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Published October 8, 2025

Nine targets have been officially selected for the Rocky Worlds Director’s Discretionary Time (DDT) program, and the initial observations are underway. The program intentionally combines the powerful capabilities of the James Webb Space Telescope and the Hubble Space Telescope to further studies of rocky exoplanets that orbit low-mass M-dwarf stars.

The observations, which formally began in July 2025 and wrap up in a few years, will carefully observe nine exoplanets — GJ 3929 b, LTT 1445 A c, LTT 1445 A b, LHS 1140 b, TOI-198 b, TOI-406 c, TOI-771 b, HD 260655 c, and TOI-244 b — along with their host stars.

Webb will measure the flux at 15-microns as each exoplanet is eclipsed by its parent star. The depth of the eclipse will probe the likelihood of a substantial atmosphere. Hubble’s UV-optical spectra will help us better characterize the host stars’ properties, particularly the likely level of flare activity. When the program is complete, Webb will have dedicated 500 hours of observing time with MIRI, and Hubble will have logged 250 hours with COS and STIS.

This program would not be possible without regular, ongoing input from the astronomical community. The program’s Core Implementation Team (CIT), led by Néstor Espinoza and Hannah Diamond-Lowe, is advised by a Science Advisory Committee (SAC) drawn from the user community. (See the full list of members.)

If you have contributions that you would like to make or data you could share about these targets, please contact us. We will not disclose any proprietary data. We also encourage the community to consider submitting General Observer (GO) programs for complementary observations of these targets with Hubble and Webb.

As they are captured, all Rocky Worlds observations will be immediately available for download and research in MAST. The program’s data products produced and released by STScI from Webb’s 15-micron MIRI observations include time-series photometry, eclipse depths and absolute fluxes, along with Hubble’s co-added, flux-calibrated, far- and near-UV spectroscopy of the host stars. 

Target Selection

The vast majority of rocky planets in the Milky Way orbit M dwarfs, which are smaller and cooler than the Sun and can be far more active, irritating nearby planets with high-energy X-rays and ultraviolet light. A key step in our search for life outside the solar system is learning whether a small planet can maintain an atmosphere in a brutal environment like this.

If a thin blanket of gas can survive ongoing stellar flares or if M dwarfs are not all as active as we think, it’s possible that rocky planets with atmospheres may be very common in the Milky Way. And that means there’s a good chance that at least some have the potential to harbor life. If we end up learning that “habitable zones” are rare around M dwarfs, the number of potentially habitable planets to explore in the future may be much smaller.

That’s why this DDT program is essential: It’s high risk — and therefore unlikely to win support through the highly competitive TAC process — but also potentially high reward. That’s also why we’ve designed this program to be both wide and shallow (3-sigma). These observations will help the community design follow-up observations that push what we know even further.

Looking for Exoplanet Atmospheres with Webb

Webb will gather time-series observations aimed at observing planetary secondary eclipses (when the planet passes behind its host star from our perspective) at 15 µm using MIRI. From these measurements we will be able to calculate each planet’s dayside thermal emission. Because each planet is tidally locked, most of a planet’s thermal emission will be coming from the dayside, which faces the telescope during the observations.

Webb will also allow us to infer the presence or absence of an atmosphere through comparisons to computer models. For example, if the planet has a thick atmosphere with winds redistributing heat, the dayside will be significantly cooler than it would without an atmosphere.

Hubble to Carefully Examine the Host Stars

Hubble will gather UV-to-optical spectra to characterize the high-energy output of the host M dwarfs using both COS and STIS. The versatile instrumentation will allow us to view the data as time series, allowing us to detect stellar flux variations that might arise, for example, from stellar flares, which can strip an atmosphere.

Hubble’s observations will also track changes in a star’s brightness to find out how energetic the flares are, how long they last, and how frequently they occur. The UV data will also help us estimate how much energy each star has emitted over its entire lifetime.

Over time, Hubble’s UV data may be combined with observations of the same targets from many other telescopes across the electromagnetic spectrum, which will help researchers better understand the stars and their environments.

These observations are essential for understanding the planet’s irradiation environment and only Hubble is currently capable of taking those observations. The broad wavelength coverage and resolution will constitute a legacy dataset for these exoplanet host stars.

What’s Ahead

We are planning a multi-day workshop at the end of 2026 to engage the broader astronomical community, but there will be plenty of information sessions and opportunities to engage before that. Feel free to contact us, at any time, via email.