From Hot Jupiters To Lava Worlds: Meet The Universe's Strangest Exoplanets

Bengaluru: The universe is vast and possibly infinite, with countless Earth-like planets orbiting distant stars. Giordano Bruno, a 16th-century Italian cosmological theorist, was among the first to propose that planets orbit stars beyond our solar system. Today, such planets are known as exoplanets or extrasolar planets—planets that orbit stars other than our Sun.

Exoplanets exhibit remarkable diversity. They range from gas giants to Neptunian worlds, terrestrial planets, and super-Earths. Many are Earth-sized and orbit within the habitable zones of their stars, making them potential candidates for supporting life. The first confirmed discovery of an exoplanet orbiting a Sun-like star came in 1995. Named 51 Pegasi b, this massive planet is similar in size to Jupiter and orbits its star in just four days. Due to its proximity, it is extremely hot and falls under the category of "hot Jupiters."

Swiss astronomers Michel Mayor and Didier Queloz made this groundbreaking discovery, earning a half-share of the Nobel Prize in Physics in 2019. Before this, planet detection beyond our solar system lacked both theoretical support and technological means. Although exploration began in the 1960s, it took nearly four decades to achieve a breakthrough. Some exoplanets orbit so close to their stars that their surfaces are molten, earning the name "lava worlds". Others, orbiting at great distances, are frozen and are called "icy planets".

Map of known exoplanet-hosting stars (gray dots) in the Milky Way. Green-yellow crossed circles highlight stars with planets located within their habitable zones. (Image Credits: Ravinder K Banyal/ IIA)

Exoplanets range in size, with Earth-sized planets nearly six times more common than massive ones. Most of these lie in the size range between Earth and Neptune and are termed "super-Earths". Unlike our solar system, where planets orbit at significant distances, most exoplanets revolve much closer to their stars, often within 0.1 AU (Astronomical Unit), leading to shorter orbital periods of just 8 to 12 days.

Like our solar system, stars in multi-planetary systems can host several planets. To date, around 700 such systems have been confirmed. Notably, Proxima Centauri, our closest stellar neighbour, is a multi-planetary system with three planets. TRAPPIST-1 is another extraordinary system featuring seven rocky planets orbiting a star cooler and smaller than the Sun.

In an interview with ETV Bharat, Professor Ravinder K Banyal, an exoplanetary scientist at the Indian Institute of Astrophysics (IIA), discussed the progress and challenges in exoplanet science. He emphasised the recent emergence of this field compared to traditional astronomy. Banyal, while talking about the most surprising discovery in exoplanet research until now, said that earlier assumptions believed planetary systems elsewhere would mirror our solar system. However, discoveries revealed a wide variety of planetary architectures, varying significantly in size and orbital distance.

Kepler Space Mission

Talking about the Kepler space mission, the professor said that the Kepler mission, launched by NASA, marked a turning point in exoplanet research. It monitored a fixed region of the sky for three years, discovering over 2,000 exoplanets that are not actually in our solar system. Kepler aimed to find Earth-like planets in the habitable zone. Earth is located approximately 149.6 million kilometres or 1 AU from the Sun and takes 1 year to go around it.

"Kepler used the transit method, detecting dips in starlight when planets passed in front of their stars. For authenticity, Kepler required at least three transit observations. Although most target stars were Sun-like, Kepler found that many were more active, causing fluctuations in starlight and complicating transit detection. Despite these limitations, including a failure in one of its reaction wheels, Kepler revolutionised our understanding of exoplanets. It provided insights into planetary frequency, size distribution, and system architecture," Banyal said.

Notably, most detected exoplanets were in compact systems, meaning they were much closer to the stars compared to the planets seen in our solar system and fell in the size range between Earth and Neptune—a size absent in our solar system," he added.

Search for Earth-sized habitable planets

Banyal further said that researchers are now focusing on finding potentially habitable planets. These are Earth-sized and situated at the right distance from their stars, allowing liquid water. That is the primary criterion for a planet to be habitable. While many such discoveries have been identified in different sub-areas of exoplanets, confirming habitability still remains a challenge.

He said that the James Webb Space Telescope (JWST) is advancing exoplanet research by analysing planetary atmospheres. Earlier efforts mainly focused on the discovery of new exoplanets; now, the emphasis is on characterisation and the planets’ atmospheres.

"JWST can analyse light filtered through a planet's atmosphere during transits or observe planet light directly when starlight is blocked, revealing molecular compositions. Future missions like PLATO and ARIEL (launch expected in two years) aim to further explore exoplanet atmospheres, especially of Earth-like planets," Banyal said.

NASA’s ambitious Habitable Worlds Observatory (HWO) is still in the conceptual phase, but it is designed to detect Earth-sized planets at 1 AU from their stars. Currently, telescopes primarily study gas giants like Saturn and Jupiter, as Earth-sized signals are too faint. The professor said that technological limitations remain a significant barrier. Earth-like planets are small, and detecting their weak atmospheric signals is challenging.

Methods to detect exoplanets

Emphasising the methods to detect exoplanets, Professor Banyal said that over six methods are used to detect exoplanets, mostly indirectly. Direct imaging, where the planet is visually isolated from the star, is rare and works only for young, massive planets far from their stars. The transit spectroscopy method is the most commonly used. It measures dips in brightness as planets transit their stars, offering information on size, orbital period, and atmospheric composition. This drop in light owing to the planet’s obstruction can be easily measured by sensitive photodetectors. When starlight passes through a planet's atmosphere, it picks up specific signatures from the atmospheric gases. These signatures reveal the types of molecules and chemical species present on the planet. However, it is more effective for large planets close to their stars, like Jupiter.

The graphic at top shows a model spectrum containing the signatures of gases the astronomers would expect to see if the exoplanets’ atmospheres were puffy and dominated by primordial hydrogen from the distant worlds’ formation (Credit: NASA/ESA/Z. Levy (STScI))

The professor also explained the radial velocity method, where a star's movement due to gravitational pull from orbiting planets causes a Doppler shift in its light. This method reveals a planet's mass and orbital period. However, it is time-consuming as it examines one star at a time. In contrast, the transit method observes thousands of stars simultaneously and helps determine a planet's relative size.

He said that to date, nearly 6,000 exoplanets have been discovered, with about 80 per cent found using the transit method. Combining data from both transit and radial velocity methods allows scientists to calculate a planet’s density, helping distinguish between rocky and gaseous planets.

Limitations of current telescopes

Banyal also highlighted the limitations of current telescopes. "Telescope aperture affects resolution; larger apertures improve clarity. Blocking starlight is another challenge, as stars outshine planets by a million times, often obscuring them," he said, adding that direct imaging uses coronagraphs to suppress starlight and capture images of exoplanets. This method is limited to massive, hot planets orbiting far from their stars. The proposed Habitable Worlds Observatory aims to overcome this with a space-based telescope equipped to block starlight and capture direct images of Earth-sized planets.

IIA's exoplanet research

Discussing IIA’s contribution in exoplanet research work and collaboration with foreign organisations, Professor Banyal said that the IIA is contributing to exoplanet research by working on a ground-based 10-meter class telescope through which massive and sophisticated instruments can be put on Earth. In a significant international collaboration, IIA is partnering with leading US institutions, such as the University of California, Santa Cruz (UCSC), Caltech, and the University of California Observatories (UCO) through the renowned Keck Observatory in Hawaii — a 10-meter optical/infrared telescope at the forefront of observational astronomy and exoplanet research.

As part of efforts to enhance Keck’s capabilities for direct imaging of exoplanets, IIA has contributed critical subsystems—including the cold-stop rotator, filter wheels, and the imager channel to the upcoming exoplanet instrument on Keck, known as SCALES. This collaboration marks a major leap in India’s role in cutting-edge astronomical instrumentation and exoplanet exploration.

Human colonisation of exoplanets is a distant dream

When asked if humans will ever colonise an exoplanet, the Professor said that colonisation remains a distant dream due to technological constraints, particularly the inability to travel faster than light.

A key goal remains identifying planets within the habitable zone, where conditions might support liquid water. Spectroscopic analysis could detect biosignatures like oxygen, methane, or water vapour. Scientists use various methods to assess habitability, including detecting techno signatures, analysing laser and radio signals, and atmospheric studies. Despite progress, challenges persist, especially in deploying large instruments in space and enhancing sensitivity to weak signals from small planets like Earth, he mused.

"I believe that exoplanet science is rapidly evolving, shifting from detection to detailed study of planetary characteristics and atmospheres. Missions like Kepler and JWST, along with ground-based efforts, have transformed our understanding of the cosmos. While we have yet to find definitive evidence of life beyond Earth, the search continues with ever-improving tools and expanding frontiers," Banyal said.