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5 mins read 02 Mar 2020

‘Pre-Teen’ Exoplanet offers UNSW researchers opportunity to test theories

UNSW researchers seize opportunity to study a rare, extremely young, recently formed exoplanet developing in a binary star system.

Illustration of a disc with dust and gas rings circulating a central star, which is the illuminating source
Newly formed solar systems – where young stars are surrounded by orbiting rings of gas, dust and materials. It is in these rings that planets form. Credit: NASA/JPL.

Researchers from the University of NSW (UNSW) have been provided the opportunity to study a new planetary system, as it starts to take shape. The recently formed exoplanet – only 1 percent the age of our Sun, forms part of a binary star system located 150 light-years away.

The findings have excited astrophysicists as it could help answer questions about early-stage planetary system formation, such as – where gas giant planets form, their migration and why our solar system is unique in its design.

The opportunity lies with the age of the system, which exists prior to external influences, like the gravitational pull of the secondary star, which would impact the location of the exoplanet or its tilt relative to the equatorial spin plane of the parent star.

“We expected the pull from the second star to tilt the rotating disk of gas and dust that once surrounded the main star – a process that would skew the orbit of the planet,” says Dr. Benjamin Montet, Scientia Fellow at UNSW Sydney and lead author of the study. 

“Surprisingly, we found no evidence the planet’s orbit was impacted. We also found the planet formed through relatively calm processes – which means it could be possible for Earth-like planets to survive in binary systems like this.”

Dr. Montet worked with an international team of researchers at the Magellan Telescopes located at Las Campanas Observatory in Chile. They used the Planet Finder Spectrograph to measure the Rossiter-McLaughlin effect, which is the relative angle between the orbit of the planet and the spin of its star.

DS Tuc Binary System

Multiple images of DS Tuc Binary System in different wavelengths from different surveys.
The DS Binary Star system captured across the 2MASS, DSS and GALEX surveys. Credit: Survey images.

DS Tucanae, also known as HD 222259, is a binary star system in the constellation Tucana – notable in southern hemisphere skies. The two stars, one similar to our Sun, classified as a class GV6 and the other a K3V class, form a visual binary – where Earth-based telescopes can distinguish the two gravitationally bound stellar objects apart.

In 2019, NASA’s orbiting exo-planet hunting spacecraft TESS (Transiting Exoplanet Survey Satellite – an all-sky survey that looks for minute dips in the light-curves of thousands of stars to identify planets which block out some light), identified a “Super-Neptune” planet orbiting the main star with a period of 8 days. Scientists have called this planet DS Tuc Ab, as it orbits many times closer than Mercury does to our Sun, and has a radius just under 6 times that of Earth (with a mass of approximately 1.3 times that of Jupiter).

“The first exoplanet searches were done in facilities in the Northern Hemisphere, and so they missed a lot of the planets far south,” says Dr. Montet.

“NASA’s TESS mission is changing that. It’s finding all these planets around stars that previously hadn’t been searched.”

Sharing the Plane

Illustration showing a blue planet close to viewer and off in the distance, a luminous orange star.
Artist impression of DS Tuc Ab, orbiting its host star. Credit: Shutterstock.

The discovery of the planet DS Tuc Ab orbits its star in a relatively flat plane, at approximately 12 degrees incline from the star’s rotational axis has astrophysicists excited. This low incline – called obliquity – suggests that the pull from the companion star did not significantly tilt the orbit of the protoplanetary disk where DS Tuc Ab formed. 

While planets in the solar system all have a low obliquity, it’s unusual for planets like DS Tuc Ab.

“Most similar planets orbit their star at random angles, sometimes reaching up to 90 degrees above the axis of their star,” Dr Montet says. 

“The DS Tuc system is the first piece of evidence that higher orbital angles don’t get defined early on in a star’s life – they are an effect that happens only later on.”

At 40 million years old, the gas giant DS Tuc Ab is considered a ‘pre-teen’ in planetary years. There are fewer than ten planets we know about that are this young. 

Its age is a unique chance for astrophysicists to study a system in development before external influences interfere.

“To find out how long planetary systems last, we need systems that are too young to go through dynamical interactions, but old enough to have formed planets. The DS Tuc system is exactly in that niche,” Dr. Montet says. 

The exoplanet’s close proximity to its host star is also of great interest to astrophysicists, as it helps uncover how planets migrate and change the dynamics of entire systems.

Even the smallest and closest planet to our Sun, Mercury, takes almost 100 days to complete its orbit. Our closest gas planet, Jupiter, takes over 4300 days.

Giant gas planets are unlikely to develop close to their stars. The current understanding is that they form further away and, over time, a force causes them to move closer to their stars. 

Scientists want to know what that force is. 

“There are two main theories about how Hot Neptunes came to be so close to their stars,” says Dr. Montet.

“One theory is that an external force – potentially a multi-body nearby collision – ‘kicks’ them closer in, where they wobble and eventually settle on a new orbit.

“Another theory is that smooth processes within the planetary disk create a force that gradually pulls the planet closer to the star.” 

Testing the obliquity can help scientists uncover which force was at play. Planets with low obliquities are understood to be formed by smooth disk processes, while more dramatic processes will lead to random or high obliquities.

The paper is published in The Astronomical Journal