Cosmic Tango: The very strange orbit of this distant planet is reminiscent of a brutal 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 thoughts conjure up systems that mirror the solar system.
A planet that orbits its parent star in a nearly circular orbit, a rocky planet nearby and a Jupiter-like giant in the depths of the ice. 

However, the more we study the cosmos, the more we begin to realise planetary systems like our own might be more of an exception than a rule.

Imagine a system with one gaseous planet, a little larger than Saturn, skimming the surface of its host star on an extremely fast orbit. It’s hellishly H๏τ and glows a dull red, baking in stellar radiation.

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

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

Introducing the HD83443 planetary system
The story of the HD83443 system begins in the late 20th century, when astronomers began obsessively observing stars similar to the Sun. They were looking for evidence of those stars wobbling back and forth under the influence of unseen planetary companions.

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

But that’s where the similarities ended. HD83443b is a “H๏τ Jupiter”: a giant gas planet skimming the surface of its host star (which is a little smaller and cooler than the Sun), and completing each lap in less than three Earth days!

For two decades since its discovery, we have continued to monitor the HD83443’s movements. In recent years, we’ve been conducting 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 describe in a paper published last month.

This world, HD83443c, takes more than 22 years to orbit its host star, and is some 200 times more distant than its hellish sibling. Since HD83443c’s “year” is so long, we needed more than two decades of observations to confirm its existence – by tracking a single lap around its host star.

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

We have been continuously monitoring HD83443 for 20 years since its discovery. In recent years we have been conducting this research at the Mount Kent Observatory at the University of South Queensland. Combining our observations with others, we have discovered a strange new planet within our star system. We discussed this in an article published last month.

The planet HD83443c takes more than 22 years to orbit its parent star and is about 200 times farther away than its Hell sibling. HD83443c’s “year” is so long that it took more than 20 years of observations of her following a single orbit around its parent star to confirm its existence. But what’s really unusual is the eccentricity of its orbit. While planets in our solar system follow circular orbits, HD83443c follows a much longer orbit reminiscent of comets in our solar system.

If HD83443c were in the solar system, it would approach the Sun to near the orbit of Mars, then circle outwards to land between the orbits of Saturn and Uranus, before falling toward the Sun. Color code:
Purple = HD83443c, Green = Earth, Red = Mars, Blue = Jupiter, Yellow = Saturn.

Aftermath of Planet Tango
Planets like Jupiter HD83443b are of particular interest to astronomers because they differ from other nearby planets. Gas giants like Jupiter begin life far from their ice-rich parent stars.
This ice allows them to grow rapidly and gain enough mass to envelop themselves in a huge atmosphere. Unlike the solar system’s giant planets, HD83443b must have moved inward as it matured to get closer to its star. What caused this migration?
Well, over the years, astronomers have discovered many Jupiters. In an attempt to understand these strange planets, several mechanisms have been proposed to explain their migration, but for the most part evidence of what caused migration in the distant past has been lost. 

However, in the particular case of HD83443b, our new findings seem to provide important evidence. A newly discovered world, HD83443c, could be the reason her brother landed in the current hellish orbit.
Imagine HD83443c and HD83443b appearing first in the ice depth of the HD83443 system. They would be buried in a giant disk of gas and dust that surrounds the star, called the protoplanetary disk.

As the planet moved through the disk, it ate the disk and interacted with the surrounding disk, gradually gaining more mass and slowly drifting inward. After all, they got too close. They didn’t collide completely, but as they swung past each other, their massive gravity acted like a slingshot, sending them both into new orbits.

HD83443b, H๏τ Jupiter, dived inward into an orbit that grazes the star’s surface at its closest approach, then circled outward again toward the original near-miss scene. The other planet, HD83443c, is being ejected in its current extended orbit.

Over the millennia, something remarkable has happened. Whenever HD83443b hovers near its parent star, its presence raises the tides of this star, which in turn raises the tides of its parent star. This essentially makes the HD83443b “slow”.

This means that HD83443b slowed down a bit each time it passed its host star. When it flew out again, it could not fly as far as before, and its trajectory slowly became circular. You will be pulled inwards until you reach your current small circular orbit, where you will spend the rest of your life. However, the HD83443c did not meet that fate. After being knocked out during the first encounter with HD83443b, HD83443b remained far enough away from its star that its orbit was unaffected. Its very slow and extended orbit is evidence of a first planetary encounter when the system was young.

Don’t you have a place like home?
The story is fascinating, but the main goal of our ongoing otherworldly quest is to find a place closer to home. Using the same tools that led to HD83443c, we are finding planetary systems like ours, including giant planets orbiting far from their parent stars. For decades we may have to gaze into the distant stars and appreciate their graceful celestial waltz. We will undoubtedly discover many more amazing systems, such as HD83443, that will further reveal the true diversity of our planetary system. 

This video, by NASA, shows the story of the first 30 years of the Exoplanet Era, and the first 5,000 known exoplanets. Future research will hopefully reveal tens of thousands more – including systems like our Solar System.