by Imagine you are a farmer looking for eggs in the chicken coop – but instead of a chicken egg, you find an ostrich egg, much larger than anything a chicken could lay.
That’s how our team of astronomers felt when we discovered a giant planet, more than 13 times heavier than Earth, around a cool, red star, nine times smaller than Earth’s Sun, earlier this year.
The smaller star, known as an M star, is not only smaller than the Sun in Earth’s solar system, but is 100 times less luminous. Such a star should not have the necessary amount of material in its planet-forming disk to give birth to such a giant planet.
Habitable Zone Planet Finder
Over the past decade, our team at Penn State designed and built a new instrument that can detect the light from these faint, cool stars at wavelengths beyond the sensitivity of the human eye—in the near-infrared—where such cool stars emit most of it. its light.
Attached to the 10-meter Hobby-Eberly Telescope in West Texas, our instrument, called the Inhabitable Zone Planet Finder, can measure the subtle change in a star’s velocity as a planet pulls into it. gravitational. This technique, called the Doppler radial velocity technique, is great for detecting exoplanets.
“Exoplanet” is a combination of the words extrasolar and planet, so the term refers to any planetary body orbiting a star that is not the Earth’s Sun.
Thirty years ago, Doppler radial velocity observations enabled him to discover 51 Pegasi b, the first known exoplanet orbiting a Sun-like star. In later years, astronomers like us improved this technique. These increasingly accurate measurements have an important goal: to enable the discovery of rocky planets in habitable zones, the regions around stars where liquid water can be sustained on the planetary surface.
The Doppler technique does not yet have the resources to detect Earth-mass planets around Sun-sized stars. But the cool and dim M stars show a larger Doppler signature for the same Earth-sized planet. The lower mass of the star causes the planet to orbit it more. And the lower luminosity results in a denser habitable zone and shorter orbit, making the planet easier to detect.
The planets around these small stars were the planets our team designed the Habitable Zone Planet Finder to discover. Our new discovery, published in the journal Science, of a giant planet closely orbiting the cool dim star M LHS 3154 – the ostrich egg in the chicken coop – was a big surprise.
LHS 3154b: The planet that shouldn’t exist
Planets form in disks made up of gas and dust. These disks attract dust grains that grow into pebbles and eventually coalesce to form a solid planetary core. Once the core is formed, the planet can gravitationally pull in the solid dust, as well as the surrounding gas such as hydrogen and helium. But it needs a lot of mass and material to do this successfully. This way of forming planets is called core accretion.
A star as low as the mass of LHS 3154, nine times less massive than the Sun, should have a low-mass planet-forming disk.
A typical disk around such a low-mass star should not have enough solid material or mass to form a core heavy enough to form such a planet. From computer simulations performed by our team, we concluded that such a planet requires a disk 10 times larger than is usually assumed from direct observations of planet-forming disks.
A different theory of planet formation, gravitational instability – where gas and dust fall directly into the disk to form a planet – also struggles to explain such planet formation without a very massive disk.
Planets around the most common stars
Bright, dim M stars are the most common stars in our galaxy. In DC comics lore, Superman’s home world, the planet Krypton, orbited an M dwarf star.
Astronomers know, from discoveries made with the Habitable Belt Planet Finder and other instruments, that giant planets orbiting close to the most massive M stars are at least 10 times rarer than those around on Sunlike stars. And we know that there are no such massive planets in close orbit around the least massive M stars – until LHS 3154b was discovered.
Understanding how planets form around our most recent neighbors will help us understand how planets form in general and how rocky life forms and evolves around the most abundant types of stars. This line of research could help astronomers understand whether M stars can support life.
This article is republished from The Conversation, a non-profit, independent news organization that brings you facts and analysis to help you make sense of our complex world.
It was written by: Suvrath Maadevan, Penn State; Guðmundur Kári Stefánsson, Princeton Universityand Megan Delamer, Penn State.
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Suvrath Madevan receives external funding from NSF, NASA, and the Heising-Simons Foundation, as well as research funding and support from Penn State.
Guðmundur Kári Stefánsson receives funding from NSF, NASA and the Heising-Simons Foundation.
Megan Delamer receives funding from NSF, NASA, and the Heising-Simons Foundation.
