Giant Eccentric Planets Can Have Habitable Companions, Creating Amazing Views

A comparison of how the giant planet HR5183b would look from a hypothetical habitable planet in the same system, compared to how Venus appears at its brightest. Teo Mocnik / UCR

Some Earth-like worlds may periodically experience an astronomical delight like nothing on Earth as a giant planet makes an approach closer than previously thought possible.

HR 5183 is a very Sun-like star a modest 103 light-years from Earth. Earlier this year it was found to have a planet at least three times Jupiter's mass with an elongated (which astronomers call eccentric) 75-year-long orbit. Such an object poses a threat to habitable worlds, as its gravity could throw them off their orbit or destroy them entirely.

Professor Stephen Kane of the University of California, Riverside, decided to test if HR 5183 made doomed Earth-like worlds entirely, modeling their prospects at a range of locations in the system. The simulations did indeed show many orbits are unsustainable, "but in certain parts of the planetary system, the gravitational effect of the giant planet is remarkably small enough to allow the Earth-like planet to remain in a stable orbit,” Kane said in a statement.

Most surprisingly, Kane and co-author California Institute of Technology graduate student Sarah Blunt report in The Astronomical Journal the location where a planet could maintain a stable orbit lies in the “habitable zone”, where liquid water, and potentially life, could exist.

We don't know if HR 5183 has an Earth-like planet, but unless Kane's simulation has an error, it's possible. Since there are almost certainly many other star systems in the galaxy with giant planets on similarly eccentric orbits, it's quite credible at least one has an inner planet that plays host to life.

If so, the inhabitants are treated to a regular visual feast. Every 75 years (the same time period as Halley's Comet) HR 5183b approaches its star. "When the giant is at its closest approach to the Earth-like planet, it would be 15 times brighter than Venus,” Kane noted. At its furthest, it would barely be visible to the naked eye under dark skies.

If the artists behind the flood of 1980s posters that included giant planets in the skies of imaginary worlds put much thought into the practicalities of the scenario they were probably imagining the view from a large moon. There is still much debate about whether such locations could host advanced life. On the other hand, the HR 5183 scenario shows a planet's skies can be dominated, if only for a few months, but a giant planet that comes close enough would have rings or moons visible to the naked eye.

HR 5183 isn't even the gas giant with the most eccentric orbit, a title that goes to the almost cometary HD 20782b, which Kane discovered three years ago from the flash of light emitted as it approaches its star.

How HR 5183's orbit would look if it was in our own Solar System. Andrew Howard/Caltech

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