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Exoplanet has stable axial tilt, like Earth – but Earth has help from the moon and other planets

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artist’s depiction of Kepler-186f/NASA Ames, JPL-Caltech, T. Pyle

The reality that earth is fine-tuned for life screams out at us from stories that seem to make a point of not emphasizing the fact. From ScienceDaily:

The researchers suggest that Kepler-186f’s axial tilt is very stable, much like the Earth, making it likely that it has regular seasons and a stable climate. The Georgia Tech team thinks the same is true for Kepler-62f, a super-Earth-sized planet orbiting around a star about 1,200 light-years away from us.

How important is axial tilt for climate? Large variability in axial tilt could be a key reason why Mars transformed from a watery landscape billions of years ago to today’s barren desert.

“Mars is in the habitable zone in our solar system, but its axial tilt has been very unstable — varying from zero to 60 degrees,” said Georgia Tech Assistant Professor Gongjie Li, who led the study together with graduate student Yutong Shan from the Harvard-Smithsonian Center for Astrophysics. “That instability probably contributed to the decay of the Martian atmosphere and the evaporation of surface water.”

As a comparison, Earth’s axial tilt oscillates more mildly — between 22.1 and 24.5 degrees, going from one extreme to the other every 10,000 or so years.

That said …

Kepler-186f is less than 10 percent larger in radius than Earth, but its mass, composition and density remain a mystery. It orbits its host star every 130 days. According to NASA, the brightness of that star at high noon, while standing on 186f, would appear as bright as the sun just before sunset here on Earth. Kepler-186f is located in the constellation Cygnus as part of a five-planet star system.

Kepler-62f was the most Earth-like exoplanet until scientists noticed 186f in 2014. It’s about 40 percent larger than our planet and is likely a terrestrial or ocean-covered world. It’s in the constellation Lyra and is the outermost planet among five exoplanets orbiting a single star.

That’s not to say either exoplanet has water, let alone life. But both are relatively good candidates. Paper. (paywall) – Yutong Shan, Gongjie Li. Obliquity Variations of Habitable Zone Planets Kepler-62f and Kepler-186f. The Astronomical Journal, 2018; 155 (6): 237 DOI: 10.3847/1538-3881/aabfd1 More.

In short, of dozens of criteria, these exoplanets meet one or two. In the same release, we also read,

Mars and Earth interact strongly with each other, as well as with Mercury and Venus. As a result, by themselves, their spin axes would precess with the same rate as the orbital oscillation, which may cause large variations in their axial tilt. Fortunately, the moon keeps Earth’s variations in check. The moon increases our planet’s spin axis precession rate and makes it differ from the orbital oscillation rate. Mars, on the other hand, doesn’t have a large enough satellite to stabilize its axial tilt. “It appears that both exoplanets are very different from Mars and the Earth because they have a weaker connection with their sibling planets,” said Li, a faculty member in the School of Physics. “We don’t know whether they possess moons, but our calculations show that even without satellites, the spin axes of Kepler-186f and 62f would have remained constant over tens of millions of years.”

So Earth has lots of axial tilt help that we don’t know that these planets have.

See also: Erth slows the deadly solar wind to a gentle breeze

From Universe Today: Do icy worlds have enough chemicals to support life?

Exoplanets break apparent rules for planet formation

Water forms superionic ice, a “new” metal-like state with H+ ions as charge carriers (kairosfocus)

and

Tour the exoplanets – virtually – courtesy of NASA

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