This is Kepler 186f, an exoplanet in the habitable zone around a red dwarf. We've found many planets in their stars' habitable zones where they could potentially have surface water. But it's a fairly crude understanding of true habitability. Image Credit: NASA Ames, SETI Institute, JPL-Caltech, T. Pyle)
A seismic shift occurred in astronomy during the Scientific Revolution, beginning with 16th-century polymath Copernicus and his proposal that the Earth revolved around the Sun. By the 17th century, famed engineer and astronomer Galileo Galilei refined Copernicus’ heliocentric model using observations made with telescopes he built himself. However, it was not until Kepler’s observations that the planets followed elliptical orbits around the Sun (rather than circular orbits) that astronomical models matched observations of the heavens completely.
As it turns out, this very quirk of orbital mechanics may be essential to the emergence of life on planets like Earth. That was the hypothesis put forth in a recent study by a team of astronomers led by the University of Leeds. According to their work, orbital eccentricity (how much aplanet’s orbit deviates from a circle) can influence a planet’s climate response, which could have a profound effect on its potential habitability. These findings could be significant for exoplanet researchers as they continue to search for Earth-like planets that could support life.
Artist's impression of Planet Nine as an ice giant eclipsing the central Milky Way, with a star-like Sun in the distance. Neptune's orbit is shown as a small ellipse around the Sun. The sky view and appearance are based on the conjectures of its co-proposer, Mike Brown.
Does another undetected planet languish in our Solar System’s distant reaches? Does it follow a distant orbit around the Sun in the murky realm of comets and other icy objects? For some researchers, the answer is “almost certainly.”
Artist’s concept of Kepler-385, the seven-planet system revealed in a new catalog of planet candidates discovered by NASA’s Kepler space telescope. Image Credit:
NASA/Daniel Rutter
NASA’s Kepler mission ended in 2018 after more than nine years of fruitful planet-hunting. The space telescope discovered thousands of planets, many of which bear its name. But it also generated an enormous amount of data that exoplanet scientists are still analyzing.
Now, a team of researchers has shed new light on a seven-planet system in Kepler’s ocean of data.
On June 5th, 2012, the NASA/JAXA Hinode mission captured these stunning views of the transit of Venus. Credit: JAXA/NASA/Lockheed Martin
Earth and Venus are often called “sister planets” because they share some key characteristics. Like Earth, Venus is a terrestrial planet (i.e. composed of silicate minerals and metals) and orbits within our Sun’s habitable zone. But of course, they are also some major differences between them, like the fact that Venus’ is atmosphere is extremely dense and the hottest in the Solar System.
This is particularly interesting when you consider that Venus is not the closest planet to our Sun (that would be Mercury). In fact, its distance from the Sun is just over 70% the distance between Earth and the Sun. And due to its low eccentricity, there is very little variation in its distance during the course of its orbital period.
Perihelion and Aphelion:
While all planets follow an elliptical orbit, Venus’s orbit is the least eccentric of any of the Solar Planets. In fact, with an eccentricity of just 0.006772 , its orbit is the closest to being circular of any of the planets. It’s average distance (semi-major axis) from the Sun is 108,208,000 km (67,237,334 mi), and ranges from 107,477,000 km (66,783,112 mi) at perihelion to 108,939,000 km (67,691,556 mi) at aphelion.
Earth and Venus’ orbit compared. Credit: Sky and Telescope
To put it another way, Venus orbits the Sun at an average distance of 0.723 AU, which ranges from 0.718 AU at its closest to 0.728 AU at its farthest. Compare this to Earth’s eccentricity of 0.0167, which means that it orbits the Sun at an average distance of 1 AU, and that this distance ranges between 0.983 and 1.0167 AUs during its orbital period.
To express that in precise terms, the Earth orbits the Sun at an average distance of 149,598,023 km (92,955,902 mi), and varies between a distance of 147,095,000 km (91,401,000 mi) at perihelion to a distance of 152,100,000 km (94,500,000 mi) at aphelion.
Mars, by contrast, orbits the Sun at an average distance of 227,939,200 km (141,634,852 mi), or 1.52 AU. But due to its high eccentricity of 0.0934, it ranges from a distance of 206,700,000 km (128,437,425 mi) at perihelion to 249,200,000 km (154,845,700 mi) at aphelion – or between 1.38 to 1.666 AUs.
Mercury, meanwhile, has the highest eccentricity of any planet in the Solar System – a surprising 0.2056. While it’s average distance from the Sun is 57,909,050 km (35,983,015 mi), or 0.387 AU, it ranges from 46,001,200 km (28,583,820 mi) at perihelion to 69,816,900 km (43,382,210 mi) at aphelion – or 0.3075 to 0.4667 AUs.
Animated diagram showing the spacing of the Solar Systems planet’s, the unusually closely spaced orbits of six of the most distant KBOs, and the possible “Planet 9”. Credit: Caltech/nagualdesign
Hence, you might say Venus is something of an oddity compared to its fellow-terrestrial planets. Whereas they all orbit our Sun with a certain degree of eccentricity (from fair to extreme), Venus is the closest to orbiting in a circular pattern. And with an orbital velocity of 35.02 km/s (126,072 km/h; 78,337.5 mph), Venus takes 224.7 Earth days to complete a single orbit around the Sun.
Retrograde Motion:
Another oddity of Venus is the peculiar nature of its rotation. Whereas most objects in our Solar System have a rotation that is in the same direction as their orbit around the Sun, Venus’ rotation is retrograde to its orbit. In other words, if you could view the Solar System from above the Sun’s northern polar region, all of the planets would appear to be orbiting it in a counter-clockwise direction.
They would also appear to be rotating on their axis in the same counter-clockwise direction. But Venus would appear to be slowly rotating in a clockwise direction, taking about 243 days to complete a single rotation. This is not only the slowest rotation period of any planet, it also means that a sidereal day on Venus lasts longer than a Venusian year.
A popular theory states that this is due to two major impacts taking place between Venus and a series protoplanets in the distant past. Much like the impact that is believed to have created the Moon (between Earth and Theia), the first of these impact would have created a moon in orbit of Venus, while a second (10 million years later) would reverseed its rotation and caused the moon to de-orbit.
Artist’s concept of a collision between proto-Earth and Theia, believed to happened 4.5 billion years ago. Credit: NASA
Every planet in our Solar System has is shares of quirks, and Venus is no exception. She’s “Earth’s Sister”, and she’s prone to extreme temperatures that do not vary. And her orbit is the most stable of any planet, also with very little variation. You might say Venus is the extremely hot-tempered sibling of Earth, and very straight-laced to boot!
NASA's Hinode X-ray telescope captured Mercury in transit against the Sun's corona in Nov. 2006. Similar views are possible in H-alpha light. Credit: NASA
Mercury is famously known for being a scorching hot world. On the side that is facing towards the Sun, conditions can get pretty molten, reaching temperatures of up to 700 K (427 °C; 800°F) in the equatorial region. The surface is also airless, in part because any atmosphere it could generate would be blown away by solar wind. Hardly surprising, considering it is the closest planet to our Sun.
But just how close is it? On average, it’s slightly more than one-third the distance between Earth and the Sun. However, its orbital eccentricity is also the greatest of any planet in the Solar System. In addition, its orbit is subject to perturbations, ones which were not fully understood until the 20th century. Because of this, Mercury goes through some serious changes during its orbital period.
Perihelion and Aphelion:
Mercury orbits the Sun at an average distance (semi-major axis) of 0.387 AU (57,909,050 km; 35,983,015 mi). However, due to its eccentricity of 0.205 – the highest in the Solar System, with the exception of Pluto (0.248) – its distance from the Sun ranges considerably. When it is at its closest (perihelion), it is 46,001,200 km (28,583,820 mi) from the Sun; and when it is farthest away (aphelion), it is 57,909,050 km (35,983,015 mi) from the Sun.
A timelapse of Mercury transiting across the face of the Sun. Credit: NASA
Orbital Resonance:
At one time, scientists believed that Mercury was tidally-locked, meaning that it kept one side facing towards the Sun at all times. However, it has since been discovered that the planet actually has a slow rotational period of 58.646 days. Compared to its orbital period of 88 days, this means that Mercury has a spin-orbit resonance of 3:2. This means that the planet makes three completes rotations on its axis for every two orbits around the Sun.
Another consequences of its spin-orbit resonance is that there is a significance difference between the time it takes the planet to rotate once on its axis (a sidereal day) and the time it takes for the Sun to reappear in the same place in the sky (a solar day). On Mercury, it takes a 176 days for the Sun to rise, set, and return to the same place in the sky. This means, effectively, that a single day on Mercury lasts as long as two years!
It’s slow rotation also means that temperature variations are extreme. On the Sun-facing side, temperatures can reach as high as 700 K (427 °C; 800°F) in the equatorial region and 380 K (107 °C; 224 °F) near the northern polar region. On the side facing away from the Sun, temperatures reach a low of 100 K (-173 °C; -280 °F) in the equatorial region and 80 K (-193 °C; -316 °F) near the northern polar region.
Diagram of Mercury’s eccentric orbit. Credit: solarviews.com
Perihelion Precession:
In addition to its eccentricity, Mercury’s perihelion is also subject to precession. What this means is, during the course of a century, Mercury’s orbit around the Sun shifts by 42.98 arcseconds (0.0119 degrees). This means that after twelve million orbits, Mercury will have performed a full excess turn around the Sun and returned to where it started.
This is much larger than the perihelion precession of other Solar planets – which range from 8.62 arcseconds (0.0024°) per century for Venus, 3.84 (0.001°) for Earth, and 1.35 (0.00037°) for Mars. Until the early 20th century, this behavior remained a mystery to astronomers, as Newtonian mechanics could not account for it. However, Einstein’s General Theory of Relativity provided an explanation, while the precession provided a test for his theory.
You might say Mercury and the Sun are pretty cozy. They dance pretty close, and the dance is powerful and full of some pretty wide swings!
At a closest average distance of 41 million km (25,476,219 mi), Venus is the closest planet to Earth. Credit: NASA/JPL/Magellan
A common question when looking at the Solar System and Earth’s place in the grand scheme of it is “which planet is closest to Earth?” Aside from satisfying a person’s general curiosity, this question is also of great importance when it comes to space exploration. And as humanity contemplates mounting manned missions to neighboring planets, it also becomes one of immense practicality.
If, someday, we hope to explore, settle, and colonize other worlds, which would make for the shortest trip? Invariable, the answer is Venus. Often referred to as “Earth’s Twin“, Venus has many similarities to Earth. It is a terrestrial planet, it orbits within the Sun’s habitable zone, and it has an atmosphere that is believed to have once been like Earth’s. Combined with its proximity to us, its little wonder we consider it our twin.
Venus’ Orbit:
Venus orbits the Sun at an average distance (semi-major axis) of 108,208,000 km (0.723 AUs), ranging between 107,477,000 km (0.718 AU) at perihelion and 108,939,000 km (0.728 AU) at aphelion. This makes Venus’ orbit the least eccentric of all the planets in the Solar System. In fact, with an eccentricity of less than 0.01, its orbit is almost circular.
Earth and Venus’ orbit compared. Credit: Sky and Telescope
When Venus lies between Earth and the Sun, it experiences what is known as an inferior conjunction. It is at this point that it makes its closest approach to Earth (and that of any planet) with an average distance of 41 million km (25,476,219 mi). On average, Venus achieves an inferior conjunction with Earth every 584 days.
And because of the decreasing eccentricity of Earth’s orbit, the minimum distances will become greater over the next tens of thousands of years. So not only is it Earth’s closest neighbor (when it makes its closest approach), but it will continue to get cozier with us as time goes on!
Venus vs. Mars:
As Earth’s other neighbor, Mars also has a “close” relationship with Earth. Orbiting our Sun at an average distance of 227,939,200 km (1.52 AU), Mars’ highly eccentric orbit (0.0934) takes it from a distance of 206,700,000 km (1.38 AU) at perihelion to 249,200,000 km (1.666 AU) at aphelion. This makes its orbit one of the more eccentric in our Solar System, second only to Mercury
For Earth and Mars to be at their closest, both planets needs to be on the same side of the Sun, Mars needs to be at its closest distance from the Sun (perihelion), and Earth needs to be at its farthest (aphelion). This is known as opposition, a time when Mars appears as one of the brightest objects in the sky (as a red star), rivaling that of Venus or Jupiter.
The eccentricity in Mars’ orbit means that it is . Credit: NASA
But even at this point, the distance between Mars and Earth ranges considerably. The closest approach to take place occurred back in 2003, when Earth and Mars were only 56 million km (3,4796,787 mi) apart. And this was the closest they’d been in 50,000 years. The next closest approach will take place on July 27th, 2018, when Earth and Mars will be at a distance of 57.6 million km (35.8 mi) from each other.
It has also been estimated that the closest theoretical approach would take place at a distance of 54.6 million km (33.9 million mi). However, no such approach has been documented in all of recorded history. One would be forced to wonder then why so much of humanity’s exploration efforts (past, present and future) are aimed at Mars. But when one considers just how horrible Venus’ environment is in comparison, the answer becomes clear.
Exploration Efforts:
The study and exploration of Venus has been difficult over the years, owing to the combination of its dense atmosphere and harsh surface environment. Its surface has been imaged only in recent history, thanks to the development of radar imaging. However, many robotic spacecraft and even a few landers have made the journey and discovered much about Earth’s closest neighbor.
The first attempts were made by the Soviets in the 1960s through the Venera Program. Whereas the first mission (Venera-1) failed due to loss of contact, the second (Venera-3) became the first man-made object to enter the atmosphere and strike the surface of another planet (on March 1st, 1966). This was followed by the Venera-4 spacecraft, which launched on June 12th, 1967, and reached the planet roughly four months later (on October 18th).
The first color pictures taken of the surface of Venus by the Venera-13 space probe. Credit: NASA
NASA conducted similar missions under the Mariner program. The Mariner 2 mission, which launched on December 14th, 1962, became the first successful interplanetary mission and passed within 34,833 km (21,644 mi) of Venus’ surface. Between the late 60s and mid 70s, NASA conducted several more flybys using Mariner probes – such as the Mariner 5 mission on Oct. 19th, 1967 and the Mariner 10mission on Feb. 5th, 1974.
The Soviets launched six more Venera probes between the late 60s and 1975, and four additional missions between the late 70s and early 80s. Venera-5, Venera-6, and Venera-7 all entered Venus’ atmosphere and returned critical data to Earth. Venera 11 and Venera 12 detected Venusian electrical storms; and Venera 13 andVenera 14 landed on the planet and took the first color photographs of the surface. The program came to a close in October 1983, when Venera 15 and Venera 16 were placed in orbit to conduct mapping of the Venusian terrain with synthetic aperture radar.
By the late seventies, NASA commenced the Pioneer Venus Project, which consisted of two separate missions. The first was the Pioneer Venus Orbiter, which inserted into an elliptical orbit around Venus (Dec. 4th, 1978) to study its atmosphere and map the surface. The second, the Pioneer Venus Multiprobe, released four probes which entered the atmosphere on Dec. 9th, 1978, returning data on its composition, winds and heat fluxes.
Artist’s impression of NASA’s Pioneer Venus Orbiter in orbit around Venus. Credit: NASA
In 1985, the Soviets participated in a collaborative venture with several European states to launch the Vega Program. This two-spacecraft initiative was intended to take advantage of the appearance of Halley’s Comet in the inner Solar System, and combine a mission to it with a flyby of Venus. While en route to Halley on June 11th and 15th, the two Vega spacecraft dropped Venera-style probes into Venus’ atmosphere to map its weather.
NASA’s Magellan spacecraft was launched on May 4th, 1989, with a mission to map the surface of Venus with radar. In the course of its four and a half year mission, Magellan provided the most high-resolution images to date of the planet, was able to map 98% of the surface and 95% of its gravity field. In 1994, at the end of its mission, Magellan was sent to its destruction into the atmosphere of Venus to quantify its density.
Venus was observed by the Galileo and Cassini spacecraft during flybys on their respective missions to the outer planets, but Magellan was the last dedicated mission to Venus for over a decade. It was not until October of 2006 and June of 2007 that the MESSENGER probe would conduct a flyby of Venus (and collect data) in order to slow its trajectory for an eventual orbital insertion of Mercury.
The Venus Express, a probe designed and built by the European Space Agency, successfully assumed polar orbit around Venus on April 11th, 2006. This probe conducted a detailed study of the Venusian atmosphere and clouds, and discovered an ozone layer and a swirling double-vortex at the south pole before concluding its mission in December of 2014. Since December 7th, 2015, Japan’s Akatsuki has been in a highly elliptical Venusian orbit.
Because of its hostile surface and atmospheric conditions, Venus has proven to be a tough nut to crack, despite its proximity to Earth. In spite of that, NASA, Roscosmos, and India’s ISRO all have plans for sending additional missions to Venus in the coming years to learn more about our twin planet. And as the century progresses, and if certain people get their way, we may even attempt to send human colonists there!
