One of the strangest things in the cosmos might be us. Among the thousands of planets known to be orbiting around other stars, a planet with Earth’s same size and similar distance from its star has yet to be found. This is likely due to the technical difficulty of finding such a sister planet, since our current telescopes can only detect planets through two main methods: wobbles and shadows. Thus, the planet that lies beneath our feet appears to be a unique and mysterious anomaly among the many cosmic wonders that have been discovered in the universe. It is this special and strange nature of our planet that makes it such a remarkable and awe-inspiring part of the cosmos; we are truly a special place in an ever-expanding universe.
The “wobble” method, or radial velocity, is an effective planet-hunting tool used to uncover the presence of orbiting planets using the subtle back-and-forth motion caused by gravity. This tugging motion between a star and its orbiting planet can be detected using special telescopes, and its size will depend on how large the planet is. This tugging motion can be measured by the dip in starlight that happens when a planet crosses the face of its star, which is known as a “transit.” The bigger the dip, the bigger the planet. This method of tracking planets allows us to discover new ones that might have been otherwise hidden from view.
Large planets are much easier to detect than small ones due to the size of their shadows, making instruments such as NASA’s TESS space telescope more capable of finding them. Small, rocky planets such as Earth are especially easily spotted when transiting small red dwarf stars, as they cast a bigger shadow that blots out more of the star’s light. This is not the case with larger stars such as the Sun, where the planet’s transit is much harder to detect. Additionally, in order to confirm an orbit around a star, a telescope would have to observe the star for an extended period of time, such as 365 days, in order to catch even one transit, and multiple transits will need to be seen in order to be sure it is actually a planet. Consequently, large planets are easier to detect than small ones, and the difficulty of detecting them increases when they are orbiting larger stars.
While the discovery of small, rocky planets orbiting red dwarf stars has been encouraging, the fact remains that distant, habitable planets around more Sun-like stars remain largely out of reach for today’s instruments. This is due to the sheer distance and faintness of these planets, which makes them hard to detect. Even if such planets were detected, their faintness in relation to their host stars would make it difficult to discern their atmospheres or other characteristics that could make them potentially habitable. Furthermore, their far distances from Earth means signals sent out to explore these planets would take a very long time to arrive and any data sent back would have a similarly long delay. All of these difficulties have placed such planets largely out of reach for today’s instruments.