We're going on an Earth hunt

Two temperate super-Earths and a cold Neptune are detected through a reanalysis of archived radial velocity data.
We're going on an Earth hunt

Exoplanet science is typically portrayed publicly as an endeavor to find a second home for human beings.  That is part of the reason why discoveries of new Earth-like planets attract significant public attention. Although for many astronomers the most important contribution of exoplanet study is to help us understand how planets form and evolve, my interest in exoplanet hunt is much closer to what the public expect, namely, to find planets that are similar to our Earth. Like the story in the famous children's book "We're Going on a Bear Hunt", I view my research as a journey to hunt Earth-like planets orbiting around Sun-like stars (so-called "exo-Earths") in order to answer whether our Earth is the only habitable planet in the universe, whether life on Earth is unique, and whether there are extraterrestrial intelligent beings. 

We are Going on a Bear Hunt

Cover of the children's book "We're Going on a Bear Hunt" written by Michael Rosen and illustrated by Helen Oxenbury published in the UK in 1989

In 2019, my advisor and also a pioneer for exoplanet detection, Dr. Paul Butler, applied his pipeline to reduce the spectroscopic data obtained a decade ago by a group of European astronomers using the Ultraviolet and Visual Echelle Spectrograph (UVES) mounted on the Very Large Telescope (VLT) in Chile. This updated UVES data show significant improvement. Paul soon published his results in The Astronomical Journal. In particular, this paper was dedicated to Sandy Keiser, a Carnegie staff passing away suddenly during the analysis of the UVES data. 

ESO VLT telescope
The ESO VLT telescope complex, Paranal, Chile Credit: ESO/H.H.Heyer

By applying my Agatha pipeline to this updated UVES data in combination with radial velocity data from other instruments, I quickly found dozens of signals, among which a cold Neptune and two temperate super-Earths were identified. These results have been published in The Astrophysical Journal Supplement Series. After the publication and during the preparation of the press release for our work, I began to realize the significance of some of these planets. For example, GJ 180 d is probably the nearest temperate super-Earth which is not tidally locked to its star. Like the Earth-Moon system, the spin period of a tidally locked planet is equal to its orbital period, leading to a very hot side with permanent day and a very cold side with permanent night on a planet. Located 0.31 au away from GJ 180, GJ 180 d is probably not tidally locked to its host and thus is more likely to be habitable than other temperate Earth-like planets that are tidally locked. 

GJ 229 Ac
Artist’s concept of GJ229Ac, the nearest temperate super-Earth to us that is in a system in which the host star has a brown dwarf companion. Illustration is by Robin Dienel, courtesy of the Carnegie Institution for Science.

Another interesting planet detected in this work is GJ 229 Ac, which is located in a binary system including an M star and a brown dwarf. It is the nearest temperate super-Earth located in such systems and provides a test case for the study of habitability in binaries. In addition, GJ 433 c is the nearest and coldest Neptune, belonging to an unexplored population of Neptune-like planets. Because these planets are close to the Earth, they are perfect candidates for direct imaging by future space facilities such as WFIRST.  

I believe that our work is only a beginning of a long journey towards the discovery of a genuine exo-Earth. Whatever barriers in front of us, I believe teamwork, precision instruments, and advanced methodology are the key elements to help us "go through them". Finally, I would like to give thanks to the other members in our team who contribute to this work. They are Stephen Shectman, Jeffrey  Crane, Steve Vogt, John Chambers, Hugh Jones, Sharon Xuesong Wang, Johanna Teske, Jennifer Burt, Matias Diaz, and Ian Thompson. 

The full article is accessible via this link: https://iopscience.iop.org/article/10.3847/1538-4365/ab5e7c

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