PLANETPLANET

In yesterday’s post we built a planetary system with an Earth that has five Suns in the sky.  Here is what it looks like:

Today we are going to pretend that we are inhabitants of this five-Sun Earth and look up at the sky. How do the Suns move across the sky?  And, with all those Suns, when does night ever fall?

What does the sky look like from the surface of this planet?  The Tatooine binary behaves like our Sun.  It rises and set every day, but instead of one star there are two buddy stars that always stay close to each other. The Tatooine stars’ orbit around each other is about 20 times their actual size, so as the stars dance across the sky and around each other, they never stray more than ten degrees apart on the sky.  The stars eclipse each other twice every orbit, about once per week. Each eclipse lasts a couple of hours. These are not eclipses like we are used to, where the Moon blocks the Sun and casts its shadow on the Earth. Rather, in these eclipses one star passes in front of another. Both objects are bright so there is no shadow. Less light but no darkness.

The two red giants look almost the same as the Tatooine binary in the sky.  Each star is as big as the Sun. The size of their orbit is the same (on the sky) as the Tatooine binary’s, so the red giants are likewise in a continual dance but a much slower one, taking a few years to complete an orbit. And like the Tatooine binary, the red giants periodically eclipse each other.  Eclipses happen every year or so and last a full 5 days!

The red supergiant also rises and sets once a day as the planet rotates.  When the alignment of their positions on their orbits is just right, then the other stars may eclipse the supergiant!  But this is a once-in-a-thousand-years kind of thing.

When would night fall on this world?  This question is surprisingly tricky.  Let’s build it up piece by piece.

First, consider just the planet orbiting the Tatooine binary.  This is the same basic setup as the Earth going around the Sun.  The Tatooine Suns rise and set as the planet rotates.  The stars appear in the sky for about half of the day.

Let’s use a clock face to visualize this.  The planet orbits around the clock, moving one hour each month.  In the image below, the white line along the outer edge of the clock face is the planet’s orbit.  The Tatooine binary is in the center.

On Earth, the Sun is in the sky half of the day (on average).  But it stays light for a little while after sunset and it gets light a little before sunrise.  This time of day is called twilight.  It is truly dark when the Sun is about 15 degrees below the horizon.  The Earth spins a full 360 degrees every 24 hours, or 15 degrees per hour.  That means that it is only dark for 10 hours of every 24 on Earth. The same goes for a planet orbiting the Tatooine binary.  [Note: in calculating the length of the night time I am making the simple assumption that our planet’s spin axis is pointed straight out of its orbital plane.  This means that the planet does not have seasons like Earth, which are caused by the 23 degree tilt in Earth’s spin axis relative to its orbit.]

Let’s add the supergiant back in next. Now we have to consider the planet’s year, its orbit around the Tatooine binary. When the supergiant and the Tatooine binary are on the same side of the sky, then night falls on the side of the planet facing away from the stars.  But when the supergiant and Tatooine binary are on exactly opposite sides of the sky, there is no night time.  When the Tatooine binary is setting, the supergiant is rising.  And when the supergiant is setting, the Tatooine binary is rising.

Back to the clock face.  Now the supergiant is off to the right on the image below.  When the planet is at the 9 on the clock, the Tatooine binary and the supergiant are lighting the exact same part of the planet, so there is a full 10 hour night time.  But when the planet is at the 3 on the clock the situation is reversed and there is no night time at all.

The region from 2 to 4 on the clock is the “night-free” zone.  When the planet is in that wedge of its orbit, there can be spells of twilight but no true darkness. This is a full 2-month stretch of the planet’s 12 month orbit. Imagine: no night time for 2 months! Over the course of the year the duration of night time simply oscillates from its maximum of 10 hours when it is at the 9 on the clock face, down to zero for 2 months, and then back.

Now let’s add the red giant binary back in to the mix. This of course makes it even harder for night to fall on our 5-Sun Earth, since we are including two more sources of light in the system.

The red giants take about 50 years for them to complete an orbit around the Tatooine binary. The supergiant is so far away that we can consider it not to move. We need to think on the 50 year timescale of the red giants’ orbit, because that is what can change the alignment between the stars.  Let’s zoom way out in time and try to visualize what happens as the red giants orbit the Tatooine binary.

As we saw, with just the Tatooine binary and the distant supergiant there is a 2 month period of the planet’s orbit during which there is no night time at all.  The red giant binary does not change anything when it is lined up with the supergiant because then the red giants and the supergiant light up the same side of the planet.

The red giants have the biggest effect when they are on the opposite side of the system from the supergiant. In that configuration the red giant illuminates the side of the planet that the supergiant can’t reach.  It looks like this:

Given the extra reach of each star because of twilight (and also because the binary is not simply a point in the sky), night cannot fall on the planet while the red giant is between the 8 and 10 on the clock face in this image. There is no night time during two twelfths of the 50-year orbit of the red giant around the Tatooine binary. That is about 8 years with absolutely zero night time on the planet.  Night exists — although it is sometimes very short — for the other ten twelfths (roughly 40 years) of the red giants’ orbit.

As the red giant binary leaves the 8-to-10 o’clock wedge, night falls once again, but only during a few months per year and not for long at a time. Here is what things look like 4 years later, when the red giant binary is at the 7 on the clock:

And another 4 years later, when the red giant binary is at the 6:

By this point there is a 3-month period in the planet’s orbit during which night lasts 4 hours or longer.  A big improvement for astronomers who like dark skies!

Over the next half of the red giant binary’s 50-year orbit, as it travels from the 6 to the 3 then up to the 12 on the clock face, night falls for at least four hours over at least six months of the year.  Of course, there are always two months with no night time when the planet passes between the Tatooine binary and the red giants (as when the planet is at the 6 in the last image above), and two months with no night time when the planet passes between the Tatooine binary and the supergiant (as when the planet is at the 3 in the last image).  The most favorable case for darkness is when the red giant binary and the supergiant are aligned and both on the same side of the sky.

Then, when the red giant turns the corner at 12 o’clock, the next 25 years on the planet are awfully bright!  (Especially those 8 years with no night!)

To sum up, the heavens really move on our 5-Sun Earth!  Not one but five Suns rise and set each day: two close pairs (the Tatooine binary and the red giant binary) and an even larger red Sun (the supergiant). The timing and configuration of the Suns rising and setting change over the course of the planet’s year and also over the course of the red giants’ 50-year orbit around the Tatooine binary.  Eclipses of all kinds and durations are a common occurrence, although they never completely darken the sky (too many Suns!).

Night is a luxury on the 5-Sun Earth. There is no full darkness for at least two months each year.  Every 50 years there is an eight year stretch with no night time. It’s almost like Isaac Asimov’s story Nightfall, where a planet enters night time for the first time in 1000 years!

Imagine living on this 5-Sun Earth and being born when the red giants’ orbit was at 11 o’clock.  You would be just four years old when the red giants reached the 8-year “no night” zone.  As a kid you heard endless stories about darkness, the stars, true night time. But you didn’t remember it, didn’t understand what it meant.  Imagine your excitement at 12 years old, awaiting the first night in nearly a decade.  The first real night of your life.

 

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The story of the Earth with five Suns is not over yet.  Tomorrow we will get astronomical and take a look at how a system like this one would evolve.

Stay tuned!