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Hot planets orbiting close to a former red-giant core. (S. Charpinet )

Astrophysicists have discovered two tiny, record-breaking planets that survived millions of years plowing through the broiling interior of a giant star.

Outside our solar system, "these are the smallest planets ever found, they are the least massive, " said University of Montreal astrophysicist Gilles Fontaine, who co-authored the study with an international team led by four of his former Ph.D students.

The planets KOI 55.01 and KOI 55.02 have radii about 0.76 and 0.87 times that of the Earth, the researchers reported online in the journal Nature Wednesday.

That makes them slightly smaller than two planets billed by NASA earlier this week as the smallest ever found.

The two planets circle a type of star known as a sub-dwarf that is much hotter than our sun and located 3,900 light years away from Earth, near the constellations Lyra and Cygnus.

"They are the hottest planets by a long shot because they are so close to their star," Fontaine said. The distance between the planets and their star is just 0.60 per cent and 0.76 per cent of the distance between the Earth and the sun. 

'If hell exists, it has to be on these two planets.' —Gilles Fontaine, University of Montreal

"They have the shortest ever orbital period found for a planet," Fontaine added, noting that a "year" lasts just 5.8 hours for one of the planet and 8.2 hours for the other, shattering the previous known planetary record of 16 hours.

Because they are so close to their star, they are "tidally locked" so that the same side always faces toward the star and the other side always faces away.

Iron rain

Naturally, the temperature on the star-facing side is extremely hot — around 8000 to 9000 C. That's hot enough to vapourize the iron that, along with nickel, make up most of the materials in the planets. In all likelihood, it forms clouds, drifts to the other side of planet, and rains back down in molten iron particles, Fontaine said.

Kepler's planet-hunting method

NASA's Kepler telescope was designed to find planets outside our solar system by searching for planets that eclipse their stars, as seen from the telescope's vantage point. That is, the planets pass in front of and behind the star in the course of their orbit. As they pass in front of the star, they block part of the star, causing a dip in the amount of light detected by the telescope.

The planets discovered by Fontaine and his collaborators do not eclipse their star — their orbits are tilted in such a way that Kepler never sees them cross in front of the star. However, the researchers used a new method that also relies on small changes in the brightness of light detected by the telescope.

"If hell exists, it has to be on these two planets."

The sub-dwarf is believed to have shrunk down just 18 million years ago from being a massive red giant – a star so big that the planets would have found themselves deep inside the star's envelope.

Elizabeth "Betsy" Green, an associate astronomer at the University of Arizona's Steward Observatory, who co-authored the research, said the two planets were likely originally giant planets like Jupiter that were closer to their star than Jupiter and further out than Earth is from our sun.

Their star would have originally been sun-like, but as it ran out of fuel, it would have expanded 100 times in size to spend the next one to five million years as a red giant, engulfing the planets at some point. 

The millions of years they spent plowing through the star's broiling atmosphere would have taken their toll. The friction would have slowed the orbiting planets down, causing them to spiral closer to the centre of the star.  It would have stripped off the gaseous and liquid outer layers of the planet, leaving just their small, solid cores.

At the same time, the friction from the planets likely helped strip the atmosphere off the star, leaving behind just the star's core in the form of a sub-dwarf star, which has a lifetime of about 100 million years.

"We think this is the first documented case of planets influencing a star's evolution," said Stephane Charpinet, an astronomer at the Institut de Recherche en Astrophysique et Planétologie, Université de Toulouse-CNRS, in France who led the research, in a statement.

It is also one of only three known cases of planets surviving immersion inside a star.

Accidental discovery

The team discovered the two planets by accident during a study of pulsating stars using NASA's Kepler space telescope. Pulsating stars expand and contract rhythmically due to pressure and gravitational forces, and studying the pulsations — a field of research known as astroseismology — can provide information about their mass, temperature, size and sometimes their interior structure.

Our sun as a red giant?

Our sun, powered by the fusion of hydrogen into helium, is expected to start running out of fuel in around five billion years. At that point, it will expand into a red giant, 100 times its current size. That will cause it to engulf its three nearest planets, Mercury, Venus and Earth.

The pulsations of the sub-dwarf were analyzed by Charpinet and Fontaine using computer models. In the process, the researchers noticed the star flickering faintly every 5.76 and 8.23 hours.

Eventually, they figured out what caused the flickering — two planets circling the star, reflecting different amounts of light as they move through different positions, as the moon does while orbiting the Earth.

"You will have a new planet, a full planet, a first quarter, a last quarter," Fontaine said. That results in light variations of just 50 parts per million, but enough to detect using the Kepler telescope's sensitive photometer.

This is the first time new planets have ever been discovered using this technique.

Based on the information gathered about the star using astroseismology, the researchers were able to figure out the size, mass, temperature and other information about the planets.