Space Telescopes and Exoplanet Exploration

Why in News?

The study of exoplanets—planets orbiting stars outside our Solar System—has entered a new phase. 

Important Point 

• The next generation of observatories will take this search further. NASA’s Nancy Grace Roman Space Telescope is currently scheduled for launch on August 30, 2026, while ESA’s latest schedule places PLATO’s launch in March 2027.
• Habitable Worlds Observatory are expected to deepen the search for potentially habitable worlds.

What is an Exoplanet?

An exoplanet or extrasolar planet is a planet located outside our Solar System. Most known exoplanets orbit stars, although planetary-mass objects can also exist without being gravitationally bound to a star. Exoplanets vary enormously in size, temperature, composition and orbital characteristics. They include:

• Rocky Earth-sized planets
• Super-Earths
• Sub-Neptunes
• Neptune-like planets
• Gas giants
• Hot Jupiters

The ultimate scientific objective is not merely to count these worlds but to understand how planetary systems form, how common Earth-like environments are, and whether conditions suitable for life exist elsewhere.

How are Exoplanets Detected?

Because stars are extremely bright and planets are comparatively small and faint, directly observing an exoplanet is difficult. Scientists therefore often detect planets indirectly through their effects on their parent stars.

1. Transit Photometry: When a planet passes between its parent star and a telescope, it blocks a tiny fraction of the star’s light.

• This temporary decrease in brightness is known as a transit.
• Star → Planet passes in front → Brightness decreases → Transit detected
• The amount of light blocked helps scientists estimate the radius or size of the planet.
• NASA’s Kepler and TESS missions have extensively used this technique. TESS continues to search for transiting planets around relatively nearby bright stars.

2. Radial Velocity or Doppler Method: A planet and its star actually orbit their common centre of mass. Consequently, the planet’s gravity causes the star to make a small periodic wobble. Scientists observe changes in the star’s spectrum:

• Moving towards us → Blueshift
• Moving away from us → Redshift
• These Doppler shifts can reveal the presence of an orbiting planet and help determine its minimum mass.

3. Direct Imaging: Instead of detecting a planet indirectly, scientists sometimes capture light coming from the planet itself. The major difficulty is the enormous brightness of the parent star.

Special instruments such as coronagraphs suppress the star’s light so that faint nearby planets can be studied. NASA’s Roman telescope will carry a Coronagraph Instrument as a technology demonstration, advancing techniques needed for future direct imaging of exoplanets.

4. Gravitational Microlensing: According to Einstein’s theory of general relativity, gravity can bend light.

• If the foreground star has a planet, the planet can create an additional signature in the magnification pattern.
• Microlensing is particularly useful for detecting planets in wider orbits that can be difficult to find using the transit method. NASA’s Roman mission will conduct a major microlensing survey.

5. Astrometry: Astrometry measures extremely small changes in the position of a star across the sky. An unseen planet can gravitationally pull its parent star, producing a tiny positional wobble. ESA’s Gaia mission, designed to map stellar positions and motions with extraordinary precision, has made astrometry an important tool for investigating planetary systems.

Historic Exoplanet Space Missions

Upcoming Exoplanet Missions

Nancy Grace Roman Space Telescope

• Agency: NASA
• Current scheduled launch: August 30, 2026
• Major Exoplanet Technique: Gravitational Microlensing
• Additional Capability: Coronagraph technology demonstration
• Roman will have a field of view at least 100 times larger than Hubble's.
• Its microlensing survey will investigate planetary systems across the Milky Way, while its coronagraph will demonstrate advanced technology for suppressing starlight and directly observing exoplanets and planet-forming disks. 
• NASA currently expects Roman to discover more than 100,000 exoplanets, including planets identified through both microlensing and transit observations. 

PLATO

• Full Name: PLAnetary Transits and Oscillations of stars
• Agency: ESA
• Current planned launch: March 2027
• Destination: Sun-Earth L2
• Number of cameras: 26
• PLATO will concentrate on terrestrial planets in orbits extending into the habitable zones of Sun-like stars. It will simultaneously study host stars through asteroseismology, allowing scientists to determine stellar properties more accurately.ESA says PLATO will observe more than 200,000 stars. 

Ariel

• Full Name: Atmospheric Remote-sensing Infrared Exoplanet Large-survey
• Agency: ESA
• Planned launch: 2029
• Ariel represents another major transition in exoplanet research.
• Rather than primarily searching for planets, it will conduct a systematic chemical census of exoplanet atmospheres.
• ESA plans for Ariel to observe approximately 1,000 exoplanets, ranging from rocky worlds to gas giants. 

Habitable Worlds Observatory

• NASA’s proposed Habitable Worlds Observatory (HWO) represents an even more ambitious stage in exoplanet science.
• Its major scientific objective would be to directly investigate Earth-like planets around Sun-like stars and search their atmospheres for potential signs associated with habitability or life.
• NASA’s technology planning currently describes HWO as a future observatory recommended for a 2040s launch timeframe, rather than a confirmed late-2030s launch.
• Its ultimate question is simple but profound: “Are we alone in the Universe?”

Key Exoplanet Missions at a Glance

Challenges

• Despite extraordinary technological advances, exoplanet astronomy faces major limitations.

• Earth-sized planets are extremely faint compared with their parent stars.
• Atmospheric signals can be extraordinarily weak.
• Stellar activity can mimic or contaminate planetary signals.
• Habitable-zone location does not guarantee actual habitability.
• Potential biosignatures may also have non-biological origins.
• Direct imaging of an Earth analogue around another Sun-like star requires extremely advanced starlight suppression.

Therefore, discovering a planet is much easier than determining whether it actually supports life.

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