How to find a planet you can’t see

Recent discoveries of exoplanets have generated excitement, raising questions about how these distant planets are found and studied. Direct imaging is difficult because exoplanets are tiny and dim compared to their bright host stars, making them easily lost in the star's glare. The Earth, for example, is 100 times smaller than the Sun in diameter and 10 billion times dimmer in reflected light, making it like a firefly next to stadium floodlights.

In the 1970s and 80s, astronomers, despite public interest in intelligent life, had not found exoplanets. Their wish list for exoplanet detection included confirming a planet's existence, determining its orbital distance to assess its temperature (habitable zone), and calculating its density to differentiate between gas giants and rocky planets. Ultimately, the goal was to find atmospheric gases like oxygen, which could indicate life.

Planets exert a gravitational pull on their host stars, causing the stars to 'wobble' around a common center of mass. Larger planets cause more significant wobbles. In 1995, Swiss astronomers used this 'wobble method' to detect the first exoplanet, 51 Pegasi b, which caused its star to oscillate every 4.23 days. This method could determine the planet's distance from its star but not its precise mass or size.

Another method is the 'transit method,' where a planet passes in front of its star, causing a slight dip in the star's brightness. This dip is very small (e.g., 1% for Jupiter and 0.01% for Earth). While initially deemed impractical, advancements in technology, especially with space telescopes like Kepler (launched in 2009), allowed for the monitoring of thousands of stars. This method provides precise size measurements, and when combined with the wobble method, allows for density calculations. Thousands of planets have been found this way, including hundreds of rocky worlds in habitable zones.

In 1999, the idea arose to study exoplanet atmospheres by analyzing starlight that passes through them during a transit. Different gases absorb different wavelengths of light, leaving unique signatures. Despite initial skepticism, the Hubble Space Telescope successfully observed the first exoplanet atmosphere. However, this technique is most effective for large planets orbiting small stars, and Earth-sized planets have signals that are currently too tiny to observe.

Direct imaging, once considered impossible, has advanced. By blocking out the star's glare with a coronograph, astronomers have captured time-lapse images of large, distant, and glowing-hot exoplanets. To improve direct imaging for smaller, cooler planets, future missions will require space-based telescopes and 'Starshades.' A Starshade is a proposed space parasol designed to block starlight, allowing a telescope to directly image Earth-like planets. This requires complex origami-like deployment and precise formation flying.

The U.S. astronomical community's 2021 Decadal Survey prioritized a telescope for observing habitable exoplanets, signaling a continued commitment to this field. While the Starshade mission was not prioritized for this decade, the journey of exoplanet discovery showcases the perseverance of scientists who defied initial doubts. The field has moved from questioning the existence of exoplanets to exploring the possibility of life beyond Earth.