astronomy

What is the Earth-like planet?

overview.

In an infinite universe, most scientists agree, the odds of life existing on a planet besides Earth are pretty high. It is unlikely, however, that familiar life forms will be found on any planet within our solar system. Life as we know it—everything from single-celled organisms to human beings—consists largely of liquid water. So a planet that harbors life can’t be too cold or water will freeze, nor can it be too hot or all the water will evaporate. Planets closer to the sun than Earth are too hot, and those farther away are too cold. The surface of Venus, for example, is hot enough to melt lead, and would vaporize any living thing, while the surface of Mars is frozen solid.

Life as we know it here on Earth also requires a magnetic field and an atmosphere, both of which protect it from the lethal radiation our parent star, the sun, emits. Earth’s magnetic field—generated by its rotating iron core—deflects the solar wind, a continuous stream of high-speed, high-energy particles coming out of the sun. (As those particles careen by the edges of Earth’s atmosphere, they sometime create the phenomenon we call the Northern Lights.) Without the magnetic field there, the solar wind might destroy all life on Earth.

As for Earth’s atmosphere, it protects life because the water, carbon dioxide and other gases in it absorb solar radiation in its harmful ultraviolet-light form. The parent stars of other solar systems would emit radiation as well, and the planets orbiting them would need the same kind of protection.

Of course, life on Earth also alters the chemical composition of the atmosphere—Earth’s atmosphere lacked gaseous oxygen until about a billion years ago, when first algae and then plants began growing here and producing it. So molecules like oxygen in the atmosphere of another planet would be one indication—not proof—that there are living things there.

What is a Planet?

Defining the term planet is important, because such definitions reflect our understanding of the origins, architecture, and evolution of our solar system. Over historical time, objects categorized as planets have changed. The ancient Greeks counted the Earth’s Moon and Sun as planets along with Mercury, Venus, Mars, Jupiter, and Saturn. Earth was not considered a planet, but rather was thought to be the central object around which all the other celestial objects orbited. The first known model that placed the Sun at the center of the known universe with the Earth revolving around it was presented by Aristarchus of Samos in the third century BCE, but it was not generally accepted. It wasn’t until the 16th century that the idea was revived by Nicolaus Copernicus.

By the 17th century, astronomers (aided by the invention of the telescope) realized that the Sun was the celestial object around which all the planets—including Earth—orbit, and that the moon is not a planet, but a satellite (moon) of Earth. Uranus was added as a planet in 1781 and Neptune was discovered in 1846.

Ceres was discovered between Mars and Jupiter in 1801 and originally classified as a planet. But as many more objects were subsequently found in the same region, it was realized that Ceres was the first of a class of similar objects that were eventually termed asteroids (star-like) or minor planets.

Pluto, discovered in 1930, was identified as the ninth planet. But Pluto is much smaller than Mercury and is even smaller than some of the planetary moons. It is unlike the terrestrial planets (Mercury, Venus, Earth, Mars), or the gas giants (Jupiter, Saturn), or the ice giants (Uranus, Neptune). Charon, its huge satellite, is nearly half the size of Pluto and shares Pluto’s orbit. Though Pluto kept its planetary status through the 1980s, things began to change in the 1990s with some new discoveries.

Technical advances in telescopes led to better observations and improved detection of very small, very distant objects. In the early 1990s, astronomers began finding numerous icy worlds orbiting the Sun in a doughnut-shaped region called the Kuiper Belt beyond the orbit of Neptune—out in Pluto’s realm. With the discovery of the Kuiper Belt and its thousands of icy bodies (known as Kuiper Belt Objects, or KBOs; also called transneptunians), it was proposed that it is more useful to think of Pluto as the biggest KBO instead of a planet.

Earth-like planets.

The discovery of a super-Earth-sized planet orbiting a sun-like star brings us closer than ever to finding a twin of our own watery world. But NASA’s Kepler space telescope has captured evidence of other potentially habitable planets amid the sea of stars in the Milky Way galaxy.

▪️Kepler-452b.

The new discovery, Kepler-452b, fires the planet hunter’s imagination because it is the most similar to the Earth-sun system found yet: a planet at the right temperature within the habitable zone, and only about one-and-a-half times the diameter of Earth, circling a star very much like our own sun. The planet also has a good chance of being rocky, like Earth, its discoverers say.

Kepler-452b is more similar to Earth than any system previously discovered. And the timing is especially fitting: 2015 marks the 20th anniversary of the first exoplanet confirmed to be in orbit around a typical star.But several other exoplanet discoveries came nearly as close in their similarity to Earth.

▪️Kepler-186f.

Before this, the planet Kepler-186f held the “most similar” distinction (they get the common moniker, “Kepler,” because they were discovered with the Kepler space telescope). About 500 light-years from Earth, Kepler-186f is no more than 10 percent larger than Earth, and sails through its star’s habitable zone, making its surface potentially watery.

But its 130-day orbit carries it around a red-dwarf star that is much cooler than our sun and only half its size. Thus, the planet is really more like an “Earth cousin,” says Thomas Barclay of the Bay Area Environmental Research Institute at NASA’s Ames Research Center, Moffett Field, California, a co-author of the paper announcing the discovery in April 2014.

Kepler-186f gets about one-third the energy from its star that Earth gets from our sun. And that puts it just at the outside edge of the habitable zone. Scientists say that if you were standing on the planet at noon, the light would look about as bright as it does on Earth an hour before sunset.That doesn’t mean the planet is bereft of life, although it doesn’t mean life exists there, either.

▪️Kepler-62f.

Before Kepler-186f, Kepler-62f was the exoplanet known to be most similar to Earth. Like the new discovery, Kepler-62f is a “super Earth,” about 40 percent larger than our home planet. But, like Kepler-186f, its 267-day orbit also carries it around a star that is cooler and smaller than the sun, some 1,200 light-years away in the constellation Lyra. Still, Kepler-62f does reside in the habitable zone.

▪️Kepler-69c.

Kepler-62f’s discovery was announced in April 2013, about the same time as Kepler-69c, another super Earth — though one that is 70 percent larger than our home planet. That’s the bad news; astronomers are uncertain about the planet’s composition, or just when a “super Earth” becomes so large that it diminishes the chance of finding life on its surface. That also moves it farther than its competitors from the realm of a potential Earth twin. The good news is that Kepler-69c lies in its sun’s habitable zone, with a 242-day orbit reminiscent of our charbroiled sister planet, Venus. Its star is also similar to ours in size with about 80 percent of the sun’s luminosity. Its planetary system is about 2,700 light-years away in the constellation Cygnus.

▪️Kepler-22b.

Kepler-22b also was hailed in its day as the most like Earth. It was the first of the Kepler planets to be found within the habitable zone, and it orbits a star much like our sun. But Kepler-22b is a sumo wrestler among super Earths, about 2.4 times Earth’s size. And no one knows if it is rocky, gaseous or liquid. The planet was detected almost immediately after Kepler began making observations in 2009, and was confirmed in 2011. This planet, which could have a cloudy atmosphere, is 600 light-years away, with a 290-day orbit not unlike Earth’s.

▪️Gliese 667Cc .

Not all the planets jostling to be most like Earth were discovered using Kepler. A super Earth known as Gliese 667Cc also came to light in 2011, discovered by astronomers combing through data from the European Southern Observatory’s 3.6-meter telescope in Chile. The planet, only 22 light-years away, has a mass at least 4.5 times that of Earth. It orbits a red dwarf in the habitable zone, though closely enough — with a mere 28-day orbit — to make the planet subject to intense flares that could erupt periodically from the star’s surface. Still, its sun is smaller and cooler than ours, and Gliese 667Cc’s orbital distance means it probably receives around 90 percent of the energy we get from the sun. That’s a point in favor of life, if the planet’s atmosphere is something like ours. The planet’s true size and density remain unknown, however, which means it could still turn out to be a gas planet, hostile to life as we know it. And powerful magnetic fluxes also could mean periodic drop-offs in the amount of energy reaching the planet, by as much as 40 percent. These drop-offs could last for months, according to scientists at the University of Oslo’s Institute of Theoretical Astrophysics in Norway.

▪️Proxima Centauri.

Scientists say that their research on the nearby star “Proxima Centauri” showed the presence of an Earth-sized planet orbiting it.What’s more, this rocky planet moves in the region that makes liquid water possible on its surface.

Data for the new planet “Proxima b” indicate that its minimum mass is 1.3 times the mass of the Earth, and it orbits at a distance of 7.5 million kilometers from the star, and it takes 11.2 days to complete one revolution.

The distance between the new planet and the star it orbits is much less than the distance between the Earth and the Sun, which is 149 million km.

But Proxima Centauri is a type of star called a “red dwarf.

“It is much less in size and glow compared to the Sun, which made the new planet enjoy the same conditions as those on Earth, despite its proximity to the star.

▪️Ross 128 b.

Ross 128 b. The newly discovered exoplanet is the second-closest found to our solar system, only 11 light-years away. And it could support life.

Announcements about exoplanets, those found outside our solar system, seem almost commonplace in this golden age of discovery for astronomers. So why is Ross 128 b unique — apart from its rather human-sounding name?The planet is about the same size as Earth, and it may have a similar surface temperature, making it a temperate world that could support life.

Every 9.9 days, it completes an orbit around its host star, Ross 128, which is what’s known as a red dwarf star: They’re the coolest, faintest and most common stars found in the universe.

Because of their plentiful nature and the fact that other exoplanets have been found around these types of stars, red dwarfs are being studied and observed with increasing frequency in the hopes of finding more exoplanets.

Astronomers found Ross 128 and its planet using the European Southern Observatory’s planet-hunting instrument, called HARPS. The High Accuracy Radial velocity Planet Searcher is based at La Silla Observatory in Chile. The astronomers detail their discovery in a new study, published Wednesday in the journal Astronomy and Astrophysics.

The astronomers believe that Ross 128 b is a good candidate for further study when the European Southern Observatory’s Extremely Large Telescope can begin searching the atmospheres of exoplanets for biomarkers in 2025.

Eight ingredients for life in space.

1. Water.

Almost all the processes that make up life on Earth can be broken down into chemical reactions – and most of those reactions require a liquid to break down substances so they can move and interact freely.

Liquid water is an essential requirement for life on Earth because it functions as a solvent. It is capable of dissolving substances and enabling key chemical reactions in animal, plant and microbial cells.

Its chemical and physical properties allow it to dissolve more substances than most other liquids. Other characteristics that make it a good habitat for life are its heat conduction, surface tension, high boiling and melting points, and its ability to let light penetrate it.

2. Carbon.

Many complex molecules are needed to perform the thousands of functions sustaining complex life. Carbon is the simple building block that organisms need to form organic compounds such as proteins, carbohydrates and fats.

Carbon’s molecular structure allows its atoms to form long chains, with each link leaving two potential bonds free to join with other atoms. It bonds particularly easily with oxygen, hydrogen and nitrogen.

The free bonds can even join with other carbon atoms to form complex 3D molecular structures, such as rings and branching trees. Carbon molecules are also strong and stable, so they are perfect to build a body with.

3. Nitrogen.

Carbon is a fundamental component of organic compounds, but it can’t do it alone. The complex proteins required for life are built up from smaller compounds called amino acids – simple organic compounds that contain nitrogen.

Nitrogen is also needed to make DNA and RNA, the carriers of the genetic code for life on Earth.

Many bacteria can convert nitrogen from the atmosphere into a form that is used in living cells.

4. Phosphorus.

Phosphorus is a key component of adenosine triphosphate (ATP), an organic substance that acts as life’s molecular unit of currency.

ATP transports chemical energy around the body’s cells, powering nearly every cellular process that requires energy.Phosphorus is a vital element in cell membranes, the layer surrounding the inside of cells that controls the movement of substances in and out.

And like nitrogen, phosphorus is necessary to create DNA and RNA.

5. Sulphur.

Sulphur is part of most biochemical processes on Earth, and most enzymes cannot function without it. It is also a component of many vitamins and hormones.

In the absence of oxygen and light, it is possible to use sulphur as an energy source. Bacteria that live under severe environmental conditions are called extremophiles and have been found to gain their energy for growth from sulphur and hydrogen alone.

6. Luck.

Having all the right chemicals on the same planet seems fortunate. And Earth – a tiny planet in the middle of an enormous universe – is lucky to have enough of the right chemicals to support a vast abundance of life.

7. Time.

The development of complex life takes billions of years, and there’s no shortcut in the journey from single-celled organisms to complex life.

Earth is 4.5 billion years old, but in its earliest stages it was far too hot to support life. The oldest fossil evidence of life comes from rocks that are 3.4 billion years old. It took a long time to evolve plants and animals from single-celled organisms.

It is possible that life exists on other planets – but it is likely such life would have a lot of evolutionary catching up to do.

8. Location.

Earth falls into the Goldilocks zone, meaning it is just the right distance from the Sun: not too hot or too cold to have liquid water on the surface.

Astronomers are searching for planets that are a similar distance from their own host stars.

Life needs an energy source to power growth – either the right amount of light from a star or chemically generated energy. Life also needs protection from certain wavelengths of solar radiation. Exposure to ultraviolet B damages DNA, but this wavelength is mostly absorbed by the ozone layer.

Reference:

Kenneth R. Lang (6-5-2017), “What are the chances of life on another planet”، now.tufts.edu”

Planets”, www.solarsystem.nasa.gov,6-12-2017

Pat Brennan (23-7-2015), “Finding Another Earth”، www.nasa.gov

جوناثان آموس (25-8-2016)، “”شبيه الأرض” يدور حول نجم قريب من الشمس”، www.bbc.com

Ashley Strickland (15-11-2017), “Newly discovered nearby planet could support life”، www.cnn.com

Paul Kenrick,Anne Jungblut (23-5-2016), “Eight ingredients for life in space”، www.nhm.ac.uk

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