Imagine you are a farmer looking for eggs in the chicken coop; But instead of a chicken egg, you found an ostrich egg, much larger than a chicken could lay.
That’s kind of how our team of astronomers felt earlier this year when we discovered a massive planet with a mass nine times less than Earth’s Sun, around a cold, faint red star 13 times more massive than Earth.
The smaller star, called the M star, is not only smaller than the Sun in Earth’s solar system, but also 100 times less luminous. Such a star should not have enough material in its planet-forming disk to give birth to such a large planet.
Habitable Zone Planet Finder
Over the past decade, our team at Penn State designed and built a new device that can detect light at wavelengths beyond the sensitivity of the human eye (near infrared), where these cool stars emit most of the light. their light.
Our instrument, called the Habitable Zone Planet Finder, attached to the 10-meter Hobby-Eberly Telescope in West Texas, can measure the slight change in a star’s speed under the gravitational influence of a planet. This technique, called the Doppler radial velocity technique, is excellent for detecting exoplanets.
“Exoplanet” is a portmanteau of the words extrasolar and planet, so the term applies to any planet-sized body in orbit around a star that is not Earth’s Sun.
Thirty years ago, Doppler radial velocity observations enabled the discovery of 51 Pegasi b, the first known exoplanet orbiting a Sun-like star. In the following years, astronomers like us developed this technique. These increasingly precise measurements have an important purpose: to enable the discovery of rocky planets in habitable zones, that is, regions around stars where liquid water could hold liquid water on the planet’s surface.
The Doppler technique is not yet capable of detecting Earth-mass habitable zone planets around Sun-sized stars. But cold and faint M stars show a larger Doppler signature for the same Earth-sized planet. The lower mass of the star causes it to be more attracted by the orbiting planet. The lower brightness leads to a closer habitable zone and a shorter orbit, making the planet easier to detect.
The planets around these small stars are the ones our team designed the Habitable Zone Planet Finder to discover. Our new discovery, published in the journal Science, of a massive planet orbiting the cool faint M star LHS 3154 (the ostrich egg in the chicken coop) came as a real surprise.
LHS 3154b: Planet that should not exist
Planets form in disks of gas and dust. These disks collect dust grains that become pebbles and eventually coalesce to form a solid planetary core. Once the core is formed, the planet can gravitationally attract solid dust as well as surrounding gases such as hydrogen and helium. But to do this successfully, a lot of mass and materials are needed. This formation of planets is called core accretion.
A star as low-mass as LHS 3154, nine times less massive than the Sun, should have a correspondingly low-mass planetary disk.
A typical disk around such a low-mass star would not have enough solid material or mass to form a core heavy enough to create such a planet. From computer simulations our team ran, we concluded that such a planet would require a disk at least 10 times larger than predicted by direct observations of planet-forming disks.
A different theory of planet formation, gravitational instability (gas and dust in the disk directly collapsing to form a planet), also has difficulty explaining the formation of such a planet without a very large disk.
Planets around the most common stars
Cold, faint M stars are the most common stars in our galaxy. In DC comics lore, the planet Krypton, Superman’s home world, orbited an M dwarf star.
Astronomers know from discoveries with the Habitable Zone Planet Finder and other instruments that giant planets in close orbits around the largest M stars are at least 10 times rarer than those around Sun-like stars. And until the discovery of LHS 3154b, we know of no such massive planets in close orbits around the smallest M stars.
Understanding how planets form around our coolest neighbors will help us understand both how planets form in general and how rocky worlds form and evolve around numerous types of stars. This line of research could also help astronomers understand whether M stars could support life.
This article is republished from The Conversation, an independent, nonprofit news organization providing facts and analysis to help you understand our complex world.
Written by: Suvrath Mahadevan, Penn State; Guðmundur Kári Stefansson, Princeton Universityand Megan Delamer, Penn State.
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Suvrath Mahadevan 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.