A gigantic planet coming towards Earth?


* ¿Un gigantesco planeta viniendo hacia la Tierra?

Currently, there is a story circulating of a gigantic red planet that is about to hit our world within a very short time. It is alleged that this story is not fiction but reality. If so then, where is this doomsday planet?

The version in question does not give many details about this new gigantic planet: only that it would be red, that would be 5 or 6 times the size of Jupiter (the largest planet in the Solar System), would pass next to the Sun, and that this would be a very short-term prophecy.


The part about "5 or 6 times bigger than Jupiter" is a bit ambiguous, because normally astronomical objects larger than Jupiter are usually classified by their mass and not their diameters.

Jupiter has a diameter of about 140 000 kilometers, 11 times more than Earth's.

Planets so large typically have a very large gravitational field, which enabled them to catch a large amount of gas in the early times, so their predominant substances are usually hydrogen, helium, methane and water.

Assuming that the new gigantic planet has the same chemical composition we could assume the same density and calculate its diameter. With five times the mass we would be talking about 5 times the volume of Jupiter, what would (we can see with a calculator) give a diameter of 240 000 kilometers. Although, paradoxically, it is likely that rather than starting to grow in diameter it would end compacting itself, precisely for having so much mass.


Very large objects could become stars if they have too much mass: the pressure will start generating nuclear reactions inside. But it takes at least 80 times the mass of Jupiter to make this happen, albeit from as little as about 10 times that mass transition objects called brown dwarfs appear, where certain very weak secondary reactions are generated. But this is not our case.

So to see the new gigantic planet, sunlight has to go up to it and then back to us. Its apparent brightness will depend on how much sunlight comes to it, the reflectivity of the substances on its surface and on the amount of illuminated area it shows to us. Assuming that the new gigantic planet has 240 000 kilometers in diameter, compared to 140 000 of Jupiter, with a calculator we see that is has about 3 times more circular area (the area that we would see from Earth). If its surface had the same reflectivity as Jupiter's and were at the same distance, it would be 3 times brighter.

The part about the "red color" is a complication because it means it could be of a different chemical composition than Jupiter, at least in its outer shell. The colors of the planets are simply the residue of light remaining after the substances on their surfaces have absorbed all they can from the light of the Sun. If they are unable to absorb a color, for example red, they return it to space. The combination of whitish and reddish clouds of Jupiter returns to space 52% of the light it receives from the Sun. Earth returns 37%. The percentage in question is called albedo.

Coincidentally, the large asteroids from the depths of the Solar System (known as Trans-Neptunian Objects) are usually reddish, although they are not gaseous but rather of solid, frozen gases. The albedo of the objects in this region is usually less than 20%, or about one-third that of Jupiter. Thus, even though the new gigantic planet would be 3 times larger, it would have a very similar overall brightness as Jupiter, by being less reflective. That if both were at the same distance, of course.


Jupiter is the third brightest object in the night sky after the Moon and Venus. It's nearly 800 million kilometers from the Sun. We are about 150 million kilometers from the Sun, but as the Earth completes its orbit very quickly, sometimes both planets are on the same side and sometimes on opposite sides, so that the latter value can be added or subtracted. But we can say that the average distance from Jupiter to us will be about 800 million kilometers.

If we double the distance to a star its brightness drops to one fourth. If we double the distance again its brightness will drop to one fourth of one fourth. With a planet there is the additional complication that the light it receives from the Sun will also decrease, so the brightness it finally returns to us will be even less. If we double the distance of bright Jupiter, to 1600 million kilometers, its brightness will drop to 1/16 of what it is now. If we quadruple it, to 3200 million kilometers, its brightness will be 1/256.

In Astronomy, the brightness of the stars are measured on a scale of magnitudes invented by the Greek Hipparchus back in the second century B.C.E. and perfected by Norman Pogson in 1856. The brightest objects are called of first magnitude, the less bright of sixth magnitude. Objects of eighth, ninth, tenth, and so on cannot be seen with the naked eye. There are objects that appear brighter than the brightest night stars, and these have negative apparent magnitude, such as the Sun, the Moon, Venus, etc.. This is the case of Jupiter, which reaches about magnitude -3.

The scale is a bit complicated because if the value falls by 1 magnitude the brightness drops 2,5 times. If it falls another magnitude we must divide by 2,5 again and so on. So, for brilliant Jupiter to become invisible to our eyes (to fall from magnitude -3 to +7) its brightness should fall about 10 thousand times. To achieve this we must move it away at least 8000 million km from here, on the frontiers of the Solar System. Thus, assuming that the purported new planet shines as much as Jupiter, if it were less than 8000 million km from here it would be visible to the naked eye.


If the object comes from the depths of the Solar System its motion most likely would be similar to that of a comet. The fastest known comets are the parabolic comets, which at the end of their fall (when they are almost touching the Sun) could reach amazing speeds of more than 2 million km/h. But when they are far away their speed is much lower. For example, the Voyager spacecraft are in the region of Trans-Neptunian Objects going at "only" 60 000 km/h, but at such a distance from the Sun that's enough velocity to escape from the Solar System.

If we take this velocity as a parameter, a gigantic planet coming in the opposite direction would take 15 years to reach Earth, from the time it becomes visible to the naked eye.

If we use binoculars we could see it long before. Good quality binoculars can reach magnitude 10, enough to perceive it at a distance of 20 times the distance to Jupiter. We could see it 30 years before it reaches Earth, so the carnage would happen not in this but in the next generation.

A portable telescope for amateurs, of a good brand and a good size, can reach magnitude 15. So an average person could see it at a distance of 63 times the distance to Jupiter, that is, when the World would still have about 1 century available.

Professional telescopes like the Hubble Space Telescope are able to perceive brightness as faint as magnitude 30, so we would see it when it were just approaching the mark of 2000 times the distance to Jupiter, that is, about 3000 years in advance.

Now it is simply a matter of the author of the prophecy telling us which way we shall look so all of us can also see the new gigantic planet. As long as it really exists, of course.

Aldo Loup.

If you want to share this article with others, you may establish an Internet link, but you cannot copy any part of this page. Copyright © 2007-2012. Reproduction prohibited. All rights reserved.

Originally published in ABC Color on 13 May 2007. Photograph: An amateur astronomer prepares herself for a night of observation from the Negev desert, Israel, in July 2005. Photo Credit: Ilan Shimony (original license, of the photograph only, obtained at: http://creativecommons.org/licenses/by-sa/3.0/deed.en). With permission from Ilan Shimony.