Mysteries of the exoplanet WASP-69b

I am an astrophysicist. My research team Published an article In the Astrophysical Journal It describes how and why WASP-69b’s tail formed, and what might help explain what other types of planets astronomers are tempted to discover outside our solar system.

The universe is full of exoplanets

When you look up at the night sky, the stars you see are suns, with distant worlds, Known as exoplanets, revolve around them. Over the past 30 years, astronomers have discovered… More than 5,600 exoplanets In our Milky Way Galaxy.

It’s not easy to spot a planet light-years away. Planets are nothing compared to the stars they orbit, both in size and brightness. But despite these limitations, exoplanet hunters have discovered an amazing variety of them — from small, rocky worlds Slightly larger than the moon Gas giants so massive that they were nicknamed “Super Jupiter“.

However, exoplanets Most common Which astronomers discovered is larger than Earth, smaller than Neptune, and revolves around its stars closer than Mercury orbits around our sun.

These most common planets tend to fall into one of two distinct groups: super-Earths and sub-Neptunian planets. Super Earth Their radius is up to 50% larger than that of Earth, while the sub-Neptunian planets usually have radii two to four times larger than Earth’s radius.

Sub-Neptunian planets, or Neptune-like planets, are very similar to super-Earths, but with a thick atmosphere. NASA-JPL/Caltech

Between these two size ranges, there is a gap known as… “radius gap” (“radius gap”)“, in which researchers rarely find planets. Planets the size of Neptune that complete their orbits around their stars in less than four days Extremely rare. Researchers call this gap “Neptune’s hot desert.”

Some fundamental astrophysical process must be preventing the formation or survival of these planets.

Planetary formation

When a star forms, it is surrounded by a large disk of dust and gas. In this disk, planets can form. As planetesimals gain mass, they can accumulate large atmospheres. However, as the star matures, it begins to emit large amounts of energy in the form of ultraviolet radiation and X-rays. This stellar radiation can “cook” and remove the atmosphere that planets have accumulated in a process called Photoevaporation.

However, some planets resist this process. More massive planets have stronger gravity, which helps them maintain their original atmosphere. Moreover, the existing planets far away Their stars are not exposed to as much radiation, and so their atmospheres suffer less erosion.

So perhaps a large portion of the super-Earths are actually the rocky cores of planets whose atmospheres have been completely removed, while the sub-Neptunian planets were massive enough to retain their inflated atmospheres.

As for Neptune’s hot desert, most Neptune-sized planets simply don’t have enough mass to resist the force of completely stripping away their star if they were orbiting too close to it. In other words, a sub-Neptune that orbits its star in four days or less would quickly lose its entire atmosphere. When we observed the atmosphere was often missing and what remained was a rocky core – a super-Earth.

To put this theory to the test, research teams like mine have collected observational evidence.

WASP-69b: A unique laboratory

Enter WASP-69b, a laboratory exclusive to the study of photoevaporation. The name “WASP-69b” comes from the way the exoplanet was discovered. It orbits star No. 69 with Planet B found in the astronomical survey Wide angle planet searching.

Although it is 10% larger Of Jupiter in radius, WASP-69b is actually closer in mass to the much lighter planet Saturn – it’s not very dense and its mass is only about 30% that of Jupiter. In fact, this planet is almost Same density From a piece of cork.

This low density is caused by Extremely closed orbit with a duration of 3.8 days About your star. Because it is so close, the planet receives a huge amount of radiation, causing it to heat up. As the gas is heated, it expands. When the gas expands enough, it begins to permanently escape the planet’s gravity.

When we observed this planet, my colleagues and I discovered helium gas rapidly leaking from WASP-69b – ca. 200 thousand tons per second. This is equivalent to the Earth’s mass lost every billion years.

During the life of the star, the planet will end up losing a total of atmospheric mass equivalent to… Nearly 15 times Earth mass. This may seem like a lot, but WASP-69b has about 90 times the mass of Earth, so even at this extreme rate, it will only lose a small fraction of the total amount of gas it is made of.

Comet-like tail

Perhaps most impressive was the discovery of WASP-69b’s helium tail, which my team discovered at least 560,000 kilometers behind the planet. Powerful stellar winds — a continuous stream of charged particles emanating from stars — sculpt their tails like this. These streams of particles hit the escaping atmosphere and form it into a tail. Looks like a comet Behind the planet.

The leaking atmosphere of WASP-69b.

Our study is actually the first to suggest that WASP-69b’s tail was this large. Previous observations of this system indicated that the planet had Just a humble tail Or himself No tail.

This difference is likely due to two main factors. First, each research group used different tools to conduct their observations, which may lead to different levels of discovery. Or there may be real volatility in the system.

A star like our Sun has a cycle of magnetic activity called the “solar cycle.” The sun’s cycle lasts 11 years. During years of maximum activity, the Sun has more flares, sunspots, and changes in the solar wind.

To complicate matters further, each cycle is unique – No two solar cycles are the same. Scientists who study the Sun are still trying to understand and predict better Our star activity. Other stars have their own magnetic cycles, but scientists do not yet have enough data to understand them.

Therefore, the observed variability in WASP-69b may be due to the fact that each time it is observed, the host star behaves differently. Astronomers will have to continue observing more of this planet in the future to get a better idea of ​​what exactly is going on.

Our direct look at WASP-69b’s mass loss tells exoplanet researchers, like me, more about how planetary evolution works. It provides us with real-time evidence of atmospheric escape and supports the theory that hot Neptune and planets in the radius gap are difficult to find because they simply do not have enough mass to retain their atmosphere. And when they lose it, all that’s left to look at is Earth’s super-rocky core.

Hey WASP-69b study It 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 fabric that makes up our universe.

* Dakota TylerPhD in astrophysics University of California, Los Angeles

This article has been republished from Conversation Under Creative Commons license. Read the Original article.