New roadmap to finding and assessing Earth-like exoplanets for habitability

Wednesday, September 01, 2021 

by Katy Cain

Since the first exoplanet around a Sunlike star was discovered in the ‘90s, astronomers have been on the search for habitable worlds like our own. We’ve confirmed more than 4,000 exoplanets since then, mostly unlike anything in the Solar System. The Kepler mission alone detected 2,662 exoplanets using the transit method—measuring the dimming of light as a planet passes in front of a star. But the transit method can’t tell the whole story when it comes to habitability.

This year, the Extreme Precision Radial Velocity Working Group (EPRV-WG), including Carnegie’s Johanna Teske, published their final report, which could transform the way we find and evaluate Earth-like exoplanets. Carnegie’s Alan Boss chaired NASA’s standing Exoplanet Exploration Program Technical Assessment Committee that reviewed and assessed the report prior to its release.

Mass is the key to habitability

In order to determine whether a planet might be able to host life, we have to know its mass—a tricky feat through a telescope looking at stars that are light-years away.

A planet’s mass can tell us a lot about its developmental history, composition, and even its atmosphere. While we can discover potentially Earth-like planets via direct observation or the transit method, there are only two known ways we might be able to directly measure the masses of these target exoplanets: astrometry and the radial velocity method.

These methods could allow scientists to both find Earth-like planets and measure their mass. However, both of these methods need significant development in order to make them work for smaller terrestrial planets like Earth. The astrometric method would have to take its measurements from space, which would require a dedicated multi-billion-dollar mission. Alternatively, the radial velocity method might allow scientists to find and assess these planets from the ground on a pre-existing network of telescopes.

Taking radial velocity to the extreme

Using the radial velocity method for finding exoplanets, scientists look for a slight wobble in their host star, which indicates a planet’s gravity pulling on the star as it orbits. Scientists can then study the periodic shift in the light coming from a star to measure its velocity, and using Keplerian laws of motion can then deduce the mass of the planet.

Today’s precision radial velocity methods can measure this wobble down to precisions of around 1 m/s, which can sense larger planets like Neptune, “super-Earth” planets, and even Earth-mass planets around small, red dwarf stars. But in order to find the smaller Earth-like planets around Sun-like stars, that precision is going to have to be less than 10 cm/s—known here as Extreme Precision Radial Velocity (EPRV).

To give humanity its best chance at building the community, instrumentation, and data needed to achieve EPRV, NSF and NASA put together the EPRV-WG. This group of scientists was charged with creating a blueprint for a community-based EPRV initiative. 

What’s in the report?

The main finding of the study is that an EPRV search for Earth-like planets would be doable with today’s telescopes, but we still have a long way to go before we can use them to readily find and measure these Earth analogs. First, stellar variability mitigation and instrument precision goals must be met. In fact, a good portion of the report is dedicated to emphasizing the importance of coordinated, large-scale, long-term research efforts to mitigate challenges with stellar variability and build confidence in EPRV data.

Various sources of RV uncertainty, grouped into three broad categories: photon noise, facility, and stellar variability. Above graphic is Figure 1-1 of the EPRV-WG report. Figure by Sam Halverson. 

The report also states that without an increase in the size and organization of the current EPRV community, the plan simply won’t work. The report highlights a focus on early-career retention of expertise as a possible solution.

The group breaks the next fifteen years of EPRV advancement into a three-stage roadmap. Stage one is five years of coordinated work needed to determine if it’s even possible to build the community architecture and mitigate the report-identified sources of systematic errors including stellar variability—things like solar flares—and instrumentation stability. If those goals can be met, the project would move on to stage two with a high chance of success.

Stage two would be a 5-10 year precursor survey of stars that we would be able to directly image through next-generation imaging missions like LUVOIR or HabEx. This survey would use existing technology and allow scientists to discover and work through problems and develop mitigation strategies in preparation for the full survey.

Finally, stage three would be a full EPRV survey, which would use upgraded instrumentation and the analysis techniques developed during Stages 1 and 2 to both detect and measure masses of Earth-like planets around target stars from the precursor survey.

This report helps further the astronomical community along the path to discovering and assessing a hidden abundance of habitable worlds in the near future.

Overall EPRV Roadmap timeline. This graphic is figure 6-1 from the EPRV-WG report. 

For more information, find the full report on the NASA website.