Imagine that you are a farmer looking for eggs in a chicken coop, but instead of eggs, you find ostrich eggs, which are much larger than eggs.
It's a bit like our team of astronomers discovering a giant planet, 13 times heavier than Earth, orbiting a cool red dwarf star that is 9 times lighter than Earth's Sun.
This tiny red dwarf, known as an M-type star, is not only smaller than the Earth's Sun, but also has only 100 times the luminosity of the Earth's Sun. Such a star should theoretically not have enough material to give birth to such a large planet in its planetary forming disk.
Over the past decade, our team has designed and built a new type of instrument at Penn State that can detect most of the light emitted by these faint, cool stars in the near-infrared wavelength range beyond the human eye.
Equipped on the 10-meter Harpy-Ebeli Telescope in West Texas, our instrument, named the Habitable Zone Planetary Finder, measures small changes in the velocity of a star as it is pulled by a planet's gravity. This technique, known as Doppler radial velocity, is ideal for detecting exoplanets.
"Exoplanet" means a celestial body of any planet size that orbits a star other than the Earth's Sun.
Thirty years ago, Doppler radial velocity observations led to the first discovery of exoplanet 51 Pegasi b orbiting a star similar to the Sun, and in the decades that followed, astronomers like us refined the technique. These increasingly accurate observations lead us to an important goal, which is to discover rocky planets in habitable areas, i.e., regions around stars, where liquid water can persist on the planet's surface.
Doppler technology has not yet been able to detect Earth-sized rocky planets orbiting stars similar in size to the Sun. But cool and faint M-type stars exhibit greater Doppler signatures for Earth-sized planets of the same size. The lower mass of the star causes it to be subject to greater traction from orbiting planets. And the lower luminosity results in closer habitable zones and shorter orbits, which also makes planets easier to spot.
Our team designed the Habitable Zone Planetary Finder to discover the planets orbiting these small stars. Our latest finding, published in the journal Science, was a huge surprise to discover that a giant planet orbiting the red dwarf star LHS 3154 was a huge surprise – like an ostrich egg was found in a chicken coop.
Planets are formed in disks made up of gas and dust. These disks collect dust particles and gradually combine them into pebbles, eventually forming a solid planetary core. Once the core is formed, the planet can gravitationally attract solid dust as well as surrounding hydrogen and helium. But it requires a lot of mass and material to do so successfully. This method of planet formation is known as core augmentation.
A star with a mass as low as LHS 3154 should have a correspondingly low-mass planet-forming disk.
A typical disk around a low-mass star usually does not have enough solid matter or mass to successfully form a core heavy enough to create a planet of this size. Through computer simulations conducted by our team, we concluded that such a planet would require at least 10 times more disks than typically estimated from direct observations of planetary forming disks.
Another theory of planet formation, gravitational instability, in which the gas and dust in the disk directly collapse to form planets, also makes it difficult to explain the formation of such planets, without a very large disk.
Cool, faint M-type stars are the most common stars in our Milky Way. In the lore of DC Comics, Superman's hometown of Kerten orbits an M-type dwarf.
Through the findings of the Habitable Zone Planet Finder and other instruments, astronomers know that there are at least 10 times fewer giant planets in close orbit around the most massive M-type stars than there are many planets around stars similar to the Sun. At present, we do not have a detailed understanding of such a huge planet around the smallest mass M-type star - until the discovery of LHS 3154b.
Understanding how our coolest neighbors form planets will help us understand how planets form and evolve. The study will also help astronomers understand whether M-type stars can support life.
Suvas Mahadwan, professor of astronomy and astrophysics at Penn State, Goodmudur Kari Stefansson, a NASA Hubble researcher in the Department of Astrophysical Sciences at Princeton University, and Megan Delamer, a graduate student in the Department of Astronomy at Penn State, may complete the work. This article is reproduced with permission from The Conversation under a Creative Commons license. Please read the original article.