Lars Buchhave_The Prevalence of Earth Like Exoplanets and the Search for Biosignatures | Carlsbergfondet
Til projektoversigt

The Prevalence of Earth-Like Exoplanets and the Search for Biosignatures

Andet forskningsprojekt | 05/02/2018

Figure 1. Artist conception of Kepler-62f, a super-Earth-size planet in the habitable zone of a star smaller and cooler than the Sun, located about 1,200 light-years from Earth in the constellation Lyra. Image credit NASA.

One of the greatest questions in the history of mankind is whether life exists elsewhere in the Universe. Astonishingly, a clear path towards resolving this question has recently emerged. Recent advances field of exoplanets have revealed an astounding prevalence of Earth-like planets, allowing us to discover and characterise temperate Earth-like exoplanets around the closest and thus brightest stars.

By Professor Lars A. Buchhave, DTU Space, National Space Institute, Technical University of Denmark

One of the greatest questions in the history of mankind is whether life exists elsewhere in the Universe. Astonishingly, a clear path towards resolving this question has recently emerged. In Earth’s atmosphere, oxygen is strongly out of chemical equilibrium due to the presence of biological life. 

Such gasses, called biosignatures, can be sought in the atmospheres of Earth-like exoplanets (planets orbiting other stars than the Sun), but abundant photons originating from the exoplanetary atmosphere are required to make such difficult measurements. 

Recent advances field of exoplanets have revealed an astounding prevalence of Earth-like planets, allowing us to discover and characterise temperate Earth-like exoplanets around the closest and thus brightest stars. Characterising the atmospheres of these nearby planets with next-generation instrumentation currently being planned or implemented will allow us to detect the presence of biosignatures in the atmospheres of these planets in the near future. 

The largest missions being planned will be capable of finding dozens of exoEarths and unambiguously detect the presence of biosignatures in their atmospheres if they are present. This allows us to determine if biological life like we know it on Earth is common in our Galaxy, or if Earth’s biosphere is rare.

“Recent advances field of exoplanets have revealed an astounding prevalence of Earth-like planets, allowing us to discover and characterise temperate Earth-like exoplanets around the closest and thus brightest stars. Characterising the atmospheres of these nearby planets with next-generation instrumentation currently being planned or implemented will allow us to detect the presence of biosignatures in the atmospheres of these planets in the near future.” - Lars A. Buchhave

The Prevalence of Earth-Like Exoplanets

The exoplanet field was set in motion about two decades ago by the discoveries of a completely unexpected type of planet now coined hot-Jupiters. We now know that these Jupiter-type exoplanets orbiting in close proximity to their host star are rare, with about 1% of stars hosting hot-Jupiters.

LUVOIR

LUVOIR (The Large UV/Optical/IR Surveyor) is a concept for a large, multi-wavelength 12-16 m in diameter space observatory. LUVOIR will search for habitable conditions and signs of life on dozens of potentially habitable worlds beyond the Solar System.

With the discovery of thousands of small transiting exoplanets by dedicated space-based missions like NASA’s Kepler Mission and ESA’s CoRoT, a new class of small planets has emerged. In contrast to the hot-Jupiters, these small planets with radii less than four Earth radii (RP < 4 R) have turned out to be astonishingly common in our Galaxy. 

We now know that the majority of stars harbour small exoplanets and large fraction of planets are found in compact multi-planet systems with very short orbital periods. In fact, nearly two-thirds of the planets discovered by Kepler range in size between 1 and 4 R
"This unexpected population of super-Earths and sub-Neptunes - larger than Earth but smaller than Neptune (3.9 R) - is completely absent in the Solar System, which furthermore lacks planets with orbital periods shorter than the 88-day orbit of Mercury." - Lars A. Buchhave

The discovery missions yield only planetary sizes and periods. These provide crucial statistical data such as occurrence rates of exoplanets, but no information on the composition and properties of the planets. 

While many of the hot-Jupiter type planets have been characterised by subsequent follow-up observations, the nature of the smaller exoplanets (1 - 4 R) is almost entirely unknown: rocky terrestrial super-Earths, mini-Neptunes with extended hydrogen/helium envelopes, and water dominated planets are all conceivable compositions.

To understand the nature of this emerging planetary regime where the majority of exoplanets and our own Earth reside, we must evolve from mere discovery to a detailed characterisation of these worlds.

Figure 2. Artists conception of LUVOIR (the Large UV/Optical/IR surveyor). LUVOIR is a large, multi-wavelength 12-16 m in diameter space observatory. LUVOIR will search for habitable conditions and signs of life on dozens of potentially habitable worlds beyond the Solar System. Image credit NASA.

Characterising Small Exoplanets

In our research group, we focus on the two most promising avenues for characterisation of these small planets: 1) determining the bulk composition of small exoplanets via precise mass measurements and 2) measuring the composition of their exoplanetary atmospheres. 

Such research will provide a critical missing link in our understanding of planet formation, evolution and the interior structure of small planets.

Biosignatures

Biosignatures are signatures in the atmospheres of exoplanets indicating the presence of biological life. In Earth’s atmosphere, oxygen is such a biosignature produced primarily by biological life and is strongly out of chemical equilibrium. There is no abiotic mechanism capable of producing oxygen in the abundance required to sustain its high contrition in Earth’s atmosphere (21%).

Precise mass measurements will allow us to distinguish between rocky and gaseous planets, and in turn, enable true estimates of how common rocky and potentially habitable worlds are in our Galaxy. 

Furthermore, studying the physics and chemistry of exoplanetary atmospheres will allow us to infer the diversity of the small planets and discover what types of planetary environments are common in our solar neighborhood. 

Such research will pave the way for the first detection of life outside the Solar System, since mass measurements (thus the scale height of the atmosphere) and a detailed understanding of atmosphere observations and modeling are required to detect biosignatures.


Lars A. Buchhave about the Grant from the Carlsberg Foundation

The Carlsberg Foundation grants received by our group have allowed us to attract top level scientists in the field of exoplanetary atmospheres and mass measurements and enabled us to upgrade the spectrograph at the Nordic Optical Telescope (NOT) to allow mass measurements of smaller planets.

As a result, the NASA TESS Mission, which is an all-sky transiting planet survey satellite to be launch in March 2018, has purchased observation time at the NOT that will be led by our group to measure the mass of the planets discovered by TESS. 

“LUVOIR will thus be able to detect signs of life outside the Solar System, if they are present, and furthermore provide an unequivocal answer to whether life as we know it on Earth is widespread in the galaxy or if Earth’s biosphere is unique.” - Lars A. Buchhave

TESS is predicted to discover close to one hundred Earth-sized planets (RP < 1.25 R), many hundreds of super-Earths (1.25 R < RP < 2 R) and thousands of sub-Neptune sized planets (2 R < RP < 4 R) orbiting bright host stars, which will be prime targets for characterisation.

Searching for Biosignatures

Currently, only a handful of planets orbit host stars bright enough to allow the photon-starved endeavor of measuring their atmospheres, and the majority of these are large Jupiter-sized planets. As targets from TESS accumulate, a unique opportunity to explore the atmospheres of smaller planets will emerge. 

With the arrival of new facilities like the 6.5 meter spaced-based infrared telescope JWST in 2019 and later the massive ground-based 39-meter ELT in 2024, we will, for the first time, be able to probe the atmospheres of super-Earth type planets, which are so abundant in our Galaxy, but we know very little about. 

Further ahead, a concept for a next generation space-based telescope is being studied for NASA’s Decadal Survey in 2020 with the prime focus of detecting biosignatures in Earth analogues orbiting solar type stars. LUVOIR (The Large UV/Optical/IR Surveyor) is a concept for an enormous 12-16 m space-based telescope, which will characterise dozens of habitable zone Earth-like exoplanets and be capable of detecting biosignatures in atmospheres of these exo-Earths. 

LUVOIR will thus be able to detect signs of life outside the Solar System, if they are present, and furthermore provide an unequivocal answer to whether life as we know it on Earth is widespread in the galaxy or if Earth’s biosphere is unique.