Travel the universe with Dr. Ethan Siegel as he answers the biggest questions of all
The first Earth-sized planet without a parent star has just been discovered.
For countless millennia, planets beyond our Solar System were mere speculation.
Today, we know of over 4,000 confirmed exoplanets, with more than 2,500 of those found in the Kepler data. These planets range in size from larger than Jupiter to smaller than Earth. However, these methods that are so useful for detecting planets around stars, like the transit or stellar wobble methods, cannot detect rogue planets. (NASA/AMES RESEARCH CENTER/JESSIE DOTSON AND WENDY STENZEL; MISSING EARTH-LIKE WORLDS BY E. SIEGEL)
Only since the 1990s has science revealed their existence.
The radial velocity (or stellar wobble) method for finding exoplanets relies on measuring the motion of the parent star, as caused by the gravitational influence of its orbiting planets. Since the planet and star both orbit their mutual center-of-mass, the star won’t remain stationary, but will “wobble” in its orbit, with periodic redshifts and blueshifts revealing the mass and period of the orbiting exoplanet. (ESO)
Today, more than 4,000 exoplanets are known, revealed from their effects on the stars they orbit.
When planets pass in front of their parent star, they block a portion of the star’s light: a transit event. By measuring the magnitude and periodicity of transits, we can infer the orbital parameters and physical sizes of exoplanets. When transit timing varies and is followed (or preceded) by a smaller-magnitude transit, it may indicate an exomoon as well, such as in the system Kepler-1625. (NASA’S GODDARD SPACE FLIGHT CENTER/SVS/KATRINA JACKSON)
But plenty of planets should have no parent stars at all.
Perhaps surprisingly, these rogue planets should be extraordinarily common.
Particular configurations over time, or singular gravitational interactions with passing large masses, can result in the disruption and ejection of large bodies from solar and planetary systems. In the early stages of a solar system, many masses are ejected just from the gravitational interactions arising between protoplanets, leading to the existence of rogue, orphaned planets. (SHANTANU BASU, EDUARD I. VOROBYOV, AND ALEXANDER L. DESOUZA; ARXIV:1208.3713)
Many young planets get ejected as solar systems form, creating “orphaned” planets.
Although we now believe we understand how the Sun and our solar system formed, this early view is an illustration only. When it comes to what we see today, all we have left are the survivors. What was around in the early stages was far more plentiful than what survives today. (JOHNS HOPKINS UNIVERSITY APPLIED PHYSICS LABORATORY/SOUTHWEST RESEARCH INSTITUTE (JHUAPL/SWRI))
Others formed as members of insufficiently massive, failed solar systems.
This star-forming region is rich in ionizing radiation, which is blowing off the remaining gas that’s attempting to collapse to form stars. The regions that fail to gain enough mass will not become stars, but rather rogue planets and rogue planetary systems: terrestrial or gas giant planets without a parent star of their own. (ESA AND NASA)
Altogether, rogue planets should outnumber the stars in our Milky Way.
The candidate rogue planet CFBDSIR2149, as imaged in the infrared, is a gas giant world that emits infrared light but has no star or other gravitational mass that it orbits. It is one of the only rogue planets known, and was only discoverable because of its large-enough mass to emit its own infrared radiation. (ESO/P. DELORME)
Direct infrared imaging only reveals high-mass rogue planets.
Rogue planets may be numerous in the galaxy, but it surprises most to learn that there are between 100 and 100,000 rogue planets for every star in our galaxy, putting the total number of planets wandering through the Milky Way at somewhere around a quadrillion. (NASA / JPL-CALTECH)
But another method — gravitational microlensing — has begun to change everything.
Any planet passing between us and a star will gravitationally bend the intervening space.
When any mass or system of masses, whether a star, planet, solar system, or something more complex passes between a telescope and its line-of-sight towards a distant star, the intervening mass distorts the intervening space, allowing for a gravitational microlensing event to occur. (NASA’S EXOPLANET SCIENCE INSTITUTE / JPL-CALTECH / IPAC)
This magnifies, distorts, and creates multiple images of the background star.
When a gravitational microlensing event occurs, the background light from a star gets distorted and magnified as an intervening mass travels across or near the line-of-sight to the star. The effect of the intervening gravity bends the space between the light and our eyes, creating a specific signal that reveals the mass and speed of the planet in question. (JAN SKOWRON / ASTRONOMICAL OBSERVATORY, UNIVERSITY OF WARSAW)
From physics, we can then infer the rogue planet’s properties.
Rogue planets may have a variety of exotic origins, such as arising from shredded stars or other material, or from ejected planets from solar systems, but the majority should arise from star-forming nebula, as simply gravitational clumps that never made it to star-sized objects. When a microlensing event occurs, we can use the light to reconstruct the intervening planet’s mass. (CHRISTINE PULLIAM / DAVID AGUILAR / CFA)
Light curve of the ultrashort microlensing event OGLE-2016-BLG-1928, which was likely caused by a free-floating, rogue planet no more massive than planet Earth. (MROZ ET AL. 2020, ARXIV:2003.01126)
Fast imaging is a necessity: the entire event lasted just 42 minutes.
From the beginning of the event, which includes the brightening of the background star, the distortion of its position, and the appearance of a second light source, until the end, only 42 minutes elapsed. Imaging the same object repeatedly just minutes or hours apart is essential for catching these extremely rapid microlensing events. (JAN SKOWRON / ASTRONOMICAL OBSERVATORY, UNIVERSITY OF WARSAW)
NASA’s Nancy Grace Roman Space Telescope will make its microlensing observations in the direction of the center of the Milky Way galaxy. The higher density of stars will yield more microlensing events, including those that reveal exoplanets as small and low-in-mass as planet Earth. (NASA’S GODDARD SPACE FLIGHT CENTER/CI LAB)
Multiple observatories can work in tandem to identify rogue planets, orbiting exoplanets, and even multi-component systems as they create microlensing events based on the bending of space along the line-of-sight to a distant object. Better, faster, higher-quality observations can reveal superior details and lower masses for exoplanets. (KOREA ASTRONOMY AND SPACE SCIENCE INSTITUTE)
Gravitational microlensing is a powerful tool for detecting exoplanets. This illustration shows the bending of light from a background source by a planetary system in the foreground. Note that peak alignment corresponds to peak magnification: the greatest apparent brightness of the background light source. (NASA EXOPLANET EXPLORATION)
Mostly Mute Monday tells an astronomical story in images, visuals, and no more than 200 words. Talk less; smile more.
