How many suns can a planet have?

US, WASHINGTON (ORDO NEWS) — A strange alien world with three suns in the sky was discovered, but it did not break any records. There are several other well-known exoplanets with three host stars and quite a bit with two suns (this last group is sometimes called the Tatooine planets, in honor of 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 all depends on the hierarchy,” said Caitlin Kratter, an astrophysicist at Arizona University who was part of the team that discovered the new three-sun planet called HD 131399Ab.

“It is becoming difficult 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 the balance between gravity from the star and what revolves around it and tries to take off, by the presence of a third star in the system, which pulls this balance onto itself.”

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

Other famous planets with three suns are also located far from binary stars in the system. For example, a planet called KELT-4Ab is so close to its main star that one orbit takes only three Earth days. But the planet is located at a distance of 330 AU from a double pair of stars – this is about eight times the average distance between Pluto and the Sun in our own solar system.

“These discoveries are actually not so surprising. As long as the other stars in the system are far enough away so as not to significantly disrupt the planet’s gravitational orbit (or to disrupt the protoplanetary disk at the beginning of evolution), such planets will form and survive, ”said Polish astronomer Maciej Konacki, who reported in 2005 on the possible planet in a three-star system called HD 188753 Ab. This world was not discovered in a subsequent study by another team in 2007, which led some astronomers to conclude that it probably does not exist.

“HD 131399 will be close to the point where the influence of stellar companions is sufficient to destabilize the planet’s orbit,” added Konatsky, “such a planet can subsequently be thrown out of the system and turn into a free-floating planet. These triple (or multiple) star systems can be one of the natural sources of free-floating planets. ”

A Strange Formation History

One of the key issues to consider is how multisolar planets first wedge into their orbits, the researchers say.

Answers are rarely straightforward, as the HD 131399Ab example shows. The planet is huge, at least four times larger than Jupiter. It is unclear whether she will have enough gas to form such a giant world at a distance of 80 AE from the main star in the system. In addition, radiation from a binary pair would take most of this material into space, which would make the accretion process even more difficult, the researchers say.

Another possibility is that the planet formed independently in a large molecular cloud that generated three stars, and not from the disk of the remaining material surrounding the main star.

“In this case, we will think of the system rather as a four-star system, where one of the objects (the planet) simply has a very small mass,” Kratter said. “However, making such a small object is also a rarity.”

The final 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 collide with each other, and ultimately such interactions led to the planet HD 131399Ab reaching 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 hide there in its bright light, Kratter said. Also, planets in this system can still evolve, given that it is so young (about 16 million years; our own solar system, for comparison, about 4.6 billion years).

According to Beatty, the second author of an article on KELT-4Ab, planets can also be pushed by gravitational tugs from stars in their systems. He noted that in most systems with close orbits of the “hot Jupiter” planets, there are two or three stars; therefore, hot Jupiters can form further in the system and then be attracted inward by the gravity of these stars.

Thousands of light years

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

Theoretically, the limit beyond which stars cease to be connected to each other is somewhere between 10,000 and 100,000 AE, Kratter said.

This means that a system consisting of one star can extend its gravitational effect on thousands of AEs, possibly with many binary stars and related planets. As an example, she cited 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 satellites orbiting closely around the planet. These moons, for example, do not affect the orbit of the earth’s moon, because they are very far from us. In addition, the gravity of the Sun does not throw satellites away from Jupiter, because there is also enough distance between the Sun and Jupiter.

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

Konatsky agreed with this assessment.

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

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