Before stars burn out, they inflate to become red giants. The outer hydrogen envelope of these massive stars expand and can swallow almost anything that stands in its path, including smaller planets.
Elizabeth Green, an associate astronomer at the Steward Observatory, was part of the research team that recently discovered two planets, roughly the size of Earth, that remained intact after being swallowed.
“They are the hottest planets ever found,” Green said, adding that the temperature of the planets’ host star is measured to be 28,000 degrees Kelvin, while the sun is only 5,770 degrees K. “We don’t know how many are like this.”
During the study of the pulsations of subdwarf B star KOI 55 of the Cygnus constellation, researchers discovered that the star is, in fact, a host star that is orbited by planets KOI 55.01 and KOI 55.02.
The star’s pulsations were being monitored via NASA’s Kepler telescope. French astronomer Stephane Charpinet of the Institut de Recherche en Astrophysique et Planétologie, who led the research team that discovered these planets, predicted stars would pulsate years ago.
Observing a star’s pulsations can aid researchers in their quest to understand what goes on inside a star.
Pressure-mode pulsations are easy to study from the ground, Green said, but it is gravity-mode pulsations that extend into the core of the star and help us to study the interior.
“Asteroseismology uses these pulsation frequencies to figure out what’s going on in a star. The holy grail will be to find a star that has both pressure and gravity modes,” Green said.
The radii of the two planets are in the same ballpark, being 0.76 and 0.87 times the size of Earth’s radius.
“What they really want to find is earth-sized planets,” Green said.
This discovery points to the theory that orbiting planets help to produce a subdwarf B star, the original star’s helium core, by pulling mass away from the star in its red giant phase.
“We don’t know if it takes two planets,” Green said. “It’s going to be very tricky to sort out what’s what. When we thought they were all due to stars, it was easier.”
Green said that her colleagues made small-scale models for their research while she provided the initial data — in this case, the spectra.
“Just getting the data is a work of art,” Green said.
Most ideas about how things in the solar system work are based solely upon observations made with telescopes and scientific laws that have been previously proven upon Earth.
The inside of the star, Green said, is so hot that one would never experience such heat on Earth, unless “you explode an atomic bomb.”