How many suns can a planet have

(ORDO NEWS) — A strange alien world with three suns in the sky has been discovered, but it hasn’t broken any records.

There are a few other known exoplanets with three host stars and quite a few with two suns (this latter group is sometimes called the Tatooine planets, after Luke Skywalker’s homeworld in the Star Wars films).

Where does all this multi-star madness end? Is it possible for alien planets to have four, five, six, or even more suns?

“It’s all about hierarchy,” said Caitlin Kratter, a computational astrophysicist at the University of Arizona who was part of the team that discovered the new planet with three suns, named HD 131399Ab.

“It’s getting hard to keep everything together,” said Thomas Beatty, a researcher at Pennsylvania State University. “Working in these orbits is like trying to balance something on top of a mountain.

This is a balance between gravity from the star and what revolves around it and tries to take off, the presence of a third star in the system, which pulls this balance on itself.

What allows the planet HD 131399Ab to endure such chaos, Kratter said, is its location. The planet orbits about 80 astronomical units (AU) from its “main” parent star and 300 AU from the other two stars in the system, which consist of a binary pair. One AE is the average distance from the Earth to the Sun – about 93 million miles or 150 million kilometers.

Other known planets with three suns are also far from the binary stars in the system. For example, a planet called KELT-4Ab is so close to its primary star that one orbit takes only three Earth days.

But the planet is at a distance of 330 AU from the binary pair of stars, about eight times the average distance between Pluto and the Sun in our own solar system.

“These discoveries are actually not that surprising. As long as the rest of the stars in the system are far enough away not to significantly disrupt the planet’s gravitational orbit (or disrupt the protoplanetary disk at the beginning of evolution), such planets will form and survive,” said Polish astronomer Maciej Konacki, who in 2005 reported on a possible planet in a three-star system called HD 188753 Ab.

This world was not discovered in a follow-up study by another team in 2007, leading some astronomers to conclude that it probably does not exist.

“HD 131399 will be close to a limit where the influence of stellar companions will be sufficient to destabilize the planet’s orbit,” Konacki added, “such a planet could subsequently be ejected from the system and become a free-floating planet. These triple (or multiple) star systems may be one of the natural sources of free-floating planets.”

Strange formation history

One of the key questions to consider is how multisolar planets wedged into their orbits in the first place, the researchers say.

The answers are rarely straight forward, as the HD 131399Ab example shows. The planet is huge, at least four times the size of Jupiter. It is not clear whether it will have enough gas to form such a giant world at a distance of 80 AU from the main star in the system.

In addition, radiation from the binary pair would carry much of this material into space, making the accretion process even more difficult, the researchers say.

Another possibility is that the planet formed independently in a large molecular cloud that gave birth to three stars, rather than from a disk of leftover material surrounding the main star.

“In this case, we would think of the system more like a four-star system, where one of the objects (the planet) just has a very small mass,” Kratter said. “However, to make such a small object is also a big rarity.”

The latest theory is that the planet formed very close to its parent star along with a companion planet or two. Over time, the orbits of the planets would have collided with each other, and eventually such interactions led to the fact that the planet HD 131399Ab came to its current location.

As for the other planet or planets in this last scenario – if they were close enough to the parent star, they could still be hiding out there in its bright light, Kratter said. Also the planets in this system may still be evolving given that it is so young (about 16 million years old; our own solar system, by comparison, is about 4.6 billion years old).

Planets could also be pushed by gravitational tugs away from stars in their systems, Beatty, co-author of the KELT-4Ab paper, says. He noted that most systems with close orbits of “hot Jovian” planets have two or three stars; therefore, hot Jupiters can form further in the system and then be pulled inward by the gravity of those stars.

Thousands of light years

Depending on where a star is in the Milky Way, it can hang gravitationally on several other stars at fairly large distances. As stars grow further apart, they are subject to gravitational interference from other stars, from mysterious dark matter, and even from the black hole at the center of the Milky Way.

Theoretically, the limit beyond which stars stop being connected to each other is somewhere between 10,000 and 100,000 AU, Kratter said.

This means that a single star system could extend its gravitational influence over thousands of AU, possibly with many binary stars and associated planets. As an example, she gave PH1, a planet with two suns that lives in a four-star system.

“Think of Jupiter in our own solar system,” she said, explaining that Jupiter has dozens of moons orbiting the planet closely.

These moons, for example, do not affect the orbit of the Earth’s moon because they are very far away from us. In addition, the Sun’s gravity does not push satellites away from Jupiter, because there is also enough distance between the Sun and Jupiter.

According to Kratter, the same can be said for planets and stars. As long as there is sufficient distance between stars, it is difficult to set an upper limit on the number of stars in one system, she said.

Konacki agreed with this assessment.

“I see no reason why (for example) five-star star systems could not host a planet as long as its gravitational stability is ensured.”

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