When one star hosts a rocky planet half the size of Venus, another world that could sport an ocean, and the hint of a planet that could host liquid water on its surface, astronomers take notice.
And new research suggests that a stellar system just 35 light-years from Earth could host all of those gems among its four or five known planets. It's just the sort of neighborhood that can inflame dreams that spotting a second Earth is just around the corner, even as astronomers wade through countless detections of exoplanets that are no more detailed than a size estimate.
"This system announces what is to come," Olivier Demangeon, a researcher at the Instituto de Astrofísica e Ciências do Espaço, University of Porto in Portugal and lead author of the new study, said in a statement released by the European Southern Observatory (ESO), whose facilities were used during the research.
"We, as a society, have been chasing terrestrial planets since the birth of astronomy," he added. "Now we are finally getting closer and closer to the detection of a terrestrial planet in the habitable zone of its star, of which we could study the atmosphere."
At the heart of the intriguing stellar system is a bright red dwarf star dubbed L 98-59, sometimes also known as TOI-175. A separate team of astronomers first identified three planets in the neighborhood, each around the size of Earth, in research published two years ago.
"This is somewhat analogous to looking at Venus, Earth and Mars in the solar system," Knicole Colón, an astrophysicist at NASA's Goddard Space Flight Center who participated in that original research but who wasn't involved in the new research, told Space.com in an email.
But Venus, Earth and Mars are very, very different from one another, and in order to understand whether that is typical of planetary systems or an unusual aspect of our own neighborhood, scientists need to get to know other families of worlds. "Just how similar are exoplanets that formed around the same star and are also close in size to each other?" Colón wrote.
At the time of the research she took part in, planetary sizes were about all the scientists could say about these worlds. Most of the observations used in that research came from NASA's Transiting Exoplanet Survey Satellite (TESS), which stares at bright nearby stars to spot the tiny dips in brightness that mark a planet passing between the star and the telescope.
The amount of light the planet blocks out is determined by its size, so that statistic is straightforward to calculate, but the number doesn't necessarily tell scientists much about what visiting a planet would be like, which depends more on its composition.
"If we want to know what a planet is made of, the minimum that we need is its mass and its radius," Demangeon said in the statement. From there, scientists can calculate a planet's overall density and compare that number with, say, the gas mixtures that make up our solar system's outer planets or the bare rock and massive metallic core of a Mercury.
Other teams have taken cracks at calculating the mass of these planets, but Demangeon and his co-authors decided to take another look at the L 98-59 system and try to calculate the mass of its planets in hopes of gathering more precise observations by using a different instrument, the Echelle Spectrograph for Rocky Exoplanets and Stable Spectroscopic Observations (ESPRESSO) instrument on ESO's Very Large Telescope in Chile.
ESPRESSO is tailored to identify potential "Earth twins" around sunlike stars, according to ESO. Like TESS and most other exoplanet hunters, ESPRESSO studies planets by carefully analyzing light from a host star.
Because of the gravitational tie between a star and its planet, a star with companions doesn't quite rotate in place; instead, the star wobbles a smidge as the planet tugs at it. By carefully plotting how the light fingerprint of the star changes over time, scientists can isolate that wobble and calculate the mass of the planet causing the wobble.
That technique, called radial velocity, can work even when scientists can't see a planet or the shadow it casts on its star's surface, so scientists can use the technique both to better understand known exoplanets and to discover new ones.
And ESPRESSO's radial velocity analysis of the L 98-59 system accomplished both feats, the researchers argue — finetuning scientists' understanding of the three known planets and identifying one or two previously hidden ones as well.
Tour of another solar system
Let's start at the heart of this nearby planetary family: the star L 98-59, which spins every 80 or so days according to the researchers. As a red dwarf, L 98-59 is much smaller than our own star, but its planets are also closer to it. So, the innermost two receive more heat and light than Mercury does, while the outer two or three receive more heat and light than Mercury but less than Earth.
Then come the planets.
The innermost world is the lightest exoplanet scientists have been able to measure using radial velocity, at just half the mass of Venus. Smaller planets are typically much more difficult to identify and study than larger planets, whatever technique scientists use, so the new research is a heartening sign that astronomers will be spotting other relatively small planets in the near future, according to the research team.
"This is a step forward in our ability to measure the masses of the smallest planets beyond the solar system," María Rosa Zapatero Osorio, an astronomer at the Centre for Astrobiology in Madrid, Spain, and a co-author of the new study, said in the ESO statement.
Both this planet and its neighbor seem to hide small iron cores, according to the scientists' analysis, and may each have a little water in an atmosphere.
The third rock from this alien sun appears to have a similar core to its two siblings, but then things get very different. Here, both water and gas appear to be much more plentiful, according to calculations in the new research, with perhaps as much as one-third of the planet's mass being water.
Those three worlds are the same first spotted in the TESS data. But with radial velocity measurements, the scientists on the new research spotted one more planet, plus a potential fifth planet that still needs firmer data.
If that data is indeed a fifth world, it would receive about as much light and heat from the star as Earth does from the sun, and would likely be the right temperature to protect water at its surface — and life in turn.
The neighborhood as a whole poses some puzzles, too, of course. Taken together, the system appears to match a category of multiplanet system that scientists have dubbed "peas in a pod," where similar-size planets are spaced at regular distances from the star.
Only a handful of other multiplanet systems are built around red dwarf stars like L 98-59, but intriguingly, one of those is the famed TRAPPIST-1 system, the study's authors note.
As usual, scientists will keep chipping away at the L 98-59 system's mysteries.
According to Colón, scientists are already using the Hubble Space Telescope to study atmospheres on all three of the innermost worlds, and the James Webb Space Telescope due to launch later this year will observe the second and third planets.
And for her, the new study is exciting just as much for what it says about exoplanet research to come as for the specifics of the planets that the astronomers were able to tease out of the data.
"This type of research really excites and inspires me, because it demonstrates the incredible synergy that exists between all the facilities that are working to discover and study exoplanets," Colón wrote.
The research is described in a paper published Thursday (Aug. 5) in the journal Astronomy & Astrophysics.
Email Meghan Bartels at firstname.lastname@example.org or follow her on Twitter @meghanbartels. Follow us on Twitter @Spacedotcom and on Facebook.