We are getting closer to finding a true Earth-like exoplanet. But finding one is only half the battle. To know for sure if we are looking at an analogue of Earth somewhere in the galaxy, we also have to image it directly. That’s the job for the Habitable Worlds Observatory (HWO), a planned space-based telescope whose primary mission is to do just that. But even taking a picture of a planet and reading its atmospheric chemistry is still not enough, according to a new paper published in advance on arXiv by Ohio State’s Kaz Gary and co-authors. HWO will need to determine the mass of the planet first.
However, it is easier said than done. The paper explains why measuring a planet’s mass with an accuracy of 10% is important to understanding whether or not a planet is actually habitable. In addition to that high level of accuracy, the models used to determine what gases the atmosphere is made of suffer from a problem that mathematicians clearly call “degeneracy”. In this case, the lack means that it is impossible to know whether the main gas is present in the planet’s atmosphere – and to distinguish between nitrogen (like our atmosphere) or CO2 (like Venus’) makes a big difference.
Currently, the standard way to measure the mass of exoplanets is radial velocity (RV). This measures the “motion” of the star as the exoplanet’s gravitational pull pulls on it. But, measuring this value is notoriously difficult. An Earth-sized exoplanet orbiting a sun-like star produces an RV signal of only 9 cm/s – a very small signal that is easily drowned out by the activity of the star itself.
Fraser discusses some of the limitations of the HWO.
To make matters worse, RV is useless for a large percentage of the stars that HWO will be looking at. About 30% of the observer’s target list consists of hot, rotating A-type and F-type stars. Those types of stars have hot photospheres with few distinguishing lines. And they rotate so quickly that the data available can easily become blurry. All this adds up to making highly accurate RV measurements impossible for about 30% of the stars that HWO expects.
Enter astrology. This alternative method uses the side-to-side motion of the target star created by a planet orbiting in relation to the stars around it. It has the great advantage of being very useful for active stars that the RV cannot handle, since watching them move in one direction is much easier than watching their signatures change.
But it comes with its challenges, which, not surprisingly, come right up. The astronomical signal for an Earth-like planet 10 parsecs away is about 0.3 microarcseconds. That’s 0.3 millionths of an arcsecond. Remember that there are 1,296,000 arcseconds in the night sky and it becomes clear how accurate this instrument must be.
NASA demonstrates HWO capabilities. Credit – NASA Goddard YouTube Channel
To detect such a small change, the High-Resolution HWO instrument would have to rely heavily on the presence of background stars. In fact, the main limit of astronomy is tied to the “photon-noise” from background stars, which, in turn, depends entirely on how many of those background stars there are. That means the direction HWO is looking at will make a big difference. If it is directed towards the edge of the galaxy, the surface of the star is weak, and the instability is high. But if it is aimed at the galactic plane, there are many stars to keep the uncertainty low.
The researchers simulated the number of stars in the background in several situations, and concluded that the best way to capture the necessary information of the star without introducing a lot of noise into the instrument was to choose the right filter to reduce the instability of the stars by balancing the opposing effects of the density of the stars and the diffraction limit. They suggest using the Gaia G array – the primary broad beam used by the European Space Agency’s Gaia spacecraft, which is currently mapping the positions of more than a billion stars in our galaxy. It hits the sweet spot between longer wavelengths like the infrared, where the HWO diffraction limit itself gets worse, and shorter wavelengths, where there aren’t many background stars to use as references.
So, with a scale, direction, and spectral band to work with, HWO only needs an observation campaign. The authors propose a 200-day astronomical observation spread throughout the 5-year primary mission of the HWO. By taking about 100 observations of each target star, HWO can successfully estimate the abundance of about 40 habitable planets in Earth-like regions to the required 10% accuracy.
The HWO itself is still a long way off, however, and probably won’t start until at least the early 2040s. But, by combining advanced photometry with highly accurate stars, we may finally get the ultimate prize that astronomers have dreamed of for centuries – another habitable world.
Learn more:
K. Gary et al – Most Habitable Planets Identified by the Habitable Worlds Observatory
UT – Optical Engineering Needed to Photograph Twin Earth
UT – HWO must be the Perfect Picometer to Watch the World 2.0
UT – HWO May Find Irrefutable Signs of Life on Exoplanets
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