What is an exoplanet?
An exoplanet is a planet outside our solar system, usually orbiting another star. They are also sometimes called "extrasolar planets", "extra-" implying that they are outside of our solar system.
Is there life on other planets?
Earth is the only planet we know of with life on it...so far. Scientists are searching the galaxy for planets similar to Earth, and signs of life. As we see on Earth, life can adapt to conditions that human beings would consider very harsh (temperature, radiation, salinity, acidity, aridity, etc). So it may be possible that life started on other worlds and adapted to conditions quite alien to what we are used to.
How many exoplanets are there?
To date, more than 3,400 exoplanets have been discovered and considered "confirmed." However, there are over 2,000 other "candidate" exoplanet detections that require further observations in order to say for sure whether or not the exoplanet is real.
Remarkably, the first exoplanets were just discovered about two decades ago. We live in an extraordinary time where in the span of a single generation, the centuries-old question "Are there planets orbiting other stars?" has been answered with a resounding "Yes!" Since the first exoplanets were discovered in the early 1990s, the number of known exoplanets has doubled approximately every 27 months.
Where are exoplanets?
In a lot of places! Most of the exoplanets discovered so far are in a relatively small region of our galaxy, the Milky Way. ("Small" meaning within thousands of light years of the solar system). That is as far as current telescopes have been able to probe.
Astronomers think that nearly every star in the universe could have at least one planet. That’s trillions of planets, waiting to be found. If you go outside on a clear night and look up at the stars, virtually every star you can see has at least one planet in orbit around it, and most likely, several.
What is the closest exoplanet to Earth?
Proxima Centauri b, which is about four light-years away. Its mass is consistent with it being slightly larger than Earth. Based on what we know about exoplanets, and planets in our solar system similar in mass to Earth, it is most likely a rocky planet. Proxima Centauri b orbits in the "habitable zone" of its star, which means it could have liquid water on its surface– if it has an atmosphere which could support it.
Calculations suggest that the one side of the exoplanet Proxima b faces its star all the time, which would make for a very different atmosphere and weather than we have on Earth. Its parent star, Proxima Centauri, is a dim red dwarf star that gives off about 600 times less light than our sun. Proxima Centauri is the closest star to the Earth after our sun, but it is still about 9,000 times further than planet Neptune.
How do we find exoplanets?
The successes of discovering exoplanets in recent decades seem to be telling us that the galaxy is teeming with trillions of exoplanets, but finding them isn't easy. Planets are typically billions of times fainter than the stars they orbit, and they are incredibly distant. The challenges of observing extrasolar planets stem from four basic facts:
- Planets don't produce any light of their own, except when they’re recently formed (i.e. young).
- They are an enormous distance from us.
- They are lost in the blinding glare of their parent stars.
- Their sizes and masses are typically much, much smaller than that of the stars they orbit.
It’s pretty rare for astronomers to see an exoplanet through their telescopes the way you might see Mars through a telescope from Earth. That’s called direct imaging, and only a handful of exoplanets have been found this way (and these tend to be young gas giant planets orbiting very far from their stars).
Most exoplanets are found through indirect methods: measuring the dimming of a star that happens to have a planet pass in front of it, or monitoring the spectrum of a star for the tell-tale signs of a planet pulling on its star and causing its light to subtly Doppler shift. NASA’s Kepler Space Telescope finds thousands of planets by observing “transits,” the slight dimming of light from a star when its tiny planet passes between it and our telescopes. Other methods include gravitational lensing, the “wobble method,” and direct imaging.
Will a person ever go to an exoplanet?
Not anytime soon, given the enormous distances between the stars and the time it would take to travel between them with our current technology. Perhaps one day a robot will visit an exoplanet like the rovers on Mars. But that day too is still very far in the future. First, we have to learn how to travel much faster and further in space than we’ve ever gone before. Keeping humans alive to complete such a long journey is probably the biggest challenge. Today, NASA is concentrating on the steps to get humans to Mars, which is our next-door neighbor. In the meantime, you can “stand” on an illustration of an exoplanet surface with our virtual reality.
Will we ever take real images of exoplanets?
Yes! In recent years, NASA has taken many some images of exoplanets that are light-years away. In the images, the planets sometimes look like faint spots next to blotted out regions (where the bright light of the planet's star has been subtracted). Examples of imaged planets orbit the bright stars Fomalhaut, HR 8799, and Beta Pictoris. The upcoming James Webb Space Telescope and WFIRST missions will image more exoplanets around nearby stars. The big challenge for NASA is to image Earth-like planets orbiting nearby stars, which tend to be about 10 billion times fainter than their stars,
How many telescopes are looking for exoplanets?
There are telescopes in space, on the ground, and even in the air that are being used to hunt exoplanets. NASA has three telescopes in space currently studying exoplanets– one observatory dedicated to discovering exoplanets (Kepler telescope), and two powerful, general-purpose observatories that conduct a wide range of astronomical observations, including exoplanet science (Hubble Space Telescope, Spitzer Space Telescope). More than two dozen telescopes on the ground are being used to discover and characterize exoplanets, ranging from small robotic observatories to large telescopes like the Keck Observatories in Hawaii. Even SOFIA, NASA's infrared observatory built into a Boeing 747-SP airplane, has conducted some exoplanet observations.
Do any exoplanets have rings?
Rings orbit all of the gas giant planets in our solar system. They range from being very dusty (Jupiter), to very icy (Saturn), to very carbon-rich (Uranus and Neptune). This hints that rings may be common around cold gas giant exoplanets orbiting other stars too. A very large ring system has been detected around one candidate exoplanet, J1407 b; however the rings cover tens of millions of miles. It may be a disk of material forming exomoons rather than ring system similar to Saturn's.
Do exoplanets have water on them?
Scientists have found water molecules in the atmosphere of more than one exoplanet, but some of those planets are very hot and probably not suitable for life as we know it. Scientists have yet to discover liquid water, like oceans or lakes. They are looking for exoplanets with liquid water in an area called the “habitable zone.”
What is the habitable zone or “Goldilocks zone”?
Imagine if Earth was where Pluto is. The sun would be barely visible (about the size of a pea) and Earth's ocean and much of its atmosphere would freeze. On the other hand, if Earth took Mercury’s place, it would be too close to the sun and its water would form a steam atmosphere, quickly boiling off. The distance Earth orbits the sun is just right for water to remain a liquid. This distance from the sun is called the habitable zone, or the Goldilocks zone. Rocky exoplanets found in the habitable zone of their star, are more likely targets for detecting liquid water on their surface. Why is liquid water so important? Life on Earth started in water, and water is a necessary ingredient for life (as we know it).
What are exoplanets made of?
The exoplanets for which astronomers are able to measure both their sizes (diameters) and masses, have densities that are suggestive of compositions ranging from very rocky (like Earth and Venus) to very gas-rich (like Jupiter and Saturn). Exoplanets are made up of elements similar to that of the planets in our solar system, but their mix of those elements may differ. Some planets may be dominated by water or ice, while others are dominated by iron or carbon.
Are there any exoplanets like Earth?
Not yet. We haven’t found a planet that can support life like Earth. So far, our home is unique in the universe. We have found many Earth-sized rocky exoplanets, some of which are in the habitable zone of their star. The next step in studying them is to analyze their atmospheres for “biosignature" molecules, which may be a sign of life.
Are other solar systems like ours?
Other solar systems, called planetary systems, come in many shapes and sizes. When scientists first started searching for planetary systems, they expected them to look like our solar system, with inner rocky planets and outer cold gas giants and ice planets. But some solar systems pack many planets into very compact orbits with short orbital periods, while others have only a few planets on orbits that last hundreds of years. Some planetary systems have Jupiter-like giants that orbit their star in a few hours, and are hot enough to melt metal. Some exoplanets even orbit multiple stars. So far astronomers have detected relatively few planetary systems with architectures like our solar system.
Do exoplanets have stars like our sun?
Yes, some do. Kepler-452b is an example of a planet that orbits a star like our sun. Both our sun and Kepler-452 are G dwarf stars, which make up only 3 percent of the stars in the universe. More common are M dwarf stars– small red stars that make up 75 percent of all the stars we know.
Other stars in our universe range from massive and hot O-type stars to the small and cool M dwarf stars like TRAPPIST-1 and Proxima Centauri. Scientists have found planets around five star types that range from red, to orange, to bluish-white. Some planets even orbit two or three stars at a time.
(Note: “Dwarf” is actually the name for a regular-sized adult star, distinguishing them from giant stars. An example of a giant star is Rigel, the blue supergiant found in the constellation Orion).
How do exoplanets get their names?
Exoplanet names can look long and complicated at first, especially in comparison to names like Venus and Mars. But they have a logic behind them that is important to scientists cataloging thousands of planets.
Astronomers differentiate between the alphanumeric "designations" and alphabetical "proper names." All stars and exoplanets have designations, but very few have proper names.
Let’s use HD 189733 b, a blue-ish planet where it may rains glass sideways, as an example. The first part of the name is usually the telescope or survey that found it. In this case ‘HD’ stands for the "Henry Draper Catalogue,” or a widely-used star catalog. The number 189733 is the order in which the star was catalogued by position (the 189,733rd star added to the catalog).
The lowercase letter 'b' stands for the planet, in the order in which the planet was found. The first planet found is always named 'b,' with future planets named c, d, e, f and so on. The star that the exoplanet orbits is usually the undeclared 'A' of the system, which can be useful if the system contains multiple stars, which themselves may be designated B, C. If a bunch of exoplanets around the same star are found at once, the planet closest to its star is named ‘b’ with more distant planets named c, d, e and so on.
In recent years, the International Astronomical Union, the international authority for the naming of celestial objects, has started the process of adopting proper names for exoplanets. The first batch of named exoplanets came from a contest open to the public, and included about 15 stars and 20 exoplanets. From the contest, the first giant exoplanet discovered in 1995 (51 Peg b) was named "Dimidium" and its host star (51 Peg) "Helvetios." Keep your eyes open for future exoplanet naming opportunities via the IAU!