by Located 163 light-years from Earth, a Jupiter-sized exoplanet named WASP-69b offers astrophysicists a window into the dynamic processes that shape planets across the galaxy. The star it orbits is baking and removing the planet’s atmosphere, and the star is knotting that atmosphere into a large comet tail that is at least 350,000 miles long.
I am an astrophysicist. My research team published a paper in the Astrophysical Journal describing how and why WASP-69b’s tail formed, and what its formation may shed light on the other types of planets astronomers usually detect outside our solar system.
A universe filled with exoplanets
When you look up at the night sky, the stars you see are suns, and distant life, known as exoplanets, orbiting them. Over the past 30 years, astronomers have discovered over 5,600 exoplanets in our Milky Way galaxy.
Detecting a light-year planet is not easy. White planets compared, in size and brightness, to the stars they orbit. But despite these limitations, exoplanet researchers have discovered an amazing variety – everything from small rocky worlds barely bigger than our own moon to gas giants so big they’ve been called “Super-Jupiters”.
However, the exoplanets most commonly detected by astronomers are larger than Earth, smaller than Neptune, and orbit their stars closer than Mercury orbits our Sun.
These ultra-common planets tend to fall into one of two distinct groups: super-Earths and sub-Neptunes. A super-Earth has a radius that is up to 50% that of Earth, and a sub-Neptune typically has a radius that is two to four times that of Earth.
Between those two radius ranges, there is a gap, called the “Radius Gap,” where researchers rarely find planets. And, Neptune-sized planets that complete orbits around their stars in less than four days are extremely rare. Researchers call that gap “Neptune’s Hot Desert.”
Some fundamental astrophysical process must be preventing these planets from forming – or surviving.
Planet formation
As a star forms, a large disk of dust and gas forms around it. In that disk, planets can form. As young planets gain mass, they can accumulate significant atmospheres of gas. But as the star matures, it begins to emit high amounts of energy in the form of ultraviolet and X-ray radiation. This stellar radiation can bake the atmospheres collected by the planets in a process called photoevaporation.
However, some planets are against this process. Larger planets have stronger gravity, which helps them hold onto their original atmospheres. In addition, the planets that are farther from their star are not hit with as much radiation, so their atmospheres do not erode less.
Thus, a significant portion of the Super-Earths may be the rocky cores of planets whose atmospheres were completely destroyed, and sub-Neptune was massive enough to retain their puffy atmosphere.
As for the Neptune Hot Desert, most Neptune-sized planets aren’t massive enough to fully resist the star’s destructive power if it orbits too tightly. In other words, a sub-Neptune orbiting its star in four days or less will quickly lose its entire atmosphere. When viewed, the atmosphere has already been lost and what remains is a bare rocky core – Super Earth.
To test this theory, research teams like mine are gathering observational evidence.
WASP-69b: A unique lab
Enter WASP-69b, a unique laboratory for the study of photoevaporation. The name “WASP-69b” comes from the way it was discovered. It was the 69th star with a planet, b, found in the Wide Angle Search for Planets survey.
Despite being 10% larger than Jupiter in radius, WASP-69b is closer to the mass of a much lighter Saturn – it is not very dense, and has only about 30% the mass of Jupiter. In fact, this planet has about the same density as a piece of cork.
This low density results from their ultra-close 3.8 day orbit around their star. Being so close, the planet receives a huge amount of energy, causing it to heat up. As a gas is heated, it expands. Once the gas expands enough, it begins to escape the planet’s gravity for good.
When we spotted this planet, my colleagues and I detected helium gas escaping WASP-69b at a rapid rate – about 200,000 tons per second. This translates to Earth’s mass lost every billion years.
Over the lifetime of the star, this planet will lose a total atmospheric mass equal to nearly 15 times the mass of Earth. This sounds like a lot, but WASP-69b is about 90 times the mass of Earth, so even at this extreme rate, it will lose only a small fraction of the total amount of gas it is composed of.
WASP-69b comedy tail
Perhaps most impressive is the discovery of the extended helium tail of WASP-69b, which my team found drifting behind the planet for at least 350,000 miles (about 563,000 kilometers). Strong stellar winds, which are steady streams of charged particles emitted from stars, form tails like this. These particles ram into the escaping atmosphere and form a comet tail behind the planet.
Our study is the first to suggest that the tail of WASP-69b was so extensive. Past observations of this system suggested that the planet had only a moderate tail or even no tail at all.
This difference probably comes down to two main factors. For one, each research group used different tools to make their observations, which may have resulted in different levels of detection. Alternatively, there could be actual variation in the system.
A star like our Sun has a cycle of magnetic activity, known as the “solar cycle”. The Sun lasts for 11 years. During peak activity years, the Sun experiences more flares, sunspots and changes in the solar wind.
To make things even more complicated, each cycle is unique – no two solar cycles are the same. Solar scientists are still trying to better understand and predict the activity of our Sun. Other stars have their own magnetic cycles, but scientists don’t yet have enough data to understand them.
So the variation observed for WASP-69b could come from the fact that the host star behaves differently each time it is observed. Astronomers will have to continue to observe this planet more in the future to better understand what exactly is going on.
Our direct observation of the WASP-69b mass loss tells exoplanet researchers like me more about how planetary evolution works. It gives us real-time evidence of atmospheric escape and supports the theory that the hot planets Neptune and Gap Gap are hard to find because they are not massive enough to retain their atmosphere. And when they lose them, all that remains to be seen is the rocky core of Super Earth.
The WASP-69b study highlights the delicate balance between a planet’s composition and its stellar environment, shaping the diverse planetary landscape we observe today. As astronomers continue to explore these distant worlds, each discovery brings us closer to understanding the complex tapestry of our universe.
This article is republished from The Conversation, a non-profit, independent news organization that brings you reliable facts and analysis to help you make sense of our complex world. Written by: Dakota Tyler, University of California, Los Angeles
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Dakotah Tyler does not work for, consult with, own shares in, or receive funding from any company or organization that would benefit from this article this, and has not disclosed any relevant connections beyond their academic appointment.
