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Last time we introduced two planets around white dwarfs and pulsars, which are also the smallest and smallest planets ever discovered. We've seen the smallest planets, but how big can the largest be?

HD 100546 is a young star more than 300 light-years away and about 10 million years old. Compared to our sun, it looks like a baby full moon. For soon after birth, it was not surrounded by a mature planet, but by a disk of gas and dust, a protoplanetary disk.

As we said in the first installment of this series, there are two main ways to discover exoplanets: transits and radial velocities.

But neither approach works well for young stars with protoplanetary disks. Because the transit method obscures the planets by the dust disk, and the radial velocity method, the unstable photosphere of the young star affects the star spectrum. What should we do?

Yes, look directly! It's just over 300 light years. It's not too far.

However, it is easier said than done. Although direct imaging method is an important way to discover exoplanets, it has many restrictions. For example, the planet had to be large enough and as far away from the star as possible so that it wouldn't be obscured by the star's light, and now there was a dust disk in the way, so it wasn't easy to see the planet directly.

But the dust pan, though a hindrance, has its advantages. Giant planets usually carve a deep, wide slit in the dust disk, and these slits in the disk are clues to where astronomers can find planets.

In 2012, astronomers studied the gap characteristics of the dust disk by means of mid-infrared interferometry. Then they discovered that there might be a supermassive planet here, with a mass of more than 30 Jupiter masses.

However, the researchers also found that this planet alone may not be enough to explain all the gaps in the dust disk. Because it is a protoplanet in the process of formation, this outrageous mass may contain some of the mass of the dust disk or other planets. And if the planet does reach that size, it probably wouldn't qualify as a planet and would qualify as a brown dwarf. In short, given the limited means of observation, there is still a lot of uncertainty about whether the planet really exists, let alone the head.

Subsequently, two other teams conducted special studies on the object. It is found that there is indeed a bright emission source in the dust disk of the star towards the north. Only this time the mass estimate has been reduced to a dozen Jupiter masses. Given the amount of water still present, the researchers think it should be a newly formed gas giant rather than a brown dwarf.

At this point, the planet is still only a candidate because we lack basic data about the orbital period of these planets.

In 2015, researchers conducted the first high-contrast imaging study using multiple bands. This time they did not give a more precise result on mass, but they did give a more precise size for the planet-6.9 times Jupiter's radius. They also calculated an orbital period of about 250 years, meaning the planet should not be particularly massive. The exoplanet in formation in the protoplanetary disk has been officially confirmed.

As a gas giant in the making, HD 100546 b is a dream for astronomers. Because our previous knowledge of planet formation has been largely speculative and imaginative, this planet is an ideal experimental specimen for planetary formation theory.

In addition, the researchers found that there seems to be another planet on the outer side of this planet, which seems to be more massive.

Six years later, in an article published in the July 2021 issue of Astronomy and Astrophysics, researchers again made detailed observations and analyses of the protoplanetary disk using the Atacama Radio Telescope (ALMA). This time, the researchers used smoothed-particle-hydrodynamic simulations of the protoplanets to accurately simulate them.

When the simulations matched the observations, they finally got accurate data on the two planets: the 6.9 Jupiter radius giant has a mass of about 3.1 Jupiter masses, while the outer planet has a mass of 8.5 Jupiter masses (or perhaps even close to 10 Jupiter masses), although the latter is still in the status of confirmation.

6.9 Jupiter times radius, which is already the ceiling of known planets in terms of size alone, but whether it is 3.1 times or 8.5 times mass, it is still some distance away from the limit.

HD 206893 c is an exoplanet 130 light-years away, with a mass of about 12.7 Jupiter masses measured by precise dynamics, so massive that its surface temperature approaches thousands of degrees.

A gas giant at thousands of degrees. What does that sound like? Does it look like a star? And there is an even more exaggerated "planet" on its outer side, which has a mass of 28 Jupiter masses, which is already a brown dwarf.

Brown dwarfs are those substars that lack mass and cannot fuse their hydrogen, so they are not classified as stars. Hydrogen cannot be ignited, but if it reaches more than 13 Jupiter masses, it is enough for its isotope deuterium to fuse. As for whether it can last and how long it can burn, this is related to the specific internal structure. Such substars that "burn but don't burn" are called brown dwarfs.

So HD 206893 b is indeed a brown dwarf, if only in terms of mass. HD 206893 is therefore a very special hybrid planetary system because it contains stars, planets and brown dwarfs. This is the first time astronomers have encountered such a situation, and it is helpful for studying the difference between brown dwarfs and supergiant planets.

In fact, there is still controversy about how to classify such critical objects. For example, as a brown dwarf that cannot become a star, although it has "failed" very much, there is a class of objects that "failed" more than it, namely "subbrown dwarfs," whose mass can even be as low as 1 Jupiter mass.

"One Jupiter mass"... Isn't that a planet?

The key lies in how it was formed: if, like Jupiter, it was born in the protoplanetary disk of a star, it is a planet; if, like a star, it was formed by the direct collapse of a gas cloud, it is a subbrown dwarf. But for a subbrown dwarf that wanders among the stars, it makes sense to call it a "stray planet"... doesn't it feel messy? That's right. Previously, it was divided according to its quality. Now, it still depends on its formation process. The key is how it was formed in the first place. It's hard to say now. So there's still debate about brown dwarfs and planets, and exactly how they're distinguished.

And that reminds me of poor Pluto. At that time, the International Astronomical Union also held a special meeting to discuss whether to kick it out of the big planet team. In the end, Pluto was demoted to a dwarf planet and the solar system resumed its eight planets.

At this time, some people will say: "You are too self-righteous, how to divide it is a human matter, what the celestial body should be or what." "

Yes, just like ornithology and birds, whether birds can fly or not is not the turn of human opinion, and it is our own business to summarize. "Perfect classification" is not an idle act of astronomers. It not only provides convenience for future research, but also represents a more comprehensive and profound understanding of this aspect, which is of great significance to future scientific development.

This article comes from Weixin Official Accounts: Linvo Says Universe (ID: linvo001), Author: Linvo

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