EXOPLANETS

When Hubble launched in 1990, we didn’t even know for certain that there were planets beyond our solar system. Today, thousands of exoplanets (planets around other stars) are known to exist. Hubble works alongside other telescopes to satisfy our curiosity about worlds beyond our solar system. Most are found by other telescopes, while Hubble is used to look more closely and probe more deeply.

Our understanding of planets beyond our own solar system is still in its infancy. Because planets in other solar systems are extraordinarily difficult to see directly, astronomers have had to come up with innovative ways to hunt for them. Only recently have our technology and techniques been up to the task of finding exoplanets.

Telescopes on the ground and in space have uncovered thousands of planets beyond our solar system. Hubble is helping to answer questions such as:

• Are there habitable planets outside our solar system?
• What does Hubble tell us about exoplanets?
• How do exoplanets form?

Are There Habitable Planets Outside Our Solar System?

For as long as we’ve gazed up at the stars, we’ve pondered this question: Is there life elsewhere in the universe? One way to begin to explore the answer is to determine what is in a planet’s atmosphere.

This quest took a huge leap forward in 2000 when Hubble studied the exoplanet HD 209458 b, the first extrasolar planet known to make “transits” across the face of its star. Hubble became the first telescope to directly detect an exoplanet’s atmosphere and survey its makeup. As a planet passes between its star and us, a small amount of light from the star is absorbed by the gas in the planet’s atmosphere, leaving chemical “fingerprints” in the star’s light.  Researchers analyzed starlight passing through HD 209458 b’s atmosphere to try to decipher what it is made of.

Since then, astronomers have used Hubble to investigate many planetary atmospheres. In the atmosphere exoplanet HD 189733 b, located 63 light-years away, Hubble detected methane. This was the first organic molecule identified in the atmosphere of a planet outside our solar system.

In 2018, astronomers Hubble conducted the first spectroscopic survey of several Earth-sized planets orbiting in their star’s habitable zone, a region at a distance from the star where liquid water, the key to life as we know it, could exist on the planets’ surfaces. Only 40 light-years away — a stone’s throw on the scale of our galaxy — scientists found seven planets orbiting the red dwarf star TRAPPIST-1. Four of these planets lie in the star’s habitable zone.

Hubble revealed that at least three of the habitable-zone exoplanets do not seem to exhibit puffy, hydrogen-rich atmospheres similar to gaseous planets such as Neptune. This means the atmospheres may be more shallow and rich in heavier gases like those found in Earth’s atmosphere, such as carbon dioxide, methane and oxygen.

Sensing minuscule modifications to a star’s light is very difficult from the ground, where our planet’s atmosphere makes stars appear blurry. (It’s why stars appear to twinkle.) The crisp vision afforded by Hubble’s position above Earth’s atmosphere, along with its ability to accurately point at an object for long periods of time give the telescope a better chance to see minute details in the light of a star so far away.

As we try to study smaller and smaller planets, infrared telescopes, such as the James Webb Telescope, will complement Hubble’s observations and improve the precision of atmospheric studies. If life does exist among the stars, the atmospheric observations made by powerful future telescopes could be how we find it.

What Does Hubble Tell Us About Exoplanets?

Exoplanets have always been — and still are — difficult to find. Seeing them directly is like looking for a firefly next to a lighthouse from a mile away. Astronomers have had to devise clever and highly precise techniques to uncover exoplanets.

Using Hubble, astronomers have inferred the possible existence of exoplanets around several other stars with disks, including TW Hydrae, HD 141569, and Beta Pictoris.

Hubble continues to make significant contributions to the search for and study of planets around other stars. Its longevity, stability and instrumental sensitivity make it a unique asset in the quest for understanding alien worlds.

How Do Exoplanets Form?

Direct visual evidence shows that planets form from circumstellar disks of gas and dust around young stars. These disks, also known as protoplanetary disks, are difficult to observe, as they surround a star that is typically 100,000 times brighter than the disk. Hubble’s exquisite resolution and sensitivity, as well as its high-contrast imaging — in which the overwhelming light from the star is blocked — have provided numerous observations of these disks. Though the disks only reflect visible light from the star, they glow in infrared light, which Hubble can also detect.

Astronomers are using both old and new Hubble observations to discover disks forming around other stars.

• Finding Disks in Archived Data: By applying new image-processing techniques, astronomers have been able to tease out images of disks previously hidden in Hubble infrared data taken years ago. Such discoveries underscore the importance of archiving astronomical observations for future astronomers.
• New Survey of Disks: Astronomers have completed large visible-light imaging surveys of dusty disks around other stars. These disks, likely created by collisions between leftover objects from planet formation, were imaged around stars as young as 10 million years old and as mature as more than 1 billion years old.

These dust-disk searches reveal surprising characteristics of planet-forming disks, and no two look the same. These are not uniform flat disks; they are three-dimensional shapes with many smaller complex features. For instance, features in one ring-like system resemble a huge spray of debris from the recent collision of two objects.
 

This small sample shows marked diversity. Because planets form within these disks, the shapes of the disks should reflect the architectures of the forming planetary systems. The Hubble results are consistent with exoplanet observations, where planets are found arranged in orbits that are very different than those seen in our solar system.

These disk surveys also yield insight into how our solar system formed and developed. In particular, a suspected planet collision may be similar to how the Earth-Moon system or the Pluto-Charon system formed over 4 billion years ago. In those cases, collisions between planet-sized bodies cast debris that then coalesced into companion moons.

With the launch of the James Webb Space Telescope, disk systems like these are being observed in more detail in the infrared, potentially revealing evidence of newly formed planets.