We’re the Cosmic 1 Percent However Our Solar System Isn’t a Total Weirdo

We’re the Cosmic 1 Percent However Our Solar System Isn’t a Total Weirdo


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Posted By Sean Raymond on Jan 05, 2021

About half of all stars seem to have “super-Earth” planets on orbits closer to their stars than Mercury is to the Sun, but we do not. Illustration by Vadim Sadovski/ Shutterstock

I s Earth special? When a grand philosophical concern, it has, over the past 20 years, become, with the discovery of thousands of worlds around other stars– our cosmic cousins– a scientific one.

One method to resolve it is to picture aliens, utilizing contemporary Earth technology, searching our planetary system for exoplanets. Which of our eight worlds would they find? The answer is Jupiter and just Jupiter Our exoplanet-searching methods search for the effect of planets on their host star, either through a cyclical gravitational yank, or by occasionally blinking out some of the star’s light. Jupiter would just be detectable (in the meantime) through a decades-long radial speed study of the Sun. We might measure Jupiter’s approximate mass and orbit. The question then becomes: How common, among recognized exoplanets, are systems similar to the Sun-Jupiter system?

About 1 percent. Gas giants with masses similar to Jupiter’s are discovered around roughly 1 in 10 stars like the Sun; however, only about 1 in 10 of those planets has a “Jupiter-like” orbit, suggesting an orbit that is significantly larger than Earth’s and close to circular. Obviously, we still have no information in the worlds or Venuses or Saturns or Neptunes around other stars like the Sun. But it’s a start.

A second way to assess our solar system’s uniqueness comes from Nanna Bach-Møller and Uffe Jørgensen at the Center for Star and Planet Formation in Copenhagen, Denmark. They base their argument on a relation in between the variety of planets in a given system and the shapes of worlds’ orbits. The key piece of their analysis, published in a paper in October in the Regular Monthly Notifications of the Royal Astronomical Society, is “orbital eccentricity.” Kepler‘s laws of orbital movement tell us that worlds orbit their host stars following ellipses. The eccentricity of an ellipse is a procedure of how extended it is. An ellipse with no eccentricity is a circle; as the eccentricity approaches 1, the ellipse ends up being definitely extended.

Orbital motion of planets as a function of their eccentricity. This animation shows five different planets, each with the same typical distance from their star however with eccentricities of 0.0, 0.2, 0.4, 0.6, and 0.8. Phoenix777/ Wikimedia Commons

While a planet on a circular orbit moves at a constant speed around the star, worlds on eccentric orbits move much faster when they are better to the star. Researchers can find this change in orbital speed over the orbit for exoplanets using various methods.

Bach-Møller and Jørgensen began by collecting the full sample of exoplanets for which scientists have already measured or approximated orbital eccentricities. They then calculated an “typical eccentricity” for all of the worlds in each system. They discovered that systems with more worlds tend to have lower eccentricities.

This is not a big surprise. Researchers can search for exoplanets just within a limited variety of orbital distances. Envision a cleaning machine-sized cardboard box. How many smaller sized boxes can you fit inside that big box if you need to put each box inside another box (like Russian dolls)? The answer is that it depends on their shapes. If your boxes are all nice and square, then you may be able to fit a dozen or more inside each other. If even one box is a stretched-out guitar-shaped box, then fewer boxes will be able to fit inside the huge box.

It’s the same idea for orbits. Planets on circular orbits can be loaded much closer together than planets on stretched-out, eccentric orbits. And the more eccentric the orbits, the fewer can fit in the variety in orbital distance that we can look for exoplanets.

The scientists put numbers on orbits-inside-orbits. They found that the number of worlds in a given system, versus the average eccentricity, follows a smooth relation– with just one exception. Systems with a single planet are a little bit off. These systems may have begun with numerous worlds on near-circular orbits that, through cumulative gravitational kicks, changed their shapes up until they crossed. This would have caused close gravitational scattering events that introduced some worlds into interstellar space!

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With eight worlds on fairly circular orbits, our solar system fits the pattern. With this trend, the scientists utilize the occurrence-rate of exoplanets with various eccentricities to approximate how numerous systems have as lots of planets as ours.

Of course, considering that we have a total census of worlds (at least within Neptune’s orbit) the solar system’s “box” is much larger than the orbital range in which exoplanets can be found.

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