A planet that shouldn’t exist baffles astronomers

The search for planets outside our solar system and exoplanets is one of the fastest growing fields in astronomy. More than 5,000 exoplanets have been detected in recent decades, and astronomers now estimate that on average, there is at least one planet per star in our galaxy.

Many current research efforts aim to detect Earth-like planets suitable for life. These efforts focus on so-called main-sequence stars like our sun stars that are powered by the fusion of hydrogen atoms into helium in their cores and remain stable for billions of years. More than 90 percent of all exoplanets known to date have been detected around main-sequence stars.

As part of an international team of astronomers, we studied a star that will look a lot like our sun billions of years from now and found that it has a planet that it was, for all intents and purposes, meant to devour. In research published this week in Naturewe expose the puzzle of the existence of this planet and propose some possible solutions.

A glimpse into our future: the giant red stars

Just like humans, stars undergo changes as they age. Once a star has used up all of its hydrogen in the core, the star’s core shrinks and the outer envelope expands as the star cools.

In this stage of red giant evolution, stars can grow to over 100 times their original size. When this happens to our sun, in about five billion years, we expect it to become large enough to swallow Mercury, Venus and possibly the Earth.

Eventually, the core becomes hot enough for the star to begin fusing helium. In this phase the star shrinks to about 10 times its original size and continues to burn steadily for tens of millions of years.

We know of hundreds of planets orbiting red giant stars. One of these is called 8 Ursae Minoris b, a planet with about the mass of Jupiter in an orbit that keeps it only about half the distance from its star as Earth is from the sun.

The planet was discovered in 2015 by a team of Korean astronomers using the Doppler wobble technique, which measures the planet’s gravitational pull on the star. In 2019, the International Astronomical Union dubbed the star Baekdu and the planet Halla, after the highest mountains on the Korean peninsula.

A planet that shouldn’t be there

Analysis of new data on Baekdu collected by NASA’s Transiting Exoplanet Survey Satellite (TESS) space telescope has yielded a startling discovery. Unlike other red giants we’ve discovered harbor exoplanets in close orbits, Baekdu has already begun fusing helium in its core.

Using the techniques of astroseismology, which studies the waves inside stars, we can determine what material a star is burning up. For Baekdu, the wave frequencies unambiguously showed that it has begun to burn helium in its core.

The discovery was staggering: if Baekdu is burning helium, it should have been much bigger in the past, so big that it swallowed the planet Halla. How is it possible that Halla survived?

As often happens in scientific research, the first thing to do was to rule out the most banal explanation: that Halla never really existed.

Indeed, some apparent discoveries of planets orbiting red giants using the Doppler wobble technique were later shown to be illusions created by long-term variations in the behavior of the star itself.

However, follow-up observations ruled out such a false-positive scenario for Halla. Baekdu’s Doppler signal has been stable for the past 13 years, and a thorough study of other indicators has shown no other possible explanation for the signal. Halla is real, which brings us back to the question of how he survived the traffic jam.

Two stars become one: a possible survival scenario

After confirming the planet’s existence, we came up with two scenarios that could explain the situation we see with Baekdu and Halla.

At least half of all the stars in our galaxy didn’t form in isolation like our sun, but are part of binary systems. If Baekdu was once a binary star, Halla may never have faced the danger of being engulfed.

If the star Baekdu was binary, there are two scenarios that can explain the survival of the planet Halla. Image credit: Brooks G. Bays, Jr, SOEST/University of Hawai’i

A merger of these two stars may have prevented both stars from expanding to a size large enough to engulf the planet Halla. If a star single-handedly became a red giant, it would have engulfed Halla, however, if it merged with a companion star, it would go straight to the helium-burning stage without becoming large enough to reach the planet.

Alternatively, Halla could be a relatively newborn planet. The violent collision between the two stars may have produced a cloud of gas and dust from which the planet may have formed. In other words, the planet Halla could be a newborn second-generation planet.

Whichever explanation is correct, the discovery of a nearby planet orbiting a helium-burning red giant star demonstrates that nature finds ways to make exoplanets appear in places where we might least expect them. The conversation

This article is republished by The conversation licensed under Creative Commons. Read the original article.

Image Credit: WM Keck Observatory / Adam Makarenko. The planet Halla may have formed from debris created by the merger of two stars.

#planet #shouldnt #exist #baffles #astronomers
Image Source : singularityhub.com

Leave a Comment