The Kepler-51 system, known for having some of the least dense planets ever found, has a fourth member. We don’t know if the newly discovered Kepler-51e shares this notable trait of its sibling planets, as we only know its mass, not size, but the discovery adds an extra level of intrigue to a truly extraordinary star system.
The star Kepler-51 looks like a younger version of the Sun, its mass is just 4 percent smaller than our star. It’s only two-thirds as bright as the Sun, but that is thought mostly to be because it’s about a seventh the age – the Sun was fainter then too. All of this makes Kepler-51’s planets even stranger.
Kepler-51b, c, and d all transit (cross the face of their star from our perspective), which allows us to measure their size. The trio’s masses have been deduced from their gravitational effects on each other, leading to the astonishing conclusion that these planets have sizes close to Jupiter’s, but masses 3-6 times that of Earth. That makes their density similar to cotton candy – it also makes them easy for a fourth member to push around.
The existence of Kepler-51e was deduced when the JWST and land-based telescopes were scheduled to observe a transit by Kepler-51d. The fact these telescopes’ precious time was dedicated to observing the event shows the importance astronomers place on understanding why the system is so different from our own. Instead of the transit proceeding as predicted, the operation only just caught the transit’s start.
“Thank goodness we started observing a few hours early to set a baseline, because 2 a.m. came, then 3, and we still hadn’t observed a change in the star’s brightness with APO,” Dr Jessica Libby-Roberts of Penn State University said in a statement. “After frantically re-running our models and scrutinizing the data we discovered a slight dip in stellar brightness immediately when we started observing with APO, which ended up being the start of the transit – 2 hours early, which is well beyond the 15-minute window of uncertainty from our models.”
Kepler-51d is the puffiest and strangest member of the system. Many extremely low-density planets orbit very close to their stars, with the heat causing their gases to expand. However, Kepler-51d has a 130-day orbit, which around a star less luminous than the Sun shouldn’t make for exceptional temperatures. Despite this, its density was thought to be just 0.0381 g/cm3, the lowest of any planet we have found. By comparison, water’s density is 1 g/cm3, Earth’s is 5.5 g/cm3, and Saturn, the least dense planet in the Solar System is 0.69 g/cm3.
Still, even planets as light as this for their size don’t float like a dandelion seed in a breeze. Only the presence of a previously unsuspected fourth planet could explain the difference in timing, just as Neptune was discovered from irregularities in Uranus’s orbit.
Using all the data collected from the system over 14 years, the authors concluded that Kepler-51e probably has a mass less than ten times the Earth’s, making it similar to the other three planets. Its orbit most likely lasts 264 days, which would make its temperature quite similar to Earth’s, although as Keplter-51 brightens it may cease to be in the habitable zone.
Since we have not seen Kepler-51e transit, we don’t know if this is yet another super-puff planet, a more typical gas giant like Neptune, or a superEarth. The researchers also note they cannot currently rule out a more massive planet on a longer orbit, although they consider this less likely.
The presence of another planet has also caused the authors to recalculate the masses of the three previous planets, revealing them as potentially slightly heavier than previously thought, particularly Kepler-51b. Since their sizes haven’t changed, that makes them a little denser, but not enough to change their status as extreme anomalies among the worlds we know.
Meanwhile, the system’s oddities increase. “Super puff planets are fairly rare, and when they do occur they tend to be the only ones in a planetary system,” Libby-Roberts said. “If trying to explain how three super puffs formed in one system wasn’t challenging enough, now we have to explain a fourth planet, whether it’s a super puff or not. And we can’t rule out additional planets in the system either.” After all, the four planets are all substantially closer to their star than Earth is to the Sun.
For planets to be as low density as the inner three, they must have tiny cores and be largely made of hydrogen and helium, like vastly more extreme versions of the gas giants in our own system. Yet that combination should see the gasses rapidly escape, particularly when close enough to the star to be provided with plenty of energy.
Early explanations for the system’s unusual features involved the planets having been formed at distances from Kepler-51 similar to Jupiter’s from the Sun, and then have migrated inwards. However, subsequent work cast doubt on this, and the question remains unsolved. Another hypothesis proposes giant rings that block the star’s light, making the planets appear larger, but can’t explain why all three have rings in just the right orientation. Since Kepler-51 has a slightly greater metal content than the Sun, it should not be that the disk around it lacked heavier elements from which cores could form.
The study is published open access in The Astronomical Journal
Source Link: System Of “Cotton-Candy” Planets Has A Lurking Fourth Member
