Astronomers spot universe’s most eccentric planet yet

Scientists have identified the most eccentric planet ever observed, careening round a star in the flattest orbit yet described.

An artist's rendering shows the planet HD 20782, the most eccentric planet ever known, passing its star in close orbit.

The most eccentric planet ever observed has been identified whipping round a star about 117 light-years from Earth.

The research, published February by a group of researchers from eight universities and five scientific institutions, also detected a flash of starlight reflected from the planet’s atmosphere as it made its closest orbital approach to its sun.

Planets in our own solar system orbit the sun in an almost circular fashion, but there are some whose orbits are far more elliptical – or “eccentric” – in nature, and this planet, HD 20782, has the most eccentric orbit ever seen.

“When we see a planet like this that is in an eccentric orbit, it can be really hard to try and explain how it got that way," said lead researcher Stephen Kane of San Francisco State University, in a press release. "It's kind of like looking at a murder scene, like those people who examine blood spatter patterns on the walls. You know something bad has happened, but you need to figure out what it was that caused it.”

The eccentricity of a planet’s orbit is measured on a scale of 0 to 1, with 0 representing a perfectly circular orbit, and figures closer to one indicative of increasingly elliptical orbits.

Earth’s orbital eccentricity, for example, is 0.017, and the most eccentric planet in our solar system – Mercury, assuming that we no longer class Pluto a planet – has an eccentricity of 0.205.

Our new friend, HD 20782, on the other hand, has an orbital eccentricity of 0.96, meaning its ellipse as it travels to and from its star is almost flat; and when it does finally return to its sun, after a 597-day orbital journey, it careens furiously round the star to slingshot back into space.

“It's around the mass of Jupiter, but it's swinging around its star like it's a comet,” said Dr. Kane.

Moreover, while the furthest point of its orbit takes this new planet 2.5 times as far from its star as the Earth lies from the sun, when it passes through its closest slingshot phase, it is a mere 0.06 of that Earth-sun distance, taking it far closer to its star than any planet in our solar system ever dares venture.

Which brings us to the second intriguing aspect of this research – the flash of starlight detected, bouncing from HD 20782’s atmosphere.

The reflectivity of a planet (how bright it appears in the sky) is partly determined by atmospheric composition: Venus and Jupiter, for example, shrouded in icy clouds, are very reflective. But if they were ever to venture closer to the sun, that ice would begin to melt.

So HD 20782 gives scientists the opportunity to study how a planet responds to the flash-frying effect of passing painfully close to a star, but for the briefest passage of time.

“The atmosphere of the planet doesn't have a chance to respond," said Kane. "The time it takes to swing around the star is so quick that there isn't time to remove all the icy materials that make the atmosphere so reflective.”

And what is the likeliest explanation for HD 20782’s eccentricity?

Perhaps there was originally another planet in the same system, and an unstable orbit caused a collision, or near-collision, ejecting one planet entirely and sending HD 20782 into its current pattern.

But HD 20782 finds itself in a binary star system, so the other possibility is the second star wandered just a little close for comfort and panicked HD 20782 off its original, more sedate, orbit.

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