The Basics

Before we found the first exoplanets — planets orbiting other stars — it seemed reasonable to suppose that other planetary systems looked like ours: small, rocky planets close to a Sun-like star, a big Jupiter and a few other gas giants farther out.

But after a quarter century of discovery revealing thousands of exoplanets in our galaxy, things look very different. In a word, we are “weird” — at least among the planetary systems found so far.

Just how weird is still a matter of debate. And weirdness is relative. We’ve detected “hot Jupiters” in scorching, star-hugging orbits around their stars, where a “year” — one trip around the star — takes only a few days. We’ve found a string of small, rocky worlds, all in Earth’s size-range, in lock-step orbits around a tiny red-dwarf star called TRAPPIST-1. We’ve seen systems with one or more planets that are larger than Earth and smaller than Neptune. The properties of these worlds are a mystery because they’re unlike anything in our solar system — and yet, they’re among the most common types of exoplanets discovered so far.

In all this variety, we’ve seen nothing yet that quite resembles our own setup: a Sun-like star with a retinue of rocky planets close in and more distant gas giants (including a domineering Jupiter).

We’re also notable for what we don’t have. Those planets larger than Earth and smaller than Neptune, for one. Systems like TRAPPIST-1 also have multiple planets in nearby orbits that are similar to each other in size and mass. For us, it’s a bit of a hodge-podge.

“Mercury and Mars are less massive than Venus and Earth,” says Yasuhiro Hasegawa, a NASA Jet Propulsion Laboratory researcher who studies the formation of planets and disks around stars. Why do we start with a small planet, Mercury, then have a relatively big Venus and Earth, then a smallish Mars?

And speaking of Mercury, why is our innermost planet so far from the Sun? You could fit the entire TRAPPIST-1 system of seven planets well within Mercury’s orbit. Many other systems detected so far also have planets in orbits far closer to their stars.

“Why is there no planet within Mercury’s orbit?” Hasegawa asks.

Nothing closer to the Sun than Mercury, a small Mars just beyond a bigger Venus and Earth, a really big Jupiter in a distant orbit. “That kind of configuration currently seems very rare,” Hasegawa says.

Some of our strangeness, of course, is likely an artifact of our limited technology. Detecting systems like ours, with planets in years-long orbits around middle-weight, yellow stars, is far more difficult with present methods than finding planets in short orbits around small red dwarfs. Such planets are easier to detect by the “transit” method, when a telescope measures a tiny dip in starlight as a planet crosses the face of its star; planets in longer orbits require far more observation time to find them.

Credit: NASA

Using observations from NASA’s Transiting Exoplanet Survey Satellite (TESS), an international team of astronomers has discovered a trio of hot worlds larger than Earth orbiting a much younger version of our Sun called TOI 451. The system resides in the recently discovered Pisces-Eridanus stream, a collection of stars less than 3% the age of our solar system that stretches across one-third of the sky.

The planets were discovered in TESS images taken between October and December 2018. Follow-up studies of TOI 451 and its planets included observations made in 2019 and 2020 using NASA’s Spitzer Space Telescope, which has since been retired, as well as many ground-based facilities. Archival infrared data from NASA’s Near-Earth Object Wide-Field Infrared Survey Explorer (NEOWISE) satellite – collected between 2009 and 2011 under its previous moniker, WISE – suggests the system retains a cool disk of dust and rocky debris. Other observations show that TOI 451 likely has two distant stellar companions circling each other far beyond the planets.

“This system checks a lot of boxes for astronomers,” said Elisabeth Newton, an assistant professor of physics and astronomy at Dartmouth College in Hanover, New Hampshire, who led the research. “It’s only 120 million years old and just 400 light-years away, allowing detailed observations of this young planetary system. And because there are three planets between two and four times Earth’s size, they make especially promising targets for testing theories about how planetary atmospheres evolve.”

A paper reporting the findings was published on Jan. 14 in The Astronomical Journal and is available online.

Stellar streams form when the gravity of our Milky Way galaxy tears apart star clusters or dwarf galaxies. The individual stars move out along the cluster’s original orbit, forming an elongated group that gradually disperses.

In 2019, a team led by Stefan Meingast at the University of Vienna used data from the European Space Agency’s Gaia mission to discover the Pisces-Eridanus stream, named for the constellations containing the greatest concentrations of stars. Stretching across 14 constellations, the stream is about 1,300 light-years long. However, the age initially determined for the stream was much older than we now think.

Later in 2019, researchers led by Jason Curtis at Columbia University in New York City analyzed TESS data for dozens of stream members. Younger stars spin faster than their older counterparts do, and they also tend to have prominent starspots – darker, cooler regions like sunspots. As these spots rotate in and out of our view, they can produce slight variations in a star’s brightness that TESS can measure.

The TESS measurements revealed overwhelming evidence of starspots and rapid rotation among the stream’s stars. Based on this result, Curtis and his colleagues found that the stream was only 120 million years old – similar to the famous Pleiades cluster and eight times younger than previous estimates. The mass, youth, and proximity of the Pisces-Eridanus stream make it an exciting fundamental laboratory for studying star and planet formation and evolution.

Credit: NASA

Since a giant planet in a scorching orbit captured public attention in 1995, a sky full of strange and exotic exoplanets – planets orbiting other stars – has only grown richer in variety and detail.

Hot Jupiters, mini-Neptunes, “super-Earths,” planets with two or three suns in their skies, rocky planets drowned in global oceans of lava, planets where it might rain glass – these make up just a short list of oddities among more than 4,300 confirmed so far in our Milky Way galaxy.

And we’ve only scratched the surface. The galaxy likely holds trillions

The search for life beyond Earth has grown up alongside the search for distant worlds. Computer simulations of possible life-bearing planets look more and more like the real thing. Deeper understanding of possible habitable worlds in our own solar system – Mars, Jupiter’s moon Europa, Saturn’s Enceladus – informs the hunt for life among the stars.

Planetary scientists, exoplanet hunters, and astrobiologists, who seek to understand the origins and requirements of life, have begun to join forces. On many fronts, NASA, with help from its academic and international partners, is leading the charge.

“I never fail to be thrilled by just the energy and innovation and creativity of the exoplanet community,” said Doug Hudgins, program scientist for NASA’s Exoplanet Exploration Program at NASA headquarters in Washington. “One of the things that makes the field as compelling as it is, is that it’s hugely important to people’s worldview, where we are as human beings. Are we alone? It’s directly addressing a fundamental question of humankind.”

Though not the first exoplanet ever found, 51 Pegasi b was the first detected in orbit around a Sun-like star. The planet ignited international excitement when it was confirmed in 1995, ushering in a new era of discovery.

A gas giant with about half the heft, or “mass,” of our own Jupiter, 51 Peg orbits its star so tightly that a year – once around the star – takes only four days.

That keeps 51 Peg infernally hot; life on this planet is out of the question. But 51 Peg showed that exoplanets could be detected by the “wobble” method, or radial velocity – tracking by telescope the gravitational jiggles a planet causes its star to make, tugging it first one way, then another.

This method resulted in dozens, then hundreds of exoplanet discoveries, and is still an important detection method. But since 2009, it’s been eclipsed by the search for shadows.

Also called the “transit” method, this approach involves waiting for a planet to cast a shadow as it crosses (or transits) the face of its star. It’s an extremely faint shadow – a dip in the star’s light that typically amounts to less than 1%.

Credit: NASA

Earth and the Moon are part of the universe, as are the other planets and their many dozens of moons. Along with asteroids and comets, the planets orbit the Sun. The Sun is one among hundreds of billions of stars in the Milky Way galaxy, and most of those stars have their own planets, known as exoplanets.

The Milky Way is but one of billions of galaxies in the observable universe — all of them, including our own, are thought to have supermassive black holes at their centers. All the stars in all the galaxies and all the other stuff that astronomers can’t even observe are all part of the universe. It is, simply, everything.

Though the universe may seem a strange place, it is not a distant one. Wherever you are right now, outer space is only 62 miles (100 kilometers) away. Day or night, whether you’re indoors or outdoors, asleep, eating lunch or dozing off in class, outer space is just a few dozen miles above your head. It’s below you too. About 8,000 miles (12,800 kilometers) below your feet — on the opposite side of Earth — lurks the unforgiving vacuum and radiation of outer space.

In fact, you’re technically in space right now. Humans say “out in space” as if it’s there and we’re here, as if Earth is separate from the rest of the universe. But Earth is a planet, and it’s in space and part of the universe just like the other planets. It just so happens that things live here and the environment near the surface of this particular planet is hospitable for life as we know it. Earth is a tiny, fragile exception in the cosmos. For humans and the other things living on our planet, practically the entire cosmos is a hostile and merciless environment.

Our planet, Earth, is an oasis not only in space, but in time. It may feel permanent, but the entire planet is a fleeting thing in the lifespan of the universe. For nearly two-thirds of the time since the universe began, Earth did not even exist. Nor will it last forever in its current state. Several billion years from now, the Sun will expand, swallowing Mercury and Venus, and filling Earth’s sky. It might even expand large enough to swallow Earth itself. It’s difficult to be certain. After all, humans have only just begun deciphering the cosmos.

While the distant future is difficult to accurately predict, the distant past is slightly less so. By studying the radioactive decay of isotopes on Earth and in asteroids, scientists have learned that our planet and the solar system formed around 4.6 billion years ago.

The universe, on the other hand, appears to be about 13.8 billion years old. Scientists arrived at that number by measuring the ages of the oldest stars and the rate at which the universe expands. They also measured the expansion by observing the Doppler shift in light from galaxies, almost all of which are traveling away from us and from each other. The farther the galaxies are, the faster they’re traveling away. One might expect gravity to slow the galaxies’ motion from one another, but instead they’re speeding up and scientists don’t know why. In the distant future, the galaxies will be so far away that their light will not be visible from Earth.

Put another way, the matter, energy and everything in the universe (including space itself) was more compact last Saturday than it is today.

Credit: NASA

After traveling several billion miles toward the Sun, a wayward young comet-like object orbiting among the giant planets has found a temporary parking place along the way. The object has settled near a family of captured ancient asteroids, called Trojans, that are orbiting the Sun alongside Jupiter. This is the first time a comet-like object has been spotted near the Trojan population.

The unexpected visitor belongs to a class of icy bodies found in space between Jupiter and Neptune. Called "Centaurs," they become active for the first time when heated as they approach the Sun, and dynamically transition into becoming more comet-like.

Visible-light snapshots by NASA's Hubble Space Telescope reveal that the vagabond object shows signs of comet activity, such as a tail, outgassing in the form of jets, and an enshrouding coma of dust and gas. Earlier observations by NASA's Spitzer Space Telescope gave clues to the composition of the comet-like object and the gasses driving its activity.

"Only Hubble could detect active comet-like features this far away at such high detail, and the images clearly show these features, such as a roughly 400,000-mile-long broad tail and high-resolution features near the nucleus due to a coma and jets," said lead Hubble researcher Bryce Bolin of Caltech in Pasadena, California.

Describing the Centaur's capture as a rare event, Bolin added, "The visitor had to have come into the orbit of Jupiter at just the right trajectory to have this kind of configuration that gives it the appearance of sharing its orbit with the planet. We’re investigating how it was captured by Jupiter and landed among the Trojans. But we think it could be related to the fact that it had a somewhat close encounter with Jupiter."

The team's paper appears in the February 11, 2021 issue of The Astronomical Journal.

The research team's computer simulations show that the icy object, called P/2019 LD2 (LD2), probably swung close to Jupiter about two years ago. The planet then gravitationally punted the wayward visitor to the Trojan asteroid group's co-orbital location, leading Jupiter by about 437 million miles.

Credit: NASA

A new NASA paper provides the most detailed map to date of near-surface water ice on the Red Planet.

So you want to build a Mars base. Where to start? Like any human settlement, it would be best located near accessible water. Not only will water be crucial for life-support supplies, it will be used for everything from agriculture to producing the rocket propellant astronauts will need to return to Earth.

Schlepping all that water to Mars would be costly and risky. That’s why NASA has engaged scientists and engineers since 2015 to identify deposits of Martian water ice that could be within reach of astronauts on the planet’s surface. But, of course, water has huge scientific value, too: If present-day microbial life can be found on Mars, it would likely be nearby these water sources as well.

A new study appearing in Nature Astronomy includes a comprehensive map detailing where water ice is most and least likely to be found in the planet’s northern hemisphere. Combining 20 years of data from NASA’s Mars Odyssey, Mars Reconnaissance Orbiter, and the now-inactive Mars Global Surveyor, the paper is the work of a project called Subsurface Water Ice Mapping, or SWIM. The SWIM effort is led by the Planetary Science Institute in Tucson, Arizona, and managed by NASA’s Jet Propulsion Laboratory in Southern California.

“The next frontier for Mars is for human explorers to get below the surface and look for signs of microbial life,” said Richard Davis, who leads NASA’s efforts to find Martian resources in preparation for sending humans to the Red Planet. “We realize we need to make new maps of subsurface ice to improve our knowledge of where that ice is for both scientific discovery and having local resources astronauts can rely on.”

In the near future, NASA plans to hold a workshop for multidisciplinary experts to assess potential human-landing sites on Mars based on this research and other science and engineering criteria. This mapping project could also inform surveys by future orbiters NASA hopes to send to the Red Planet.

NASA recently announced that, along with three international space agencies, the signing of a statement of intent to explore a possible International Mars Ice Mapper mission concept. The statement brings the agencies together to establish a joint concept team to assess mission potential as well as partnership opportunities between NASA, the Agenzia Spaziale Italiana (the Italian Space Agency), the Canadian Space Agency, and the Japan Aerospace Exploration Agency.

Credit: NASA

A greenhouse is for growing plants. It is made of glass or clear plastic to let in lots of sunlight. But why not just put the plants outside? A greenhouse stays warmer than the air outside. Instead of cooling off at night, it traps some of the heat inside to keep the plants warm. Even in the winter, with no heat source but the Sun on a clear day, a greenhouse stays warmer than the air outside. In the summer, if a greenhouse gets too hot, the gardener can open the windows and doors and maybe turn on a fan.

Greenhouse Earth?

A greenhouse is terrific if all you want to do is grow heat-loving plants. But what if Earth's atmosphere started to behave like a too-hot greenhouse? Don't forget, we cannot open Earth's windows or doors to cool it off. Earth as a closed-up greenhouse would soon grow to be ghastly!

If you made our Gummy Greenhouse Gas models, you may wonder why the molecules you made with gumdrops are called greenhouse gases. Here is why: If the atmosphere contains too much of these gases, the whole Earth becomes a hotter and hotter greenhouse. The atmosphere holds onto too much of the heat at night instead of letting it escape into space. Then, the next day, the Sun heats Earth's surface even more.

If the atmosphere works too well as a greenhouse, each day gets a little warmer and a little warmer. We may not be able to measure this effect from day to day or even year to year. But over tens of years, a few degrees of warming starts causing changes. For example, ice melts in the North and South Pole regions. All this new liquid water raises the sea level. Cities built on coastlines could someday be under water!

When the oceans get warmer, weather is affected everywhere. Some places have more severe storms and other places have hardly any rain at all. And many other changes could occur that would be bad for humans and other living things.

Our burning desires

Some of the greenhouse gases in the atmosphere are caused by humans. Whenever we burn anything, such as—

—we pollute our atmosphere with carbon dioxide and carbon monoxide. Although carbon monoxide does not act as a greenhouse gas, it is poisonous to breathe.

Our livestock (cows and chickens, for example) also pollute the atmosphere with methane from their digestion process.

Ozone is made when the Sun cooks carbon monoxide, such as from our car and truck exhaust, with other chemicals in the atmosphere.

Good ozone, bad ozone

In the case of ozone, it's all about location, location, location.

Scientists have divided the atmosphere into different layers, each with a name. The layer closest to the ground, where we live and fly in jets, is called the troposphere [TRO-po-sphere]. Above that layer is the stratosphere [STRAT-o-sphere], which goes to about 30 miles high. (Three more layers above that have names too, but we won't talk about those right now.)

Credit: NASA

A galaxy is a huge collection of gas, dust, and billions of stars and their solar systems, all held together by gravity.

We live on a planet called Earth that is part of our solar system. But where is our solar system? It’s a small part of the Milky Way Galaxy.

A galaxy is a huge collection of gas, dust, and billions of stars and their solar systems. A galaxy is held together by gravity. Our galaxy, the Milky Way, also has a supermassive black hole in the middle.

When you look up at stars in the night sky, you’re seeing other stars in the Milky Way. If it’s really dark, far away from lights from cities and houses, you can even see the dusty bands of the Milky Way stretch across the sky.

There are many galaxies besides ours, though. There are so many, we can’t even count them all yet! The Hubble Space Telescope looked at a small patch of space for 12 days and found 10,000 galaxies, of all sizes, shapes, and colors. Some scientists think there could be as many as one hundred billion galaxies in the universe.

Some galaxies are spiral-shaped like ours. They have curved arms that make it look like a pinwheel. Other galaxies are smooth and oval shaped. They’re called elliptical galaxies. And there are also galaxies that aren’t spirals or ovals. They have irregular shapes and look like blobs. The light that we see from each of these galaxies comes from the stars inside it.

Sometimes galaxies get too close and smash into each other. Our Milky Way galaxy will someday bump into Andromeda, our closest galactic neighbor. But don’t worry. It won’t happen for about five billion years. But even if it happened tomorrow, you might not notice. Galaxies are so big and spread out at the ends that even though galaxies bump into each other, the planets and solar systems often don’t get close to colliding.

Credit: NASA