Introduction
From the start of the HPF project, we have talked about how the instrument was designed to discover small exoplanets in the habitable zones of cool, nearby stars. In the meantime, we have shared a lot of science with you here. Exciting examples include the discovery of unexpectedly large, close-in planets around M dwarf stars, characterization of exoplanets discovered by the Kepler, TESS, and Gaia spacecraft, and a deeper understanding of the lowest-mass stars in the Milky Way Galaxy.
But until now, those potentially Earthlike habitable-zone planets have not appeared here. Why? Because finding them is a challenging, data-intensive pursuit, requiring hundreds of observations carried out over years. Today, though, we are thrilled to announce that we have discovered a potentially terrestrial-mass exoplanet orbiting within the habitable zone of the very nearby star Gliese 251. What’s more, the system is close enough to Earth that 30-meter telescopes currently in development could potentially obtain images of the planet and measure its atmosphere, searching for signs of life in the process.
The HPF Exoplanet Survey
Since HPF was first commissioned for science operations, we have been conducting a survey of nearby M dwarf stars, searching for new exoplanets using the radial velocity technique. Thanks to generous support from HPF’s host telescope–the Hobby-Eberly Telescope (HET)–and its partner institutions The University of Texas at Austin and Penn State University, we have been able to use approximately 20 HET nights per year to monitor about 50 stars for evidence of gravitational wobbles from orbiting planets.
Exoplanet surveys are an exercise in patience. As we watch our targets over years, we find candidate signals that do not pan out with repeated observations, signals that turn out to originate from the stellar surfaces rather than planets, and planetary orbits that take years just to complete a full cycle. Then, there are the signals that hold up. They require continued data collection to reach statistical significance, careful analysis to rule out false positives, and physics-based interpretation to place in a real-world context.
Despite beginning in 2019, the HPF survey is actually just now getting to the point that many of those most promising signals are reaching the point of bona fide exoplanet detections. Today’s post describes our most exciting one to date.
Artist’s conception of the Gliese 251 exoplanet system. Gliese 251 is a nearby M dwarf star, hosting two super-Earth exoplanets. The newly-discovered outer planet, Gliese 251c, is a terrestrial-mass exoplanet candidate within the liquid-water habitable zone. Image credit: Michael Marcheschi
Gliese 251
Gliese 251, or GJ 251, is a favorite target of the HPF exoplanet survey. It is a bright, nearby M dwarf, towards the hotter end of stars in our survey. At 18 light-years from Earth, it is the 74th-closest star system to our own. In a galaxy of roughly 100 billion stars, that makes us practically next-door neighbors!
GJ 251 was shaping up to be an early discovery from our survey, as we saw a number of periodic signals in the early HPF data of the star. The most obvious corresponded to a super-Earth exoplanet in a 14-day orbit around the star, but our colleagues observing the star with the CARMENES spectrometer announced the discovery of this planet before we could solidify our detection.
Immediately upon combining the CARMENES data with our own, we realized that there was more to the story. In particular, we saw evidence for a second planetary signal at a period of about 54 days. Crucially, such an orbit would place the planet right in the middle of the liquid-water habitable zone! Given such an exciting potential discovery, we intensified our observations of GJ 251, both with HPF and its optical-wavelength sister spectrometer NEID.
The Infrared Advantage
Part of the challenge of establishing the signal at 54 days as a planet candidate was separating it from nearby signals created by variability in the host star’s atmosphere. We have discussed the challenge of stellar variability many times on this blog; as stars rotate, irregularities on their surfaces produce signals that can be confused as exoplanets. These signals appear preferentially at the star’s rotation period, and fractions thereof.
Gliese 251 has a rotation period of just over 100 days, so it produces signals near that of the candidate habitable-zone planet. Here, HPF had an advantage over other exoplanet spectrometers thanks to its infrared wavelength coverage. Features like spots on stellar surfaces appear less prominently in infrared wavelengths than in visible light, so we expect that stellar signals will have smaller amplitudes in HPF data, while true planets will continue to stand out. In the case of GJ 251, we noticed that in the full collection of data, a forest of signals appeared near periods corresponding to the habitable zone. However, when restricting our analysis to HPF and another infrared instrument SPIRou, only the signal at 54 days remained. This was the key piece of evidence that a high-value exoplanet was hiding among the stellar noise.
A Potentially Habitable Planet
As a very nearby, bright M star, GJ 251 has been observed previously by several exoplanet search campaigns. For our analysis, we combined a total of almost 700 observations from five different telescope/spectrometer combinations, taken over more than 20 years. We used a statistical modeling method that accounted for the fact that stellar noise appears at different strengths depending on the color of a given spectrometer. In the end, we found that the data favored a solution with two exoplanets: the previously-known planet at 14 days, and a second super-Earth on a 54-day orbit.
The new planet, Gliese 251c, has a minimum mass of just under 4 Earth masses, placing it within the “super-Earth” class of planets like its companion planet GJ 251b. However, unlike planet b, GJ 251c orbits in the very middle of the liquid-water habitable zone, where planets receive the right amount of stellar radiation to potentially host liquid water on their surfaces. The planet’s mass–which is potentially consistent with a rocky surface like Earth–and location within the habitable zone makes it extremely interesting from the perspective of looking for life outside the Solar System. It is exactly what HPF and its exoplanet survey were designed to look for!
Orbits of the planets GJ 251b and GJ 251c around their host star. The liquid-water habitable zone is shown in green.
Of course, simply orbiting within the habitable zone is not enough to make a planet habitable. A planet must also have the right atmospheric conditions to maintain suitable surface pressures and temperatures. Together with our colleagues from the CHAMPs Consortium, we created computer models for a variety of potential atmospheric compositions for GJ 251c, and predicted how they would react with the star’s incoming radiation. What we found is that if this planet has an exact copy of Earth’s atmosphere, it will not actually be habitable for Earth-like life; the atmosphere would be too thin to provide sufficient insulation, causing the planet to freeze. On the other hand, if the planet has an atmosphere dominated by carbon dioxide–similar to Venus and Mars in our Solar System–the resulting surface temperatures will be much more Earth-like.
Outcomes of climate simulations for GJ 251c. On top, we show the approximate appearance of the planet based on four potential atmospheric compositions. Below, we show what the spectra of each of these atmospheres might look like to a future Earth-based telescope.
How will we learn more?
It could be argued that what we have learned about GJ 251c so far raises more questions than it answers. What is its true size? The radial velocity technique used to discover the planet only offers a lower limit on its mass. Which–if any–of the atmospheres we simulated is most similar to the real one? Is it habitable…or inhabited?
The good news is that the Gliese 251 system is close enough to us that we could potentially answer some of these questions in the future. The closer a planet system is to Earth, the more separated it is from its host star on our sky. That separation is crucial for efforts to take direct images of exoplanets, as even the closest habitable-zone planets will be just at the limits of our biggest telescopes.
Currently, there are three telescopes in development that are, collectively, known as “30-meter class telescopes” or, alternatively, “extremely large telescopes.” The ability to resolve objects with tiny separations on the sky–like habitable-zone exoplanets–is fundamentally dependent on telescope size, so these behemoth telescopes are the best bet to get images and spectra of exoplanets like GJ 251c. While the planet’s small size will still make this a challenging prospect, GJ 251c currently stands as one of the very best prospects for direct detection of a terrestrial, potentially habitable exoplanet. With images and spectra directly from the planet, we can evaluate whether the planet has liquid water, a temperate climate, and potentially even atmospheric signatures of life.
One wrinkle to the story is that Gliese 251 sits relatively far north in the sky. Two of the 30-meter telescopes are slated to be built in Chile, and thus will work best in the southern sky. GJ 251 is best suited for observations with the Thirty Meter Telescope, which is set to be built in the northern hemisphere. It is crucial that we also get a 30-meter telescope in the north to learn more about GJ 251c!
Want the full story?
The full details of our study on Gliese 251 were published today in The Astronomical Journal, in an article led by HPF Team Member Corey Beard. We encourage you to check it out!
Looking to the Future
The HPF project and the discovery described here were only possible through support–both direct and indirect–from the National Science Foundation and NASA. Both of these organizations are facing budget cuts from the US federal government that, if enacted, will devastate efforts to find more exoplanets like GJ 251c, and eventually to search them for signs of life.
If you find the work shared here to be valuable, we encourage you to contact your elected representatives to request that basic scientific research in the US be supported. Our search for life elsewhere in the Universe depends on the continued investment in an open, robust scientific research community.
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