New exoplanet survey method finds high rates of closely orbiting planets

Up until now, exoplanet surveys have mostly focused on nearby, bright stars that are sun-like or are red dwarfs, which are known to frequently host planets. While astronomers have discovered thousands of planets this way, a new study, published in the Monthly Notices of the Royal Astronomical Society, seems to have found a more efficient way of detecting planets that orbit close to their stars.

Low magnetic activity as a clue for evaporating planets

Methods for detecting planets involve measuring the properties or changes in light coming from their host stars. Most planets that have been detected have been high-mass planets that orbit close to their host star, because they induce a larger effect on the light detected from Earth, making them easier to find. However, such a close orbit has detrimental effects on these planets. Scientists have noticed that some of these close-orbiting stars are also losing mass due to irradiation from the nearby star.

This mass, in the form of gas and dust, leaves the planet’s surface and atmosphere and ultimately ends up enveloping the star. While it’s not enough to obscure the star significantly, the dust and gas do affect the light leaving the star. In particular, it appears to lead to the reduction of measurable magnetic activity by absorbing certain emission lines. Thus, astronomers can detect which stars are stealing material from their planets by looking for which stars have “anomalously low” magnetic activity through changes in these emission or absorption lines.

The researchers involved in the new study decided to use this idea to target closely orbiting planets more efficiently. First, they target stars with apparently low magnetic activity, and then apply the usual methods of planet detection, particularly, the radial velocity method, which detects how a planet wobbles around from the gravitational effects of planets.

“This potentially provides an efficient short-cut to finding particularly rare and interesting exoplanets orbiting bright, nearby stars,” the study authors write.

The Dispersed Matter Planet Project

The team calls their project the Dispersed Matter Planet Project (DMPP). DMPP began their study by surveying 24 stars with more than 10 radial velocity observations each, using High Accuracy Radial velocity Planet Searcher (HARPS) and Echelle SPectrograph for Rocky Exoplanets and Stable Spectroscopic Observations instrument (ESPRESSO) spectrographs. They also compared planet occurrence rates with other major radial velocity surveys, correcting for differences in orbital assumptions.

The researchers discovered seven new planets in five different star systems and were also able to update orbital parameters, like orbital periods and planet masses, for several known systems. Of the newly discovered planets, all were significantly larger than Earth and had orbital periods of 63 days or less (six had less than 37 day periods).

The team says their method resulted in much higher detection rates than other radial velocity surveys. “DMPP is approximately 40% more efficient purely in terms of observations per planet detection. However, we note that this is calculated assuming a standard integration time for each of the observations in the aforementioned survey,” the study authors write.

Occurrence rates for planets with periods of less than 50 days and masses of 3–10 Earth masses was 83%. For planets of 10–30 Earth masses, occurrence rates were 27%, and 13.9% for 30–100 Earth masses. The team notes that the results suggest that our galaxy may be teeming with close-in, evaporating planets, especially around certain types of stars.

Improved photometric techniques, such as those from upcoming missions, may detect transits of these planets as well. The team involved in the study hopes to find additional planets with continued monitoring and more systematic observations in the near future.

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