Mooley Lab - Blog-exoplanet-2dec2024

In search of another earth

In Feb. 14 1990, NASA’s Voyager 1 took – The Pale Blue Dot, a photograph of our Earth, 6 billion kilometers from the sun; about which Carl Sagan wrote:

“Look again at that dot. That’s here. That’s home. That’s us. On it everyone you love, everyone you know, everyone you ever heard of, every human being who ever was, lived out their lives. The aggregate of our joy and suffering, thousands of confident religions, ideologies, and economic doctrines, every hunter and forager, every hero and coward, every creator and destroyer of civilization, every king and peasant, every young couple in love, every mother and father, hopeful child, inventor and explorer, every teacher of morals, every corrupt politician, every "superstar," every "supreme leader," every saint and sinner in the history of our species lived there – on a mote of dust suspended in a sunbeam.”

What could be more humbling than realizing how insignificant we are in the grander picture of the Universe? What could be lonelier than knowing that Earth is the only home we could possibly have? Perhaps we are not as alone as we think we are. Who knows—there might be another Earth with life, waiting to be discovered. This curiosity has driven humankind to explore beyond our Solar system for our cosmic neighbors and in 1995 the very first planet orbiting a main sequence star was discovered, an extra solar planet – Exoplanet.

51 Pegasi b was the first confirmed exoplanet detection which orbits a G type star like our sun. It had an orbital period of 4 days and a size greater than Jupiter, with half its mass. Missions like Kepler, TESS and ground-based observatories have identified about 5788 confirmed exoplanets (as of 27 Nov 2024).

How do we discover them?

The main techniques used by these telescopes are:

· Radial Velocity: The star and planet orbit their common centre of mass. This wobble of star causes Doppler shift in the light when observed by us. This periodic variation in radial velocity is used for the detection of exoplanets.

· Transit method: When a planet passes directly between a star and the observer, the star’s light gets dimmed by a measurable amount.

· Direct imaging: As the name suggest, taking the picture of exoplanets after removing or blocking the extreme brightness of the star.

These techniques have an inherent bias towards heavier planets. There are other emerging techniques like gravitational lensing and astrometry.

Distribution of exoplanets based on their orbital periods and masses, categorised by discovery methods. The majority of discoveries are made using the transit and radial velocity techniques, highlighting their effectiveness in detecting planets of varying sizes and orbital distances.

Source of Fig 1: https://science.nasa.gov/exoplanets/discoveries- dashboard/

With the tremendous increase in the number of exoplanet discoveries, the focus has shifted towards finding habitable worlds. Conventionally a habitable world is one where we can find liquid water on the surface. A habitable zone is a region around the star where a planet has just the right amount of surface temperature to have liquid water on the surface. As most of the planets use their star as the main energy source, the habitability zone is necessarily decided by the distance from the star. Smaller stars have narrower Habitable zone closer to them compared to stars like our sun owing to their lower luminosity. The star type also has a say in deciding if it can host a habitable world. O type and B type are so short lived that it is hard to expect life to form and thrive in such short span. F, G and K type star are the most promising types. Although M type show potential their low luminosity with close orbit to star expose planets to harmful radiation.

The habitable zone, light blue depicts the conventional habitable zone while yellow and dark blue shows the inward and outer extensions of habitable zone. A few known exoplanets with mass less than 10 Earth masses and radius less than 2.5 Earth radii (from Rein H., http://arxiv.org/abs/1211.7121 (2012)) are shown in symbols.

Source of Fig 2: Seager, Sara. (2013). Exoplanet Habitability. Science (New York, N.Y.). 340. 577-81. 10.1126/science.1232226. https://doi.org/10.1126/science.1232226

The diversity of exoplanets and improved understanding of the factors affecting habitability has led to redefining the habitable zone. The presence of water vapour and other gases in the atmosphere were also taken into consideration. The surface temperature of the planet is mainly controlled by the greenhouse gases present in the planets atmosphere. The most important of them is the molecular Hydrogen. The potency of Hydrogen as a greenhouse gas is far more than carbon dioxide. As Hydrogen is a light gas, it is likely to be found on massive and colder planets. This extends the outer edge of habitable zone beyond the conventional limits. The inner edge is determined by the presence of water vapour. For Sun-like systems, the updated definition of the habitable zone has shifted it from 0.99–1.7 AU to a much broader range of 0.5–10 AU.

From measurements of mass and radius, we can estimate the density and type of exoplanets. but it’s still hard to probe their atmospheres. Direct imaging and transit spectroscopy are the methods used to observe the atmospheres. The next step in the search for extraterrestrial life involves identifying biosignature gases—gases produced by life processes. To be detectable remotely, these gases must accumulate in significant amounts in the atmosphere. Only globally mixed and spectroscopically active gases will appear in the observed spectrum. On Earth, oxygen and ozone are the two dominant biosignature gases.

Even with these advancements, there is no guarantee that a planet in the habitable zone is habitable, for example our Venus. Habitability is very planet specific. There is no Universal habitability condition applicable to all planets. We may be still far from finding life beyond Earth but we have come a long way. Who knows? In the coming years, we might discover that Earth is not the only home we have ever known!

References:

· https://science.nasa.gov/exoplanets/

· Sara Seager, Exoplanet Habitability. Science 340,577-581(2013). DOI:10.1126/science.1232226

· Perryman M., 2018, The exoplanet handbook. Cambridge University press.

· Rosen, Philip & Liu, Christina & Wen, Qianzhuang & Chen, Yanbei. (2024). Analysis of Habitability and Stellar Habitable Zones from Observed Exoplanets. 10.48550/arXiv.2408.09296.

· Mazevet, S., Affholder, A., Sauterey, B., Bixel, A., Apai, D., & Ferriere, R. (2023). Prospects for the characterization of habitable planets. Comptes Rendus. Physique, 24(S2), 1-16.

Posted by Ajisha C Sajayan on Dec 2nd, 2024

Phd student

Optical Astronomer