Jupiter moved inward, it pulled all the planetesimals it encountered along the way into orbital resonances and carried them toward the Sun. But as those planetesimals got closer to the Sun, their orbits also became elliptical. Those new, more elongated orbits caused the planetesimals, mostly on the order of 100 kilometers in radius, to sweep through previously unpenetrated regions of the disk, setting off a cascade of collisions among the debris. During this period, every planetesimal would have collided with another object at least once every 200 years, violently breaking them apart and sending them decaying into the Sun at an increased rate. The model predicts that the super-Earths would have been shepherded into the Sun by a decaying avalanche of planetesimals over a period of 20,000 years.
When Jupiter tacked around, some fraction of the planetesimals it was carrying with it would have calmed back down into circular orbits. Only about 10 percent of the material Jupiter swept up would need to be left behind to account for the mass that now makes up Mercury, Venus, Earth, and Mars. And it would take millions of years to form the terrestrial planets.
Thank you for listening. This is 365 Days Of Astronomy
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365 Days of Astronomy
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