Cosmic tango: this extremely strange planet's orbit points to a violent and chaotic past

If you close your eyes and imagine a planetary system orbiting a distant star, what do you see?

For most people, such thinking gives rise to systems that mirror the Solar System: planets orbiting their host stars in nearly circular orbits – rocky planets are closer, and giants like Jupiter in the icy depths.

However, the more we study the cosmos, the more we begin to realize that planetary systems like ours may be more the exception than the rule.

Imagine a system with one gas planet, slightly larger than Saturn, tracing the surface of its parent star in a very fast orbit. It is extremely hot and glows a dull red, baked in stellar radiation.

Then imagine another, more distant giant planet, larger than Jupiter, moving on a distant and highly elongated orbit that makes it look more like a comet than a traditional planet.

It doesn’t sound like home, does it? But that’s what we found.

Introducing planetary system HD83443

The story of the HD83443 system begins in the late 20th century, when astronomers began to obsessively observe stars similar to the Sun. They are looking for evidence of stars wobbling under the influence of an unseen planetary companion.

Using the 3.9-meter Anglo-Australian Telescope at Siding Spring Observatory near Coonabarabran, the researchers discovered a planet orbiting the star HD83443. This planet, HD83443b, is as large as the gas giants Saturn and Jupiter.

But that’s where the similarities end. HD83443b is a “hot Jupiter”: a gas giant planet that glides across the surface of its host star (which is slightly smaller and cooler than the Sun), and completes each rotation in less than three Earth days!

For the two decades since its discovery, we have continuously monitored the movement of HD83443. In recent years, we have done this work at the University of Southern Queensland’s Mt Kent Observatory.

By combining our observations with others, we discovered a strange new planet in the system, which we described in a paper published last month.

This world, HD83443c, takes more than 22 years to orbit its parent star, and is about 200 times farther than its evil sibling. Because HD83443c’s “years” are so long, it took us more than two decades of observations to confirm its existence – tracking one loop around its parent star.

But what’s really unusual is the eccentricity of its orbit. While the planets in the Solar System follow nearly circular orbits, HD83443c follows a much more elongated path like the comets in our Solar System.

The result of planetary tango

Planets like “hot Jupiter”, HD83443b, are of great interest to astronomers because they are unlike anything close to home. Gas giants like Jupiter begin their lives far from their parent stars where ice is abundant.

The ice allowed them to grow rapidly, gaining enough mass to envelop themselves in the enormous atmosphere.

Unlike the Solar System’s giant planets, when HD83443b grew to maturity, it must have migrated inward to end up close to its parent star. What caused this migration?

Well, over the years, astronomers have discovered many hot Jupiters. In trying to understand the strange planets, several mechanisms have been proposed to explain their migration – but in many cases, evidence for the causes of such migrations is lost in the past.

However, in the specific case of HD83443b, it seems that our new discovery may have provided evidence of firearms. The newly discovered world, HD83443c, may be the reason its sibling ended up in hellish orbit this time.

Imagine HD83443c and HD83443b first forming in the ice depths of the HD83443 system. They will be buried in a large disk of gas and dust surrounding the star, which is called the “protoplanetary disk”.

As the planets move through the disk, they eat from it, grow larger, and slowly drift inward as they interact with the disk around them.

Eventually they came too close together. They didn’t actually collide, but when they swung past each other, their immense gravitational pull acted like a catapult, catapulting them both into new orbits.

HD83443b, a hot Jupiter, was hurled inward into a gliding orbit toward the star’s surface on its closest approach, before swinging back outward toward the initial site of the near collision. Another planet, HD83443c, was flung out into its current elongated path.

Over thousands of years, something extraordinary happened. Each time HD83443b swings closer to its parent star, its presence increases the tides on the star, and in turn the parent star causes the tides to rise above it. This will essentially “apply the brakes” to the HD83443b’s motion.

This means HD83443b loses a bit of speed each time it swings past its parent star. As it flew back outside again, it failed to travel as far as before and its orbit slowly coiled. He was dragged inside until he reached his current little circular orbit – where he would spend the rest of his life.

The HD83443c, however, did not suffer such a fate. After being flung outward during its initial encounter with HD83443b, it remains so far from the central star that its orbit is never affected.

Its extremely slow, elongated orbit is evidence of the planet’s early encounters since the system was young.

Is there no place like home?

The story is interesting – but the main goal of our ongoing search for alien worlds is to find a place that is more like home.

We used the same tools that brought us to HD83443c to find planetary systems like ours – with giant planets in orbits far from their parent stars. We may need to gaze at the stars in the distance for decades, watching their graceful heavenly waltzes.

We’re sure to find more surprising systems similar to HD83443, which reveal more about the true diversity of planetary systems out there.

This article is republished from The Conversation, the world’s leading publisher of research-based news and analysis. A unique collaboration between academics and journalists. It was written by: Adriana Errico, South Queensland University; Brad Carter, South Queensland Universityand Jonti Horner, South Queensland University.

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Brad Carter receives funding from the Australian Research Council.

Jonti Horner received funding from the Australian Research Council in 2016 to help build Minerva-Australis, the University of Southern Queensland exoplanet detection suite used to detect HD83443 c.

Adriana Errico does not work for, consult with, own stock in or receive funding from any company or organization that would benefit from this article, and does not disclose any relevant affiliations outside of their academic designation.

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