Planets outside our solar system

discovery: Two planets roughly the size of Earth orbit a red dwarf star just 16 light-years away — close in astronomical terms. The planets, GJ 1002 b and c, lie within the star’s habitable zone, the orbital distance that could allow liquid water to form on a planet’s surface if it had the right kind of atmosphere.

Key facts: Whether red dwarf stars are likely to host habitable worlds is a subject of scientific debate. On the downside, these stars — smaller, cooler, but longer-lived than stars like our Sun — tend to glow more frequently in their youth. Such flares can strip the atmosphere of closely orbiting planets, and the planets orbiting GJ 1002 are already close. Planet B, with a mass slightly greater than that of Earth, is the closest of the two. Her year, once around a star, lasts only 10 days. Planet C, about a third larger than Earth, takes about 20 days to orbit the star.

On the plus side, however, GJ 1002 appears to be mature enough to weather youthful tantrums, and now seems calm. It’s even possible that the early ignition helped create a variety of molecules on planetary surfaces that could be used later, during the star’s quiet period, by any advanced life forms that might have been present.

details: An international team led by Alejandro Suárez Mascareño of the University of La Laguna, Spain, discovered the new planets using radial velocity measurements – that is, detecting “wobbles” of the parent star caused by gravitational tugs from orbiting planets. As the planets move toward the far side of the star, they pull the star away from us, causing the star’s light to shift toward the red end of the spectrum. As the planets move toward the star’s near side, they pull the star in our direction, turning its light more blue. The planetary tugs on GJ 1002 are very small, about 4.3 feet (1.3 meters) per second — the equivalent of moving at about 3 miles per hour (4.8 kilometers per hour). Such small movements are hard to detect.

The radial velocity method, which also reveals the size of planets, has yielded more than 1,000 confirmed exoplanet detections. However, most detections have been made using the “transit” method — observing a slight dip in starlight as a planet transits in front of its star — with nearly 4,000 confirmed observations.

To make the radial velocity measurements, the science team relied on instruments called spectrometers, which measure variations in light. The spectrometers used to discover GJ 1002 b and c were part of two collaborative observing programs: the Echelle Spectrograph for Rocky Exoplanets and Stabilized Spectroscopic Observations (ESPRESSO), the Calar Alto High-Resolution Search for Exoplanets with Near-Infrared M Dwarfs and the Echelle Spectrophotometer (Carmens). ).

Fun facts: The new planets join 10 others in a somewhat exclusive category: small worlds in the “conservative” habitable zone that are less than 1.5 times the size of Earth or less than five times the size of Earth. If we relax the membership criteria a bit – slightly larger planets are in the “optimistic” habitable zone – the group expands to about 40 exoplanets, or planets outside our solar system. The conservative habitable zone is a stricter boundary of the region around a star that may allow planets to harbor water; Optimistic habitable zones expand those boundaries a bit. Any estimate of the habitable zone is an approximation. To date, none of the atmospheres of these worlds has been fully analyzed—and many may not have atmospheres at all.

Finders: A paper on the discovery, “Two moderately earth-mass planets orbiting nearby star GJ 1002” by A. Suárez Mascareño and his team, has been accepted for publication in the Journal of Astronomy & Astrophysics. The planets were entered into the NASA Exoplanet Archive on December 22, 2022.