Astronomers find a planet, named Halla, that shouldn’t exist

Picture this: our Sun, a peaceful, life-sustaining star, suddenly swelling up to 100 times its current size, wrapping the Earth in a fiery embrace. This is what should have happened to a planet named Halla.

This isn’t just science fiction, but a future reality. Not only our Earth, but many other planets in distant galaxies also await this fiery end. As with our Sun, most of their stars are inching towards their inevitable death.

But, all isn’t gloom and doom! A team of astronomers from the University of Hawaiʻi Institute for Astronomy (UH IfA), led by Marc Hon, a NASA Hubble Fellow, have made an astonishing discovery.

A planet named Halla escaped a fiery star named Baekdu

They found a planet that has dodged this lethal outcome. It is a Jupiter-like planet named Halla.

Halla orbits Baekdu, a red giant star. It orbits at a distance that’s half of what separates the Earth from our Sun.

Utilizing the facilities of the W. M. Keck Observatory and Canada-France-Hawaiʻi Telescope (CFHT) located on Hawaiʻi Island, the scientists learned that despite the star’s dramatic evolution, Halla continues to exist.

This discovery was made possible through the analysis of data gathered from NASA’s Transiting Exoplanet Survey Satellite (TESS).

Baekdu enters the red giant phase

Observing the oscillations within Baekdu, they noted that the star is currently burning helium in its core. This is an indication that it had once inflated massively into a red giant. This happened immediately before returning to its present size.

In its red giant phase, Baekdu would have expanded to 1.5 times the distance of Halla’s orbit. This should have seemingly doomed the planet to a fiery demise before shrinking to its current size. It is now just a tenth of that distance.

The details of this intriguing study can be found in the latest issue of the journal Nature.

Marc Hon, the leader of the study, commented, “Planetary engulfment usually spells disaster for the planet, the star, or both. Yet, Halla has managed to hold its ground near a star that should have swallowed it. This makes Halla a remarkable survivor.”

Confirming that Halla survived Baekdu’s storm

The planet Halla was first discovered back in 2015 by a team of Korean astronomers. They used the radial velocity method. This technique observes the movement of a star caused by the gravitational tug from its orbiting planet.

Building on this discovery, the UH IfA team carried out further observations from 2021 to 2022. They used the High-Resolution Echelle Spectrometer (HIRES) at the Keck Observatory and the ESPaDOnS instrument at CFHT.

They confirmed that Halla’s near-perfect circular orbit, taking 93 days to complete, had remained stable for more than a decade.

Daniel Huber, an IfA astronomer and the study’s second author, shared, “These observations confirmed the planet’s existence. The question is: how did the planet survive?”

He further stressed, “The data from various telescopes on Maunakea was crucial in understanding this.”

Being 0.46 astronomical units (AU, the distance between the Earth and the Sun) away from its star, Halla is categorized as a ‘warm’ or ‘hot’ Jupiter-like planet. These types of planets are believed to have started on larger orbits. They then gradually shift closer to their stars.

However, with a rapidly evolving star like Baekdu, survival for Halla through this path would be extremely unlikely.

So, what saved Halla?

One possibility is that Baekdu was originally not one, but two stars. This scene would be similar to Tatooine from Star Wars. The merger of these two stars might have prevented any single star from growing large enough to engulf the planet.

Another theory posits that Halla is a newborn, a ‘second generation’ planet. It may have been born from the gas cloud resulting from the violent collision between the two stars.

“Given that most stars exist in binary systems, we can’t rule out the possibility of planets forming around them. It’s quite possible that we could find more planets around highly evolved stars, thanks to these binary interactions,” explained Hon.

Whatever the truth may be, the discovery of Halla challenges our understanding of how planets and stars evolve. As Earthlings, this gives us a glimmer of hope amidst the fiery fates that await many worlds, including ours.

More about the life cycle of stars

Stars lead fascinating lives, and their life cycles are an amazing spectacle. Let’s start from the very beginning.

First, a star is born within a vast cloud of gas and dust, known as a nebula. Gravity pulls these particles together.

Being a protostar

This causes the cloud to collapse under its own weight. As it contracts, the center of the cloud heats up, forming a protostar.

As the protostar continues to accumulate mass from the surrounding cloud, its core temperature rises. When it becomes hot enough, nuclear fusion begins. This marks the birth of a new star.

Hydrogen atoms start fusing together to create helium, releasing a huge amount of energy. This stage of a star’s life is known as the “main sequence” stage. This is the stage where our Sun currently resides, lasting for several billion years.

What happens next?

Eventually, the star exhausts its hydrogen fuel. What happens next depends on the star’s mass. For a star like our Sun, and like Baekdu, it starts burning helium and expands into a red giant. At this stage, the star’s outer layers grow so large that they can engulf nearby planets.

After the red giant phase, the star sheds its outer layers into space. This creates a beautiful cloud of gas known as a planetary nebula.

What’s left behind is the hot, dense core, or a white dwarf. Over billions of years, this white dwarf cools down and fades away.

Larger stars die more dramatically

For stars larger than our Sun, the process is more intense. After they exhaust their fuel, they too become red giants.

However, their cores are hot and dense enough to fuse heavier elements, all the way up to iron. Eventually, these massive stars explode in a supernova. These are powerful blasts that can outshine an entire galaxy.

The supernova explosion leaves behind either a neutron star, which is incredibly dense, or a black hole if the original star was very massive. These remnants signify the end of the star’s lifecycle.

The material expelled in the supernova eventually finds its way into new nebulae, ready to start the star formation process again. It’s a beautiful cosmic cycle of life, death, and rebirth.