New star activity catalog could sharpen hunt for habitable worlds Sadie Harley Scientific Editor Andrew Zinin Chief Editor Searching for habitable worlds beyond our solar system involves more than having a planet orbit within its star's habitable zone, the region where temperatures could be just right for liquid water to exist on the surface. On Earth, where water comprises approximately 75% of the planet's surface, life is abundant. But what about the exoplanet's star, specifically its activity and rotation?

How could this influence how exoplanets are identified for current and future missions? Now, an international team of scientists is discussing a new method for cataloging stars that could be used to fine-tune the search for habitable exoplanets. In a study, published on the arXiv preprint server, the team focuses on refining the exoplanet target list for the planned Habitable Worlds Observatory (HWO).

HWO is a planned high-profile mission not slated to launch until the 2040s, but scientists are eager to build its target list long before the spacecraft launches. This is because HWO's primary science objective is to directly image Earth-sized exoplanets that orbit in their star's habitable zone, making HWO arguably one of the most anticipated missions in the search for habitable exoplanets. For the study, the researchers conducted an extensive literature review of past studies discussing scientific knowledge of stars throughout the galaxy, specifically stars' activity and rotation properties.

A star's activity includes space weather that could cause flares and sunspots, while rotational properties are important for understanding a star's magnetic field activity. Scientists have determined that both characteristics could interfere with identifying Earth-sized exoplanets that could be orbiting in a star's habitable zone. In the end, the researchers combined all the star data they collected from the literature review into a new Activity and Rotation Catalog (ARC), whose primary purpose is to narrow potential HWO targets.

A secondary purpose for ARC will be to address unanswered research questions regarding stellar activity and rotation properties. This is because the researchers found that while about 70% of stars in target systems for HWO have had their stellar and magnetic activity measured, only about 20% have had their activity cycles measured. For context, our sun has a cycle of about 11 years in which its stellar and magnetic activity increases and decreases in one large cycle.

Essentially, only about 20% of HWO target stars have had long-term data measured. The study notes in its summary: "Understanding and constraining stellar magnetic activity is important for interpreting observed planetary atmospheres with future direct imaging missions, such as the HWO. Stellar activity can mimic or hide planetary signatures, and can affect our ability to interpret spectra that includes contributions from both the star and the planet.

In this work, we aimed to assess our current understanding of stellar activity and rotation in preparation for HWO and other future direct imaging missions." As noted, HWO isn't slated to launch until the 2040s, but this isn't stopping the scientific community from discussing and dreaming about the types of Earth-like exoplanets this much-anticipated future space telescope could identify.