There's nothing like brotherly love to make you feel all warm inside. The same may be true for planets, according to a new study published in the Monthly Notices of the Royal Astronomical Society in July.

According to the new study, the gravitational pull of a nearby planet can help an older planet maintain heat as it ages. As planets age, they cool down, eventually losing their atmosphere and ability to retain heat. Astronomers believe this is what happened to Mars, which once probably had liquid water but has now iced over. This process of cooling down and losing atmosphere makes planets uninhabitable.

However, astronomers at the University of Washington and the University of Arizona believe they have found a set of circumstances under which planets can escape, or prolong, this process. According to their study, the gravitational pull of a companion planet could generate enough heat to stave off this internal cool-down. This is known as tidal heating. The push and pull of the companion planet against the older planet can create heat, which in turn can extend the length of a planet's habitability for carbon-based life.

The research study explained that this phenomenon can be seen on Jupiter's moons Io and Europa. They also said that this could be true for exoplanets as well. They demonstrated how the effect could work on Earth-sized exoplanets that orbit stars that are smaller than our sun. They used planets that are in their solar system's habitable zone, the temperature zone scientists think is able to support carbon-based life. The habitable zone is the zone inside of which planets can sustain liquid water.

"When the planet is closer to the star, the gravitational field is stronger and the planet is deformed into an American football shape. When farther from the star, the field is weaker and the planet relaxes into a more spherical shape. This constant flexing causes layers inside the planet to rub against each other, producing frictional heating," said Rory Barnes, who works at the University of Washington.

The companion planet is important because it keeps the inner planet's orbit ovular. Planets need a noncircular orbit to have the tidal heating effect.

This research study suggests that when scientists look for habitable exoplanets in the future, they should also take into account whether there are other nearby planets. These companion planets might extend the length of an older planet's ability to support life.

Barnes said that the combined effect of the ancient planet's own tectonics and tidal heating generated by the outer companion might let older planets which a companion planet hold some of the longest-lived surface habitats in the universe.

"Perhaps in the distant future, after our sun has died out, our descendants will live on worlds like these," he said.

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