Introduction

A team of astronomers led by R. Di Stefano have found a possible exoplanet in another galaxy. What’s more is their results hint at a planet under very extreme conditions. Such conditions are not common with other exoplanets.

Up to now, most exoplanets where found around stars in our galaxy with most of them orbiting main-sequence stars. A few exoplanets were found in other galaxies using gravitational lensing. Unfortunately, such a method does not uncover much about the planets apart from their mass.

The new exoplanet dubbed, M51-ULS-1 b, orbits what is most likely a black hole or a neutron star siphoning gas off a blue supergiant star into an accretion disk. This produces large amounts of X-rays creating an extreme environment for the planet.

An exoplanet in the middle of a galactic collision

Di Stefano’s team found M51-ULS-1 b 23 million years from earth in the Whirlpool galaxy, also known as M51. An interesting fact about this beautiful spiral galaxy is that it is in the process of colliding with a dwarf galaxy called NGC5195.

The collision is deforming M51’s spiral arms and causing rapid star formation in certain parts of the galaxy. This has created many binary star systems with young massive stars.

In many of these binaries, the more massive star runs out of fuel first. They then turn into supergiants and then explode leaving behind neutron stars and black holes.

These compact objects start pulling material from their binary partners into disks. Friction causes the material to heat up to extreme temperatures. This causes them to produce X-rays. This is the case for the binary system M51-ULS-1 in which M51-ULS-1 b orbits.

A giant X-ray machine

At first glance, M51-ULS-1 looks like a single star. However, closer examination shows that this is in fact a binary star system. One of the stars has turned into a neutron star or a black hole and can only be detected by it’s powerful gravitational effect on the other star.

This object’s gravitational pull is so strong that it is pulling material from the second star into an accretion disk. This accretion disk emits vast amounts of high energy X-rays due to the temperature of the material. This means M51-ULS-1 produces a blackbody spectrum.

Like many X-ray sources, M51-ULS-1 experiences changes in brightness. However, one of these changes in brightness has drawn the attention of Di Stefano’s team due to it having the signs of an exoplanet transiting the X-ray source.

Detection methods

Exoplanets in other galaxies are difficult to find because stars in them are very close together in the sky. This makes identifying individual stars difficult. In turn, this makes it next to impossible to apply the transit method or the radial velocity method to find exoplanets.

With X-ray sources like M51-ULS-1, this is not a problem as they are far brighter then the other stars. This makes it easy to study them individually. The blackbody spectrum of M51-ULS-1 makes it easy to confirm the presence of a planet. This is because astronomers can predict the effects of objects passing in front of the source in such a spectrum.

To discover an exoplanet, Di Stefano’s team employed the transit method. The transit method involves looking for drops in star brightness due to a planet passing in front of the star. This is similar to the retired Kepler space telescope. The only difference being that Di Stefano’s team used X-rays while Kepler used visible light and infrared.

Detecting the right drop in brightness

Detecting a planet around an X-ray source such as M51-ULS-1 is not easy. For one, such sources vary in brightness for many reasons. One can easily misattribute a drop in brightness to a transiting planet when it is in fact due to something else.

X-ray sources like M51-ULS-1 change their apparent brightness when thicker portions of the accretion disk block part of the X-ray source. They also experience “off states” where the inherent brightness of the X-ray source changes. Given that M51-ULS-1 is a binary system, Di Stefano’s team also have to contend with transits caused by the star that is donating material to the disk.

The drop in brightness studied by Di Stefano’s team did not change the spectrum of M51-ULS-1 the way a star or a cloud of gas would. Furthermore, the light curve of the drop in brightness is symmetric. This further rules out the possibility that an irregularly shaped cloud of material caused the drop in brightness.

Finally, Di Stefano’s team saw that the drop in brightness lasted for 20 to 30 minutes. All known X-ray “off states” last more than 27 hours ruling this out as the cause for the drop in brightness.

Other things that could look like an exoplanet

Di Stefano’s team also had to contend with objects that look similar to planets due to their size and lack of brightness. White dwarfs, red dwarfs and brown dwarfs are some of these objects. Luckily, there are ways to rule out such objects… Well, almost.

White dwarfs are so massive that their gravity would focus light coming from the source. This causes an increase rather than the drop in brightness seen by Di Stefano’s team. So this cannot be a white dwarf.

Red dwarfs could be a possibility until you remember that M51-ULS-1 is only 10 million years old. The problem is that 10 million year old red dwarfs are predicted to be 10 times the diameter of Jupiter. The size of the object estimated from the transit by Di Stefano’s team is most likely one third the diameter of Jupiter and thus too small to be a red dwarf.

Di Stefano’s team cannot rule out the possibility that this object is a brown dwarf due to uncertainties in their measurements. However, they point out that brown dwarfs are less likely to orbit stars than planets. This together with the object’s most probable size makes it more likely a planet than a brown dwarf

Exoplanet orbit and physical properties

Based on the planet’s velocity from the transit as well as rough measurements of the total mass of M51-ULS-1, Di Stefano’s team deduced that the planet has an orbit radius between 10 to 100 times the distance between the Earth and the Sun (10 to 100 AU).

From this, we can deduce that the planet orbits both stars. Astronomers call this a circumbinary orbit. The planet’s orbital radius has to be more than 3 times the orbits of the binary pair or else it’s orbit would not be stable.

Di Stefano’s team found that the planet receives 1 million times more energy than Earth receives from the Sun. There are exoplanets that receive similar amounts of energy. These are “Hot Jupiters” orbiting within 5% the distance between Earth and the Sun around many Sun-like stars.

Such planets are very large in size due to intense heat. Their atmospheres are constantly being stripped away by intense radiation from their star.

The same thing could be happening to the planet around M51-ULS-1 even though the planet could be as far from the binary pair as Pluto is from the Sun. What’s more is that more of the energy in this case comes in the form of highly ionizing X-rays. These X-rays are certainly stripping away vast amounts of material from the planet.

Perhaps it could be that the planet was larger and more massive in the past.

How this exoplanet got to where it is?

Di Stefano’s team points out that there is little understanding of the evolution of planets orbiting systems such as M51-ULS-1. However, one can still speculate on how the planet survived and end up where it is.

According to Di Stefano’s team, the planet may have started out orbiting within several AUs from the binary pair while the black hole or neutron star was still a main sequence star. When this star evolved into it’s supergiant phase, it lost a great deal of mass. This caused the planet to enter a bigger orbit.

If the more massive star became a neutron star, the planet would have had to survive the resulting supernova. This is not impossible as many binary systems exist where one star has exploded and the other is still present. Given this fact, it’s not implausible for a planet with a big orbit to survive a supernova according to Di Stefano’s team.

If the more massive star became a black hole, the effect on the planet would have been less. This is because the more massive star may have undergone a failed supernova event which is less destructive.

Conclusion

The discovery of the planet around an X-ray binary in another galaxy has shown that planets can be found in very extreme conditions. It also opens up more possibilities in learning about exoplanets. According to Di Stefano’s team, it could also uncover any relationship between the properties of a galaxy and the planetary systems within it.

References

https://arxiv.org/abs/2009.08987